Handbook-milk-powder-manufacture.pdf

In 1968 Dr. Pisecky joined Niro Atomizer A/S in Denmark (now GEA Process Engineering A/S). He worked with Niro until his retirement in 1992 as the manager of the Dairy Development Group, with responsibilities for developing new processes, equipment design, and assisting in dairy plant commissioning. Dr. Pisecky has been an active member of the International Dairy Federation (IDF) since 1958. Vagn Westergaard gained his M.Sc. in Dairy Technology in 1969 and joined Niro Atomizer A/S (now GEA Process Engineering A/S) immediately thereafter holding different positions including in the Research and Development Division, as well as Division Manager of the Dairy Division. During his time at GEA Process Engineering A/S he worked part time as assistant professor teaching evaporation and spray drying technology at the Vet. & Agric. University in Copenhagen, Denmark. He partly retired in 2007. Ejnar Refstrup gained his M. Sc. in Dairy Technology in 1970, after which he joined Niro Atomizer A/S (now GEA Process Engineering A/S). In 1971. He went back to academia and gained his Ph. D. in 1974. After 6 years as assistant professor at the Vet. & Agric. University in Copenhagen, Denmark, he rejoined GEA Process Engineering A/S in 1980. He partly retired in 2011.

GEA Niro book design [3].indd 1

Edited by: Vagn Westergaard & Ejnar Refstrup

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edition

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DR. ING. JAN PISECKY

HANDBOOK OF

MILK POWDER MANUFACTURE

Beginning in 1951, Dr. Pisecky worked for the Milk Nutrition Industry (the monopoly manufacturer of milk-based powdered baby food in Czechoslovakia) as a consultant on vitaminization of baby food. He became leader of the research department in 1953 and over the years established himself as a world authority on dairy technology. In 1964 he became the director of the Dairy Research Institute of Czechoslovakia.

HANDBOOK OF MILK POWDER MANUFACTURE

Dr. Ing. Jan Pisecky was born in 1924 in Prague, Czechoslovakia. He graduated as an Engineer of Chemical Technology in 1949 and continued his studies at the Institute for Special Analytical Methods of the Technical University in Prague, where he received his doctorate in 1953.

DR. ING. JAN PISECKY Edited by: Vagn Westergaard & Ejnar Refstrup 24/02/12 15.04



Preface to Second Edition Dr. Ing. Jan Pisecký started working for GEA Process Engineering A/S in 1968 (at that time the company was known as Niro Atomizer). Soon thereafter he became head of the Dairy Research and Development Division. A position he held until his official retirement in 1992. However, Jan Pisecký continued to work as a consultant until his final retirement in 1999. During this time he wrote the book:

Handbook of Milk Powder Manufacture Published in 1997 In 2010 the book was sold out, and it was decided to update it and include the newest technology within evaporation and spray drying and at the same time describe new plant types. Dr. Ing. Jan Pisecký contacted the undersigned - both former “disciples” of him. We were both very proud and honoured, when he asked us to be in charge of the updating, now present here as the Second Edition of Jan´s book. We appreciate the valuable co-reading of the new chapter about evaporation by Jerry van Loon GEA Process Engineering, France, and we wish to thank Tessy Jakubczyk and Betina Grewal, GEA Process Engineering A/S who put everything in order ready to be sent to the book printer. Needless to say, but Jan Pisecký - the father of us all – had the final word to the text in this new updated second edition. We owe it all to him. Copenhagen, Denmark. February 2012 Vagn Westergaard and Ejnar Refstrup

Publisher and distributor

GEA Process Engineering A/S (GEA Niro) Gladsaxevej 305 DK-2860 Soeborg Copenhagen, Denmark in 2012

ISBN No. 87-87036-74-6

With 193, figures, drawings, photos and tabels.

The selection and presentation of material and the opinions expressed in this publication are the sole responsibility of the author. All rights reserved. No part of this publication may be produced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise without the prior written permission of the copyright owner. GEA Process Engineering A/S [email protected] www.niro.com

Printed by Rosendahls, Oddesundvej 1, 6715 Esbjerg N, Denmark, www.rosendahls.dk

List of contents

List of contents 1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 2. Evaporation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.1. 2.2. 2.2.1. 2.2.1.1. 2.2.1.2. 2.2.1.3. 2.2.1.3.1. 2.2.1.3.2. 2.2.1.3.3. 2.2.1.4. 2.2.1.4.1. 2.2.1.4.2. 2.2.2. 2.2.2.1. 2.2.2.2. 2.2.2.3. 2.2.3. 2.2.3.1.

Basic principles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Main components of the evaporator. . . . . . . . . . . . . . . . . . . . . . . . . . . . . Heat exchanger for preheating. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Spiral-tube preheaters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Straight-tube preheaters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Preheaters to prevent growth of spore forming bacteria . . . . . . . . . . . . Direct contact regenerative preheaters. . . . . . . . . . . . . . . . . . . . . . . . . . . Duplex preheating system. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Preheating by direct steam injection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . Other means to solve presence of spore forming bacteria. . . . . . . . . . . Mid-run cleaning. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . UHT treatment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pasteurizing system including holding . . . . . . . . . . . . . . . . . . . . . . . . . . . Indirect pasteurization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Direct pasteurization. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Holding tubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Product distribution system. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Dynamic distribution system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3 4 5 5 6 7 7 8 8 8 9 9 9 9 9 11 11 11

2.2.3.2. Static distribution system. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.4. Calandria(s) with boiling tubes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.5. Separator. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.5.1. Separators with tangential vapour inlet . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.5.2. Wrap-around separator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.6. Vapour recompression systems.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.6.1. Thermal Vapour Recompression – TVR. . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.6.2. Mechanical Vapour Recompression - MVR. . . . . . . . . . . . . . . . . . . . . . . . 2.2.7. Condensation equipment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.7.1. Mixing condenser. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.7.2. Surface condenser. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.8. Vacuum equipment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.8.1. Vacuum pump. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.8.2. Steam jet vacuum unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.9. Flash coolers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.10. Sealing water equipment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.11. Cooling towers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Evaporator design parameters. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.1. Determination of heating surface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.2. Heat transfer coefficient. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

11 13 15 15 15 16 16 17 21 21 21 21 22 22 22 23 23 24 24 25 I

2.3.3. 2.3.4. 2.4. 2.4.1. 2.4.1.1. 2.4.1.2. 2.4.1.2.1. 2.4.1.2.2. 2.4.1.2.3. 2.4.1.2.4. 2.4.2.

Coverage coefficient. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Boiling temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Evaporation parameters and its influrence on powder properties. . . . . Effect of pasteurization. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bacteriological requirements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Functional properties of dried products. . . . . . . . . . . . . . . . . . . . . . . . . . Heat classified skim milk powders. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . High-Heat Heat-Stable milk powders. . . . . . . . . . . . . . . . . . . . . . . . . . . . Keeping quality of whole milk powders . . . . . . . . . . . . . . . . . . . . . . . . . . Coffee stability of whole milk powders. . . . . . . . . . . . . . . . . . . . . . . . . . . Concentrate properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

25 26 27 27 28 28 28 29 29 30 30

3. Fundamentals of spray drying . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 3.1. 3.1.1. 3.1.2. 3.1.3. 3.2. 3.2.1. 3.2.2. 3.2.3. 3.2.4. 3.3. 3.4. 3.5. 3.6. 3.7.

Principle and terms. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Drying air characteristics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Terms and definitions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Psychrometric chart.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Drying of milk droplets. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Particle size distribution. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mean particle size. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Droplet temperature and rate of drying. . . . . . . . . . . . . . . . . . . . . . . . . . Particle volume and incorporation of air. . . . . . . . . . . . . . . . . . . . . . . . . . Single-stage drying. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Two-stage drying. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Expansion of air bubbles during drying . . . . . . . . . . . . . . . . . . . . . . . . . . Extended Two-stage drying. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fluid bed drying . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

33 33 36 37 38 38 39 41 42 43 44 45 47 48

4. Components of a spray drying installation . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

Drying chamber. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

Hot air supply system. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Feed supply system. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Atomizing device. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Powder/fines recovery system. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fines return system. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Powder after-treatment system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Final product conveying, storage and bagging off system. . . . . . . . . . . instrumentation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Drying chamber. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Hot air supply system. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.1. Air supply fan. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.2. Air filters. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.3. Air heater. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II

51 51 51 51 51 51 51 51 54 57 57 57 58

List of contents

4.2.3.1. Indirect heater. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.3.2. Direct heater . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.4. Air dispersers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Feed supply system. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.1. Feed tank. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.2. Feed pump . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.3. Concentrate heater. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.4. Filter. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.5. Homogenizer/High-pressure pump. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.6. Feed line . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4. Atomizing device. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.1. Rotary wheel atomizer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.2. Pressure nozzle atomizer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.3. Two-fluid nozzle atomizer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5. Powder recovery system. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.1. Cyclone separator. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.2. Bag filter. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Advantages of the SANICIP™ filter: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.3. Wet scrubber. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.4. Combinations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.6. Fines return system. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.6.1. For wheel atomizer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.6.2. For pressure nozzles. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.7. Powder after-treatment system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.7.1. Pneumatic conveying system. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.7.2. Fluid bed system. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.7.3. Lecithin treatment system. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.7.4. Powder sieve. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.8. Final product conveying, storage and bagging-off system. . . . . . . . . . . 4.9. Instrumentation and automation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

58 59 60 61 62 62 63 64 64 64 64 65 68 70 70 71 72 74 75 76 76 77 77 78 78 79 81 82 82 84

5. Types of spray drying installations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86 5.1. 5.1.1. 5.1.2. 5.1.3. 5.2. 5.2.1. 5.2.2. 5.2.3. 5.3. 5.3.1. 5.3.2. 5.3.3.

Single stage systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Spray dryers without any after-treatment system. . . . . . . . . . . . . . . . . . . Spray dryers with pneumatic conveying system. . . . . . . . . . . . . . . . . . . . Spray dryers with cooling bed system. . . . . . . . . . . . . . . . . . . . . . . . . . . . Two stage drying systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Spray dryers with fluid bed after-drying systems. . . . . . . . . . . . . . . . . . . TALL FORM DRYER™. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Spray dryers with Integrated Fluid Bed. . . . . . . . . . . . . . . . . . . . . . . . . . . Three stage drying systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . COMPACT DRYER™ type CDI (GEA Niro). . . . . . . . . . . . . . . . . . . . . . . . Multi Stage Dryer MSD™ type. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Spray drying plant with Integrated Filters and Fluid Beds - IFD™. . . . .

86 86 87 87 88 88 89 90 91 92 93 94 III

5.3.4. Multi Stage Dryer MSD™-PF. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.5. FILTERMAT™ (FMD) integrated belt dryer. . . . . . . . . . . . . . . . . . . . . . . . 5.4. Spray dryer with after-crystallization belt . . . . . . . . . . . . . . . . . . . . . . . . . 5.5. TIXOTHERM™. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Choosing a spray drying installation. . . . . . . . . . . . . . . . . . . . . . . . . . . . .

95 95 96 96 98

6. Technical calculations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 6.1. 6.2. 6.3.

Evaporation and product output. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 Heating of atmospheric air. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100 Mixing of two air stream. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101

6.4. 6.5. 6.6. 6.7. 6.8.

Dry air rate, water vapour rate and air density. . . . . . . . . . . . . . . . . . . . . Air velocity in ducts. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Air flow measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Barometric distribution law. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The heat balance of a spray dryer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

102 103 103 106 106

7. Principles of industrial production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110 7.1. 7.2. 7.3. 7.3.1. 7.3.2. 7.3.3. 7.3.4. 7.4. 7.4.1. 7.4.2.

Commissioning of a new plant. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Causes for trouble-shooting. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Production documentation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Production log sheets. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . General maintenance log book. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Product quality specification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Operational parameter specification. . . . . . . . . . . . . . . . . . . . . . . . . . . . . Product quality control. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Process quality control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Final quality control. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

110 110 111 112 115 117 117 117 117 119

8. Dried milk products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121 8.1.

Regular milk powders. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121

8.1.1. 8.1.2. 8.1.3. 8.1.4. 8.1.4.1. 8.1.4.2. 8.1.5. 8.2. 8.2.1. 8.2.2.

Regular skim milk powder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Concentrate properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Regular whole milk powder. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Concentrate properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Whole milk powder with high free fat content. . . . . . . . . . . . . . . . . . . . . Butter milk powder. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sweet butter milk powder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acid butter milk powder. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fat filled milk powder. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Agglomerated milk powders. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Agglomerated skim milk powder. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Agglomerated whole milk powder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

IV

122 122 123 123 124 125 125 125 125 126 127 127

List of contents

8.2.3. 8.2.4. 8.2.5. 8.3. 8.3.1. 8.3.2. 8.3.3. 8.3.4. 8.3.5. 8.3.6. 8.3.7. 8.3.8. 8.3.9. 8.4. 8.4.1. 8.4.2. 8.4.3. 8.4.4. 8.4.5. 8.4.6.

Instant whole milk powder. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Agglomerated fat filled milk powder . . . . . . . . . . . . . . . . . . . . . . . . . . . . Instant fat filled milk powder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Whey and whey related products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ordinary sweet whey powder. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ordinary acid whey powder. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Non-caking sweet whey powder. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Non-caking acid whey powder. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fat filled whey powder. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hydrolysed whey powder. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Whey protein powder. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Permeate powders . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mother liquor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Other dried milk products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Baby food. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Caseinate powder. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Coffee whitener. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cocoa-milk-sugar powder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cheese powder. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Butter powder. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

128 129 129 129 130 131 132 132 133 133 133 134 134 134 135 137 137 138 139 139

9. The composition and properties of milk. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140 9.1. Raw milk quality. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2. Milk composition. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.3. Components of milk solids. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.3.1. Milk proteins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.3.2. Milk fat. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.3.3. Milk sugar. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.3.4. Minerals of milk. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.4. Physical properties of milk. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.4.1. Viscosity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.4.2. Density. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.4.3. Boiling point . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.4.5. Acidity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.4.5. Redox potential. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.4.6. Crystallization of lactose. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.4.7. Water activity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.4.8. Stickiness and glass transition. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

140 140 142 142 143 143 145 145 145 150 152 152 153 153 156 159

10. Achieving product properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163 10.1. 10.2. 10.3.

Moisture content. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163 Insolubility index. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165 Bulk density, particle density, occluded air. . . . . . . . . . . . . . . . . . . . . . . . 166 V

10.4. Agglomeration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.5. Flowability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.6. Free fat content. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.7. Instant properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.7.1. Wettability. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.7.2. Dispersibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.7.3. Sludge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.7.4. Heat stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.7.5. Slowly dispersible particles. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.7.6. Hot water test and coffee test. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.7.7. White Flecks Number (WFN). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.8. Hygroscopicity, sticking and caking properties . . . . . . . . . . . . . . . . . . . . 10.9. Whey Protein Nitrogen Index (WPNI). . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.10. Shelf life. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

174 177 178 180 181 183 183 184 187 190 190 191 195 196

11. Analytical methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199 11.1. Moisture content. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.1.1. Standard oven drying method (IDF Standard No.26-1964 [32]) . . . . . . . 11.1.2. Free moisture. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.1.3. Total moisture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.1.4. Water of crystallization. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2. Insolubility index. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.3. Bulk density. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.4. Particle density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.5. Scorched particles. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.6. Wettability. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.7. Dispersibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.8. Other methods for determination of instant properties. . . . . . . . . . . . . 11.8.1. Sludge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.8.2. Slowly dispersible particles. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.8.3. Hot water sediment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.8.4. Coffee test. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.8.5. White flecks number. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.9. Total fat content. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.10. Free fat content. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.11. Particle size distribution. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.12. Mechanical stability. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.13. Hygroscopicity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.14. Degree of caking. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.15. Total lactose and -lactose content. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.16. Titratable acidity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.17. Whey Protein Nitrogen Index (WPNI). . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.18. Flowability (GEA Niro [31]). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.19. Lecithin content. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VI

199 199 200 200 200 200 201 202 204 204 205 207 207 208 208 208 209 209 210 211 212 212 213 214 215 216 217 218

List of contents

11.20. Analytical methods for milk concentrates. . . . . . . . . . . . . . . . . . . . . . . . . 11.20.1. Total solids. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.20.2. Insolubility index. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.20.3. Viscosity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.20.4. Degree of crystallization. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

219 219 220 220 221

12. Troubleshooting operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222 12.1. 12.2. 12.3. 12.4. 12.5. 12.6.

Lack of capacity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Product quality. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Deposits in the system. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fire precaution. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Principles of good manufacturing practice. . . . . . . . . . . . . . . . . . . . . . . . The use of computer for quality control and trouble-shooting. . . . . . . .

222 225 227 232 238 240

References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 242 Index. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 244

VII

VIII

HANDBOOK OF MILK POWDER MANUFACTURE

1. Introduction Transforming liquid milk into dry powder requires removal of almost all the water, the amount of which exceeds by many times that of the final product. During this water removal process, significant changes of the properties, physical structure and appearance take place. Milk is a sensitive product and its quality can be seriously affected especially by the influence of heat or bacterial activity. Thus it is obvious that a single water removing process cannot have optimum performance throughout the whole duration of dehydration. Therefore it is necessary to apply successively several methods, each being chosen with respect to the properties of the material processed at each individual process step, while taking into account both product quality and economy. Two main water removal processes used in milk powder industry are: - vacuum evaporation which removes the first part of water and transforms a thin liquid into relatively high viscous milk concentrate, and - spray drying which transforms the concentrate into powder by atomization and dispersion of small droplets into a flow of hot air. This air supplies the heat necessary for evaporation of water from the droplets and carries away the water vapour.

These two main processes can be further supplemented by: - membrane processes (reverse osmosis) are, in some cases, suitable for removing the very first portions of water prior to transportation and further evaporation possibly also removing some monovalent salts (nanofiltration). The concentration of high molecular substances like whey proteins is interesting, but not dealt with in this book. - fluid bed drying which is used for removing the very last portions of water, i.e. transforming the moist powder leaving the spray dryer into a final dry powder. Modern spray dryers are often equipped with an integrated fluid bed in the bottom of the chamber, for either final after drying/ cooling or for intermediate drying followed by a second fluid bed.

1

The technology of milk powder manufacture is very complex. There are a vast number of different products and compositions, and each product can be produced according to various quality requirements. Moreover there are a great number of qualitative criteria defining not only the composition and overall quality from the general hygiene and health hazard point of view, but also optimum suitability for a given application - so-called functionality. As with every industrial production, occasionally some irregularities may occur which become evident as abnormal behaviour of the plant. Such behaviour often occurs without any obvious reason, i.e. without any apparent deviation of the production line set-up or operational parameters. The consequence of that may be lack of capacity, excessive loss of product, high consumption of energy, product quality degradation etc. If this occurs it is necessary to mobilize all efforts to find the reasons for the problems, to solve them and to re-establish regular operation conditions, i.e. to conduct a troubleshooting operation. The objective of this book is to provide for a milk powder technologist the guidelines for trouble-shooting actions. However one cannot discuss trouble-shooting without knowledge of the fundamentals of evaporation and spray drying and the principles of milk powder manufacturing technology. Even well established and well controlled milk powder production facilities cannot fully avoid some production irregularities. An appreciation of basic principles can, to a great extent, reduce their occurrence, and moreover provide means to facilitate the trouble-shooting action. Therefore this book draws the attention to these aspects. Even if this book is focusing on dairy products, the principles described herein covevering equipment and processes may be used as well in drying other products whether food, chemical or pharmaceutical.

2

2. Evaporation

2. Evaporation 2.1. Basic principles Evaporation is a process by which a liquid is brought to its boiling point by external heating thereby transforming the water into vapour, which escapes from the surface of the liquid. The rate of evaporation depends primarily: a) On the rate of heat transfer from the heating surface into the liquid b) On the surface area of the liquid exposed to the heating surface and c) On the rate of vapour removal from the surface of the liquid. The evaporation of water from milk requires special attention because of heat sensitivity. Therefore the evaporation has to be carried out under vacuum to: a) Reduce the boiling point to below the temperature which would cause heat damage to the milk components, (especially proteins) and b) Enable multi-stage evaporation by selecting a cascade of vacuum levels. The water contents of the most important liquid dairy products, their concentrates for spray drying and dried powders are given in Table 2.1. From these figures it is obvious that a substantial part of the water is removed by vacuum evaporation and only a fraction by spray drying and possibly fluid bed after-drying.

Table 2.1. Water content of main milk products. Product

Water content % Liquid

Concentrate

Powder

Whole milk

87-88

48-52

2-3

Skim milk

91-92

48-52

3-4

Sweet whey

94-95

40-60

2

Beside these three main dairy products, others are also processed by evaporation and spray drying, like buttermilk, acid whey, permeates from ultrafiltration, mixtures of dairy liquid products with other components to produce special formulations e.g. milk formulas and icecream mixes. Therefore, whenever the expression 'milk' is used in the following text, it has to be considered as a general designation which may mean any of the other products mentioned.

3

There are two main reasons for using evaporation prior to spray drying: a) It has a positive influence on many qualitative properties of the final powder, b) It is a far more economical water removing process than spray drying. Consequently, removing as much water as possible by evaporation improves the overall heat economy of the process. Having these reasons in mind, the basic principles for the design of an evaporator are:

a) Using a level of vacuum, which will reduce the boiling temperature to below the temperature that would cause heat damage to the milk,



b) Providing sufficient evaporative surface for the liquid to achieve fast evaporation rates in order to reduce the exposure time to heat,



c) Providing sufficient heating surface to achieve high rate of heat transfer, d) Keeping a low temperature difference between the heating surface and boiling point of the liquid, ensuring at the same time constant coverage of the surface by liquid and avoiding local overconcentration and scorching.

2.2. Main components of the evaporator The main components of an evaporation plant are: •  Heat exchanger for preheating the liquid either indirect or direct •  Pasteurizing system including holding tubes •  Product distribution system •  Calandria(s) with boiling tubes •  Separator for separation of the vapour from the evaporated liquid •  Vapour recompression systems •  Vacuum equipment •  Flash coolers •  Sealing water equipment •  Cooling towers.

4

2. Evaporation

2.2.1. Heat exchanger for preheating As the milk to be evaporated has a temperature of 5-10°C it has to be heated to the boiling temperature of the first effect in order to enable evaporation. The milk is therefore first passed through a vapour cooler/preheater, placed between the last effect’s separator and the condenser, thereby saving cooling water as well. From the vapour cooler the milk is passed through the preheating section of the last effect and then backwards to the first effect, before it enters either the pasteurization system or directly into the boiling section of the first effect. The preheating system can technically be carried out in different ways: •  Spiral-tube preheaters •  Straight-tube preheaters •  Preheaters to prevent growth of spore forming bacteria •  Direct contact regenerative preheaters •  Duplex preheating system •  Preheating by direct steam injection •  Other means to solve presence of spore forming bacteria.

2.2.1.1. Spiral-tube preheaters The spiral tubes are placed inside the heating room in the calandria surrounding the falling-film tubes, thus being heated by vapour. The system is simple, but does not offer the possibility of inspection for deposits or leakage. In modern evaporators they are not used any longer. See Fig.2.1.

Fig. 2.1. Spiral-tube preheater © GEA Niro

5

2.2.1.2. Straight-tube preheaters The straight-tube preheaters are placed vertically outside the evaporator and like the spiral tubes heated by vapour from the corresponding calandria. The vapour connection is at the top of the calandria, so that uncondensable gasses can easily be extracted. See Fig. 2.2. This ensures an optimum utilization of the heating surface of the evaporation tubes. With this system inspection and manual cleaning are possible, if in rare cases it should prove necessary. The heat transfer surface in the preheater is arranged in groups of parallel tubes with small diameter resulting in a large surface. Each group of tubes is connected by normal dairy fittings at the end. Due to the parallel flow, the holding time is very short. The viscosity of the final concentrate is therefore lower in evaporators equipped with straight-tube preheaters.

Evaporator with straight-tube preheaters © GEA Niro



 Fig. 2.2. straight-tube preheater © GEA Niro

The large surface of the preheaters and the temperature level prevailing during operation (565°C) offers, however, optimal growth conditions for mesophilling and thermophilie bacteria. After 14-16 h of operation a bio-film is formed on the inner surface of the preheaters, where they can form spores. Unless special attention is paid, one cannot expect a 20 hour production without increase of mesophile and thermophile bacteria and their spores during the last 4-5 hours of a 20 hour production.

6

2. Evaporation

The table below indicates typical growth temperatures and inactivation temperatures/time of spore forming bacteria, their vegetative cells and spores.

Spore forming Bacteria

Growth temperatures (°C)

Usual inactivation in milk by heat

Minimum

Optimum

Maximum

Vegetative cell

Spore

B.Stearothermophilus

30-45

55-60

60-70

12 s 85°C

8-15 m 121°C

B. Cereus

5-20

30-37

45-48

10 s 72°C

0.5 m 121°C

B. Coagulans

15-25

35-50

55-60

20 s 72°C

3-5 m 121°C

15

30-45

50-55

20 s 72°C

3-5 m 121°C

6-20

30-40

45-55

20 s 72°C

3-5 m 121°C

C. Botulinum

3

25-40

48

20 s 72°C

3-4 m 121°C

C. Perfringens

8-20

45

50

20 s 72°C

1-4 m 121°C

20 s 72°C

1-4 m 121°C

B. Licheniformis B. Subtilis

C. Tyrobutiricum

2.2.1.3. Preheaters to prevent growth of spore forming bacteria Spore forming bacteria are bacteria which under adverse growth conditions, such as too high or too low temperature or lack of nutrition, transform themselves into a dormant state - they sporulate and become extremely heat resistant. When growth conditions become favourable again, they re-vegetate and develop. It has been found that the development of spore forming bacteria in evaporators takes place in the preheaters, as that is the only place where bio-films are formed. To ensure production of powder during a 20 hour operation without problems the following type of preheaters can be used:

2.2.1.3.1. Direct contact regenerative preheaters By using a direct contact regenerative preheater of similar design as the direct contact regenerative flash chambers (see section 2.2.2.), the heating from 5°C to 40°C and from 40°C to 70°C can be done in fractions of a second without heat surfaces where biofilms can be formed. The milk is pumped to the inlet of the direct contact preheater(s), where vapour from one of the calandrias is introduced by means of live steam through a small thermo-compressor. See Fig. 2.3. 7

 Fig. 2.3. Direct contact regenerative preheater © GEA Niro By applying this technology it is possible to operate the plant for 20 hours or more without growth of mesophile and/or thermophile bacteria and their spores at reduced steam consumption.

2.2.1.3.2. Duplex preheating system By installing duplex preheaters, see fig 2.4., it is possible to have a continuous run of 20 hours, as the preheaters are cleaned before the critical level has been reached. Additional costs for cleaning and effluent treatment must be taken into account. Further, the investment is higher, but the actual direct production costs and time are not affected.

2.2.1.3.3. Preheating by direct steam injection As mentioned, the spore forming bacteria only develop in biofilms in the preheaters. Therefore, an obvious solution would be to by-pass the preheaters, where temperatures are between 5 and 70°C. This will, however, result in increased overall steam consumption, as direct steam injection is necessary to bring up the temperature from 5°C to the pasteurization temperature, and further the water from dilution of the condensing steam has to be evaporated again.

Fig. 2.4. Duplex preheaters © GEA Niro

2.2.1.4. Other means to solve presence of spore forming bacteria

If for some reason, one does not want to use the above described method, but still wants to operate the plant for 20 hours without problems with spore forming bacteria, the following measures can be implemented:

8

2. Evaporation

2.2.1.4.1. Mid-run cleaning If the evaporator is cleaned after 10 hours, the problem is solved, but approx. 10% of effective production time is lost, and further there are expenses for cleaning agents and waste disposal.

2.2.1.4.2. UHT treatment By heating the milk to 140°C in 4 sec. after the preheaters, the problem is solved, however, the dead cells are still traceable, and it will not be possible to make powders with “tailor-made” functional properties. Further, there will be additional steam consumption, and the maximal running time is depending on the milk quality.

2.2.2. Pasteurizing system including holding 2.2.2.1. Indirect pasteurization The indirect heaters are working as ordinary heat exchangers, either the plate, straight-tube or spiral-tube type. If temperatures up to 110°C are wanted, it is recommended to have two heaters, where one is in operation while the other one is being cleaned. The advantage of the indirect heating is that the product will not be mixed with the condensating steam and neither will the product be diluted. The disadvantage is that it takes a long time for the product to be heated in the interval from 80°C to 110°C resulting in a concentrate with high viscosity. This is because the whey proteins, when unfolded, will react with each other and the k-casein. For improved efficiencies one or more regeneration systems can be incorporated.

2.2.2.2. Direct pasteurization The direct pasteurization is done in two different ways, either by direct steam injection, where the live steam is mixed into the milk using a Tangential Swirl Heater (TSH), see photo. It offers a controlled and short residence time with no mechanical impact, even at temperatures >120°C. It can operate 20 hours or more without intermediate cleaning. Alternatively, milk is sprayed into a steam atmosphere (infusion) at a sufficient pressure. The steam must be of good quality, i.e. for use in products for human consumption. Culinary steam boilers, where milk condensate is heated up in an indirect coil-type heater by means of live steam, can be used. The advantage of direct pasteurization is the short time it takes to reach the desired temperature.

9

Tangential swirl heater © GEA Niro

The direct heating will further have a less pronounced effect on the denaturation of the whey proteins at the same pasteurization temperature/time.

Whey protein denaturation

Thiamin loss

Direct system

35%

0.5 - 0.8%

Indirect system

65%

1.4 - 4.4%

Fig. 2.8. Indirect contact regenerative  pasteurizer with flash chambers  © GEA Niro

10

Fig. 2.9. Direct contact regenerative pasteurizer with flash chambers © GEA Niro

2. Evaporation

As for the indirect preheating, regenerative flash chambers are used, if high pasteurizing temperatures are needed. The temperature of the milk will drop due to the evaporation, and the vapours are used for preheating prior to the pasteurizer. The regenerative flash chamber can be either indirect as shown in Fig. 2.8., or direct contact as shown in Fig. 2.9. The direct contact regenerative system is preferable for its short residence time and there is no heat contact surface, where deposits can develop. The pasteurization temperature will of course have a direct influence on the total steam consumption, which will increase by increasing the temperature. For the same pasteurization temperature the direct pasteurization will result in higher steam consumption compared with indirect pasteurizing due to the need of evaporation of the extra water formed by the condensation. However, the additional steam used is - after flashing off - used as heating medium in the subsequent calandrias and some of the applied energy is reused.

2.2.2.3. Holding tubes The holding is practically always done in horizontally placed holding tubes, with specific length and diameter to give the desired holding time. There are usually several tubes of the same length but with various diameters, the combination of which enables the holding time to be varied. For instance four tubes corresponding to holding times 0.5, 1, 2 and 4 minutes allow the holding time to be varied from 0.5 to 7.5 minutes in half minute intervals.

2.2.3. Product distribution system It is very important that the product to be evaporated is distributed evenly into all the tubes in the calandria to get a good coverage. The distribution system is therefore given special attention when designing an evaporator. In principle there are two different systems: •  Dynamic distribution system. •  Static distribution system.

2.2.3.1. Dynamic distribution system In the dynamic distribution system, the necessary kinetic energy for distribution is obtained by a pressure drop of the product over a full-cone nozzle. As the product is superheated in relation to the pressure inside the tubes, flash vapour will instantaneously be formed. The mixture of product and vapour is sprayed into the inlet of the tubes thus being covered by product. This system is very inflexible as to capacity variations and not used in dairy evaporators designed for various milk products with different solids content.

2.2.3.2. Static distribution system In the static distribution system the incoming superheated product is first separated in flash vapour and product. The product enters a distributor plate placed inside an open cone, as the product enters the calandria. The cone is placed above a distributor bowl with a number of holes. Here a certain level of product is maintained. The product flows through the holes in the plate by gravity. Each hole is placed just above the area between the tubes. Thus the product 11

flows onto the tube plate and then over the edge down along the surface of each tube. The flash vapour also enters the tubes and pushes the product against the inner surface of the tubes giving it its initial velocity. See Fig. 2.10.

Static distribution system © GEA Niro

Fig. 2.10. Static distribution system, here shown for one tube only © GEA Niro This distribution system is much more flexible with respect to capacity, as an increase in the level in the distributor bowl - as a result of increased capacity - will make the product flow through the holes at a higher velocity, thus maintaining the level. During CIP of the evaporator and especially the pasteurizing equipment, some jelly lumps of milk protein deposits may cause blocking of the holes in the distributor plate. To avoid this, a self-cleaning hydro cyclone may be installed in the product line between the discharge from the flash vessel of the regenerative pasteurizer and the inlet to the first calandria. See Fig. 2.11.

Fig. 2.11. Self-cleaning hydrocyclone installed between the discharge of the flash vessel and the inlet to the first calandria © GEA Niro

12

2.2.4. Calandria(s) with boiling tubes The liquid to be evaporated is evenly distributed on the inner surface of the tubes. The liquid will flow downwards forming a thin film, from which the boiling/evaporation will take place because of the heat applied by the steam. The steam will condense and flow downwards on the outer surface of the tube. A number of tubes are built together side by side. At each end the tubes are fixed to tube plates, and finally the tube bundle is enclosed by a jacket, see Fig. 2.12. The steam is introduced through the jacket. The space between the tubes is thus forming the heating section. The inner side of the tubes is called the boiling section. Together they form the so-called calandria. The concentrated liquid and the vapour leave the calandria at the bottom part, from where the main proportion of the concentrated liquid is discharged. The remaining part enters the subsequent separator tangentially together with the vapour. The separated concentrate is discharged (usually by means of the same pump as for the major part of the concentrate from the calandria), and the vapour leaves the separator from the top. The heating steam, which condenses on the outer surface of the tubes, is collected as condensate at the bottom part of the heating section, from where it is discharged by means of a pump.

Evaporator calandria © GEA Niro

Fig. 2.12. Calandria with boiling tubes © GEA Niro

In order to understand the heat and mass transfer, the basis for the evaporation, it is necessary to define various specific quantities. Feed (A) means a liquid product supplied to the evaporator to be evaporated (B) and concentrate (C) is the resulting product. And thus: A=B+C 

[2,1]

The evaporation ratio (e) is a measure for the evaporation intensity and can be defined either as the ratio between the amount of feed and concentrate or the ratio between the total solids (TS) percentage in the concentrate and in the feed.

13

e=

A TSConcentrate = [2,2] C TSfeed

If the concentrations or the evaporation ratio are known the quantities A, B or C can be calculated, if one of them is known.

Given quantity Quantity to be treated A

To be found

Formula

B

B = A∗ C Evaporated quantity B

A

C

Concentrate quantity C

e-1 e

C=A ∗

1 e

A=B ∗

e e-1

C=B ∗

1 e-1

A

A = C∗ e

B

B = C ∗ (e - 1)

Where: A: feed in kg/h B: evaporation in kg/h C: concentrate in kg/h e: evaporation ratio Since milk, due to the protein content, is a heat-sensitive product, evaporation (i.e. boiling) at 100°C will result in denaturation of these proteins to such an extent that the final product is considered unfit for consumption. The boiling section is therefore operated under vacuum, which means that the boiling/evaporation takes place at a lower temperature than that corresponding to the normal atmospheric pressure. The vacuum is created by a vacuum pump prior to start-up of the evaporator and is maintained by condensing the vapour by means of cooling water. A vacuum pump or similar is used to evacuate incondensable gases from the milk. At 100°C the evaporation enthalpy of water is 539 Kcal/kg and at 60°C it is 564 Kcal/kg. As the milk has to be heated from e.g. 6°C to the boiling point, and as energy, approx. 20 Kcal/ kg, is required for maintaining a vacuum corresponding to a boiling point of 60°C, we get the following energy consumption figures, provided we estimate the heat loss to be 2%:

14

2. Evaporation

Boiling temperature

°C

100

60

Heating

Kcal/kg

94

54

Evaporation

Kcal/kg

539

564

Vacuum

Kcal/kg

-

20

Net energy consumption

Kcal/kg

633

638

Heat loss, approx.

Kcal/kg

15

15

Total energy consumption

Kcal/kg

648

653

Corresponding to about 1.1 kg of steam/kg of evaporated water.

2.2.5. Separator The role of the separator is to separate vapour from the evaporated liquid. Milk evaporators are working exclusively with centrifugal type separators.

2.2.5.1. Separators with tangential vapour inlet As the vapours generated from the evaporation are used as heating media in the “next” calandria, any product must be separated, since it would otherwise contaminate the condensate and further represent a loss. The majority of the concentrate is discharged from the bottom of the calandria below the tube bundle. Due to the high vapour velocity some of the concentrate will be carried along with the vapour as small droplets. The separation is done in a separator with tangential vapour inlet; see Fig. 2.13., connected to the calandria below the tubes.

V apour

C alandria

Special care is taken to design the separator to avoid product carry-over at lowest possible pressure drop, as a drop in the pressure is equal to drop in heating enthalpy in the following calandria with an all-over drop in the efficiency as a result.

2.2.5.2. Wrap-around separator C lassical S eparator

Milk C oncentrate

Fig. 2.13. Separator with tangential vapour inlet © GEA Niro

To reduce space requirements a new development has taken place with the design of the Wrap-around separator, see Fig. 2.14. It is integrated into the base of the calandria. It has the same high efficiency as the classical separator with a low pressure drop. It is typically used on big calandrias with 15

MVR compressors connected to the wrap-around separator with a very short vapour duct minimizing the pressure drop. The saving in floor space is typically around 30%.

Calandria

2.2.6. Vapour recompression systems. Wrap-around Separator New Type

Vapour

2.2.6.1. Thermal Vapour Recompression – TVR

One way of saving energy is by using a thermo-compressor which will increase the temperature/pressure level of the Milk Concentrate vapour, i.e. compress the vapour from a lower pressure to a higher pressure by Fig. 2.14. Wrap-around separator © GEA Niro using steam of a higher pressure than that of the vapour. Thermo-compressors operate at very high steam flow velocities and have no moving parts. The construction is simple, the dimensions small, and the costs low. See Fig. 2.15. Vapour

Mix of vapour / steam

Live steam

(1) Nozzle

(2) Diffuser

Fig. 2.15. Thermo-compressor © GEA Niro In the live steam nozzle (1) the pressure of the inflowing steam is converted into velocity. A jet is therefore created which draws in part of the vapour from the separator connected to the calandria. In the diffuser (2) a fast flowing mixture of live steam and vapours is formed, the speed of which is converted into pressure (temperature increase) by deceleration. This mixture can now be used as heating steam for the evaporator. In Fig. 2.16 a flow sheet of a three-effect evaporator with thermo-compressor is shown. The best efficiency in the thermo-compressor, i.e. the best suction rate, and thereby a good economy, is obtained when the temperature difference (pressure difference) between the boiling section and the heating section is low.

16

2. Evaporation

Fig. 2.16. Three-effect evaporator with thermo-compressor © GEA Niro Thermo-compressors must be adapted to the operating conditions. But these conditions can vary, be it, for example, that the heat resistance of the heating surfaces increases during operation due to deposits on the heating tubes. The suction rate will then decrease considerably. In evaporators that have to serve various capacities a number of thermo compressors with different characteristics are used. Further, a thermo-compressor, which has been designed for a higher live steam pressure, can draw a larger amount of vapour from the separator than one built for a lower pressure. For simplification we will in the following use an efficiency of 1:2, but new designed thermo-compressors will under certain conditions operate with an efficiency of 1:3. By adding a thermo-compressor we have then in a three-effect evaporator by means of 1 kg live steam evaporated 5 kg of water, i.e. the saving of steam is as great as that obtained by addition of two effects in multi-effect evaporation. Dividing a given total ∆t between the first and last effect in multi-effect evaporators requires an enormous heating surface and consequently an expensive installation.

2.2.6.2. Mechanical Vapour Recompression - MVR As an alternative to the thermo-compressor, the mechanical vapour compressor has during the last fifteen years found extensive use in evaporators in the dairy industry. The applied energy for the compressor is usually electricity, but also diesel and gas motors are used. Other processes may require steam at low pressure, and the compressor may be driven by a steam turbine acting as a reducing valve. All determined by local price policy for energy. As a rule of thumb, however, an MVR solution is profitable, if the price/kW < price/kg steam x 3. However, the decision as to which type of compressor to use, is nowadays influenced by the end product quality - the milk powder - and in the MVR evaporator there is a very short residence time, resulting in low viscosity of the concentrate.

17

MVR Recompressor © GEA Niro

The mechanical vapour compressor is a fast revolving high pressure fan (~3000 rpm) capable of operating under vacuum. At low boiling temperatures the volume of the vapours is enormous. Consequently, there is a limit as to the lowest temperature levels used in practice. As the energy applied to the compressor is utilized most efficiently by low compression ratios, the obtained temperature/pressure increase is limited. Therefore, a large heat transfer surface is required tending to increase the capital costs of the equipment.

Fig. 2.17. One-effect MVR evaporator © GEA Niro

18

As it is essential to operate an MVR unit at a low overall temperature difference between the vapour evolved from the product and the heating medium as a result of the compression, it is a must that the boiling point elevation of the product is kept at a minimum, as this would otherwise even further minimize the temperature difference available for the evaporation. This, too, limits the maximum concentrations aimed at in evaporators of this kind. Fig 2.17., illustrates a one-effect MVR evaporator. The incoming cold milk is first preheated by concentrate then by condensate from the heating section of the calandria followed by a final pasteurization by means of live steam. The vapour is compressed in the MVR unit and used as heating medium, as it releases the latent heat by condensation.

2. Evaporation

A vacuum pump, together with a small amount of cooling water, maintains the desired vacuum in the system. As it can be seen no energy leaves the plant in form of warm condensate, and only a minor part via the cooling water (depending upon the pasteurization temperature desired). The MVR evaporator is in this context very often used as pre-condenser of milk products for transport purposes, where the required solids content is in the range of 30-35% and thus the boiling point elevation is limited. With the concentrate leaving the plant at low temperature, this kind of installation is a strong competitor to hyperfiltration. The vapour is by the MVR fan sucked from the separator and the compressed vapour is desuperheated by spraying water into the outlet of the compressor. The compressed vapour is condensed on the heat exchanger surface in the subsequent calandria, where it is discharged as condensate. Simultaneously, water is evaporated from the milk and separated as vapour in the separator. The MVR evaporator offers much better capacity flexibility / turn-down capability, as only the RPM on the fan needs to be adjusted.

Fig. 2.18. Combined MVR/TVR evaporator © GEA Niro Usually, the MVR evaporator is combined with a TVR unit, if solids contents suited for a spray drying plant are aimed at, see Fig. 2.18. The steam consumption per kg evaporated water is of course less than in a multi-effect evaporator, but if the MVR unit is driven by an electric motor, the electrical energy consumption will be bigger. As only very little cooling water is required, this combination offers a very attractive solution, however, a higher investment should be anticipated. Under special energy price conditions it is advantageous to replace the TVR unit with an additional MVR unit to compress the vapour over the last effect, see Fig. 2.19. It is therefore recommendable that each case be studied carefully taking local conditions such as steam, electricity and cooling water prices into consideration.

19

Fig. 2.19. Evaporator with 2 MVR fans © GEA Niro

Comparison of energy consumption in different evaporators 5-effect TVR

7-effect TVR

1-effect MVR/ 2-effect TVR

Skim milk

Skim milk

Skim milk

Capacity, kg/h

15,000

15,000

15,000

Solids in/out, %

9/50

9/50

9/50

12,300

12,300

12,300

Pasteurization temp., °C

90

90

90

Holding time, sec.

30

30

30

1,610

1,190

375

10

10

10

13,400

13,400

12,800

Condensate temp., °C

54

51

22

Power consumption - MVR, kW - Motors, kW

75

75

150 50

Cooling water cons., m3/h

32

3.5

2 *)

28/35

28/35

12/50

Power cons. cool. tower, kW

10

2.5

-

Residence time, min.

12

18

6

PRODUCT

Evaporation, kg/h

Steam consumption, kg/h Steam pressure, bar Condensate, kg/h

Cooling water temp in/out, °C

*) to be used only if the temperature of the raw milk is above 5°C. 20

2. Evaporation

2.2.7. Condensation equipment In multi-effect evaporators - be it a TVR or MVR plant or combinations hereof - any “subsequent” calandria – operated at a lower boiling temperature - is used as condenser for the ”warmer” vapour coming from the separator of the previous calandria. Water is used as cooling medium in a condenser to condense the vapour from the last calandrias separator either indirectly (shell and tube surface condenser) or directly (spray mixing condenser). Surface condensers are more expensive and need 10-15% more water. The type of condenser has no effect on the performance of the evaporator. In plants processing products containing volatile acids, surface condensers are preferred to avoid contamination of cooling water by acid.

2.2.7.1. Mixing condenser In a mixing condenser numerous nozzles and plates are installed in order to get a good mixing of the vapour and the cooling water, see Fig. 2.20. The water and condensed vapour are removed at the bottom. As there will be the same vacuum in the mixing condenser as in the last effect, the pump to remove the water and condensate should be capable of discharging from this vacuum. Another solution is to place the mixing condenser barometrically high, i.e. about 11 meter above the pump. The water will run into a well from where it is pumped away, either to a cooling tower or to a natural water reservoir. The advantage of the mixing condenser is low investment costs and lower cooling water consumption. The disadvantage is that condensate is mixed with the cooling water which may have the effect that the cooling tower is contaminated. Since there is an open connection between the product in the last effect and the, possibly contaminated, cooling water they represent a bacteriological hazard and thus should be avoided.

Fig. 2.20. Mixing condenser © GEA Niro

2.2.7.2. Surface condenser

The surface condenser is working and built according to the same principle as an ordinary straight tube heat exchanger. The advantage of a surface condenser is that cooling water and vapour condensate remain separate. As only the vapour condensate has to be pumped out of the vacuum, it has never been considered to place it barometrically as is the case for the mixing condenser. Surface condensers should always be used in plants where acid products such as acid whey are evaporated in order to separate acid vapour condensate from the cooling water.

2.2.8. Vacuum equipment The vacuum in the last effect of the evaporator is a function of the power of the vacuum equipment and the amount of cooling water and the temperature to maintain the vacuum once created. The vacuum in the first and intermediate effects is created by the subsequent calandria acting as a condenser for the vapours from the previous effect. Any change in the evaporation rate in one effect, due to fouling for example (decrease of K factor), therefore 21

means that less vapour is condensed. This results in increased boiling temperature in the previous effect, the ∆t decreases and so does the overall evaporation capacity. Each effect is connected to the condenser to ensure the de-aeration of incondensable air and gas. Saturated steam which is used as heating steam contains also considerable amount of air and other non-condensable gases. So does the product to be concentrated. It amounts usually to about 0.5% and increases especially in multi-effect evaporators up to 1%. The noncondensable gases reduce the heat transfer coefficient considerably. Therefore it is important to provide effective degassing of the calandrias. The heating steam may contain some milk solids creating deposits on the steam side of the tubes, due to incomplete separation of entrained droplets from vapour in the separator. This also reduces the heat transfer. To create and maintain (due to the incondensable gases and leaks) the vacuum in the evaporator, two types of pumps are used: •  Vacuum pump •  Steam jet vacuum unit.

2.2.8.1. Vacuum pump Vacuum pumps such as the water-ring pump are used. Normally two units are installed; they are both used for quick start-up of the plant, while only one is used during production to maintain the vacuum. Only stainless steel material should be considered, as bronze - even it is cheaper - has a very short lifetime, especially if the plant has to process whey, due to corrosion.

2.2.8.2. Steam jet vacuum unit The steam jet vacuum unit is in principle designed like the thermo-compressor discussed earlier. This system has a low maintenance cost, but the additional steam requirement should be taken into consideration.

2.2.9. Flash coolers Very often the required concentrate temperature is lower than the one obtained from the last effect. The concentrate can naturally be passed over a cooling surface, such as a plate heat exchanger, but as the viscosity is high at this stage, it is not recommended. Instead, flash coolers are used. The system is simple and consists only of a vacuum chamber (vacuum created by steam jet vacuum units) into which the concentrate is sprayed. See Fig. 2.21. Depending upon the vacuum the concentrate will flash and due to the evaporation a cooling will take place simultaneously resulting in a slight increase of the solids content.

22

2. Evaporation

The flash cooler is mainly used for whey, where it is especially advantageous, as the cooling takes place instantaneously, thereby avoiding problems with crystallization of the lactose, which would create blockages between the plates.

2.2.10. Sealing water equipment All falling-film evaporators have transport pumps for passing the milk concentrate from one effect to another. The amount of pumps depends on the number of effects, and whether the effects are split or not. As the pumps work under vacuum efficient sealing is necessary to avoid any air leaking making it difficult to maintain the vacuum. This sealing is done with water. Each pump requires about 100200 l/h of sealing water of which normally ½-1 l/h enters the milk flow. The sealing water system may be designed, Fig. 2.21. Flash cooler so that each pump is furnished with a small funnel to see if © GEA Niro there is any excessive waste of sealing water and - which is more important - if a pump is suddenly using more water than normal, which means that the sealing ring is wearing out.

2.2.11. Cooling towers Many factories are placed near lakes, rivers or other natural water reservoirs, and the amount and temperature of cooling water are therefore no problem, provided the increased temperature in the return water is not causing any environmental problems. However, not all factories have got access to unlimited water supply, and the situation where the cooling water requirement cannot be covered may arise. The problem could be solved by recycling the water, but hot water is not a good cooling medium, so the vacuum in the evaporator would soon disappear. By installing a cooling tower, see photo, this problem is overcome. In the cooling tower the water is cooled (how much depends on local conditions for ambient temperature and wet-bulb temperature) by evaporation, as the water is distributed over a big surface, and a fan ensures the necessary air turbulence. The flow of water goes from the cooling tower to the condenser from where it is pumped back to the cooling tower.

23

Cooling towers © GEA Niro

Due to the evaporation of water in the cooling tower water has naturally to be added, but the amount is low. When a mixing condenser is used the extra water requirement is practically nil, as the condensed vapours are mixed with the water. It is recommended at certain intervals to renew the recycled water completely to avoid excessive bacteria and algae growth.

2.3. Evaporator design parameters 2.3.1. Determination of heating surface Saturated steam is used exclusively as the heating medium for evaporation of milk. The essential aspect to consider when designing a milk evaporator is to estimate the heating surface. Generally it must be large enough to secure the required heat transfer but not excessively large to keep still good coverage of the over-all tube surface by the evaporating liquid. It is calculated by following equation:

A=

Q [2,3] K * (t s - t m )

Where: A is the heating surface in m², Q is the amount of heat required for given duty in kcal/s or J/s or W,

t s is the tube wall temperature on the steam side in °C, tm is the tube wall temperature on the milk side in °C, K is the heat transfer coefficient in kcal/m² / °C / s or J/m² / °C / s or W/m² / °C.

The amount of heat required for evaporation Q is calculated from the required duty, i.e. the amount of water to be evaporated W (kg/h) and the specific heat of evaporation I under given conditions (vacuum and temperature): 24

2. Evaporation

Q=

W∗I 3600



[2,4]

2.3.2. Heat transfer coefficient The heat transfer coefficient is the most critical factor. The numerical values of K are influenced by a number of external factors as well as the properties of the evaporating liquid at any stage of the process (i.e. density, viscosity, surface tension, temperature, boiling point elevation, heat conductivity, specific heat) properties of the heating steam, tube wall material, surface treatment and cleanliness, velocity of the film flow, thickness of the film etc. The numerical value of the heat transfer coefficient varies between 3000 and 100 W/m²/°C for low viscous liquids and clean surfaces to high viscous liquids and dirty surfaces respectively. Therefore, under continuous operating conditions in a multi-effect evaporator the heat transfer coefficient decreases from stage to stage due to rising viscosity and formation of deposits (mainly calcium phosphates and precipitated proteins) on the heating surfaces. The heat transfer coefficient for skim milk is about 2500 W/m²/°C in the first effect and drops down to below 1000 in the last effect. For whole milk values are about 15% lower.

2.3.3. Coverage coefficient Burnt deposits in the tubes occur especially if the tube surfaces are not completely covered due to a low average flow of liquid per tube or to poor distribution. The increased demand for big multi-effect evaporators requiring bigger heating surface in order to obtain better specific consumption figures, can be met by using more tubes. This would, however, mean that less liquid is getting into each of the tubes, and the produced film becomes too thin. At high solids contents the viscosity will increase, the film will not flow any more, and there will be a risk of burnt deposits. This will result in a concentrate with small jelly lumps, very often discoloured and found in the powder as “scorched particles”, as they will not dissolve when the powder is reconstituted. In extreme cases the tubes block completely and manual cleaning is necessary. The designer therefore operates with the so-called coverage coefficient defined as:

Product kg / h at the lower end af the tubes [2,5] Periphery of the tubes The coverage problem was some years ago overcome by recirculating part of the feed from the outlet of the calandria to the inlet of same, thus increasing the amount of liquid sufficient to cover all the tubes. From a technical point of view this is the ideal solution, as it is cheap and simple, but from a product point of view it should not be tolerated, as it means that part of the product is exposed to the high temperature for a long uncontrollable time. This means that the final concentrate will get increased viscosity and possibly protein denaturation, both resulting in a powder with an inferior solubility. 25

In modern falling-film evaporators, the so-called “singlepass” evaporators, the problem is solved by dividing the effects with low coverage coefficient in two or more separate calandrias with the same boiling temperature and often one combined separator. See Fig. 2.22. Another method is to split the calandria by dividing it into two or more sections in a “multi-flow” evaporator. The product is pumped to one section, from the outlet of which it is pumped direct to the next section, and so forth. Having passed through the last section it is pumped to the next effect. This system is almost as cheap as the recirculation, but has the advantage of the divided calandria and no circulation is necessary. The trend today is to manufacture the calandria with longer tubes in order to obtain more heating surface per tube and to combine it with calandrias with two or more “splits” maintaining the coverage coefficient at a safe level. About forty years ago the evaporators were equipped with 3-4 m tubes and operated with a temperature difference of about 15°C, whereas evaporators 15 years ago had tubes with a length of up to 14 m and a temperature difference down to Fig. 2.22. Two calandrias with 2°C. Today most new evaporators have tube lengths up to 16 m. The advantage is that less product passes are needed one separator © GEA Niro to obtain sufficient coverage, fewer pumps, and reduced residence time. The disadvantage is that there will be an increased pressure drop of the vapour over the longer tubes, and that is on the account of overall evaporation capacity. Tube length up to 18 m has been tried, but the speed of the vapour becomes so high when it leaves the boiling tubes that the concentrate gets “atomized” and ends up in the heating side of the subsequent calandria and is then discharged as condensate with high BOD level. When designing an evaporator/spray dryer the main product is therefore always selected, and the evaporator calandrias are designed, so that optimal coverage coefficients are ensured, also for the other products. As mentioned above, the vapour generated from the evaporation, contains all the applied energy (less heat loss). The applied energy can thus be reused if the vapour condenses in a subsequent calandria operated at a lower product boiling pressure. The energy applied to the system can therefore be reduced to 50% if a second calandria is installed and 33% if a third calandria is used and so forth. But the vapour needs to be separated from the evaporated product before reused.

2.3.4. Boiling temperature A very important factor for evaporator design is the selection of the boiling temperature throughout the whole evaporator profile. The principle of multi-effect evaporation requires a temperature cascade of steam temperatures and boiling temperatures from stage to stage. Most common is the so-called feed-forward system, in which each subsequent evaporator stage has lower values of both heating and boiling temperature, than the previous stage. Milk is a heat sensitive liquid and thus the maximum permissible boiling temperature in the 26

2. Evaporation

first effect has an upper limit. Usually this is 66 to 68°C. It is somewhat higher for whole milk than for skim milk. Due to increasing concentration during evaporation the viscosity of the concentrate rises as well. This increase is further supported by the temperature drop and therefore there is also a limit for minimum permissible temperature. Therefore the available working temperature range is about 25-30°C which means that the temperature drop between the individual stages, which depends on number of stages, is in practice 10 to 3°C. The evaporation capacity of an evaporator is:

C = K ∗ S ∗ ∆t 

[2,6]

Where: C = evaporation capacity K = heat transfer coefficient S = heat surface t = temperature difference between the boiling temperature in the first and last effect. Thus the capacity of an evaporator can be increased by more surface or higher boiling temperature in the first effect. It is not recommended to use higher temperature than 6668°C, as discussed above. The thermo-compressor is incorporated between the separator and the shell of the first effect (mono-thermal compression), the separator of the second and the shell of the first (bi-thermal compression), or between the separator of the third and the shell of the first effect (tri-thermal compression). The influence on the steam economy and the investment costs is significant. However, one major drawback in multi-effect evaporators is the long residence time, where the product is exposed to heat. Although it is at low temperature, it will have a negative effect on the viscosity of the concentrate.

2.4. Evaporation parameters and its influrence on powder properties 2.4.1. Effect of pasteurization The temperature obtained from the last preheater is in multi-effect evaporators lower than the boiling temperature in the first effect. Additional preheating is therefore necessary to obtain the minimum required 2-3°C above the boiling temperature of the first effect. A separate preheater heated by live steam, usually via a thermo-compressor, is then used. However, some products may require higher temperatures, but the primary purpose of the heat treatment in an evaporator, apart from bacteriological requirements, is not ”pasteurization”, but obtaining a tool to get functional properties in the final powder. The reasons for the heat treatment are: •  Bacteriological requirements •  Functional properties of dried products: •  Heat classified skim milk powders •  High-heat heat-stable milk powders •  Keeping quality of whole milk powders •  Coffee stability of whole milk powders.

27

2.4.1.1. Bacteriological requirements A pasteurization directly before the evaporation will naturally influence the bacteria count in the final powder, and the higher the temperature and the longer the holding the more efficient the killing. The heat treatment applied should under any circumstances meet or exceed legal requirements.

2.4.1.2. Functional properties of dried products 2.4.1.2.1. Heat classified skim milk powders Skim milk powder is often produced according to a fixed degree of denaturation of the whey proteins and is classified according to the whey protein nitrogen index (mg WPNI/g powder) which expresses the content of undenaturated whey proteins. Different temperature and time combinations have an influence on the index as shown in Fig. 2.23., as well as % denaturation of H-lactoglobulin in milk in Fig. 2.24.

Fig. 2.23. mg WPNI/g powder as a function of the pasteurization intensitive. A relation between temperature and time © GEA Niro

28

Fig. 2.24. % Denaturation of H-lactoglobulin © GEA Niro

2. Evaporation

2.4.1.2.2. High-Heat Heat-Stable milk powders This type of powder is used for reconstitution for making evaporated, sterilized milk, especially in the Far East. After reconstitution to 25-27% TS the product has to be sterilized using temperatures of 120°C or higher during 20 min. The heat stability of the recombined product is controlled by the pasteurization temperature/time combination prior to the evaporation and drying. A direct contact heating system gives a better result.

Pasteurization

Temperature Interval °C

Indirect

°C

From 60 to 80

Direct

°C

From 80 to 110 *)

Direct

°C

From 110 to 125

Holding time in min.

2-4

*) In the heating interval from 80 to 110°C a very fast heating is important to avoid interaction between the whey proteins, in order to produce low viscous products with good heat stability.

2.4.1.2.3. Keeping quality of whole milk powders When producing whole milk powder one problem is the shelf-life, as the fat easily becomes oxidized, if the powder is not packed using an inert gas. As a lot of powder is shipped in bags, it is not possible to protect the powder effectively, and antioxidants are in most cases not permitted.

Fig. 2.25. Development of free -SH groups as a function of pasteurization temperature © GEA Niro

By pasteurizing (direct) the milk prior to the evaporation to 90-95°C and keeping the temperature for ½-1 min., some natural antioxidants will be formed, as -SH groups, originating from the amino acids cystine, cysteine and methionine. They are liberated and will act as antioxidants. Higher pasteurization temperatures will form more -SH groups, but they will react with casein and not be found in free form. See Fig. 2.25. The free -SH groups will at the same time give the milk a cooked flavour, which, however, is liked by many consumers. 29

2.4.1.2.4. Coffee stability of whole milk powders To produce instant whole milk powder with good reconstitution properties in cold water and at the same time with a good “coffee stability” - that is no coagulation should take place when the powder is added to hot coffee as a “whitener”. It is recommended to use a temperature/ time combination to achieve a WPNI of > 3.5 mg/g, which corresponds to approx. 45% denaturation of -lactoglobulin, see further Fig. 2.24. For further and a more elaborate reading please see chapter 10. Achieving product properties. The pasteurization can be carried out in different ways, either: •  Indirect in plate-, spiral- or straight-tube heat exchangers •  Direct steam injections into the milk or milk into a steam atmosphere.

2.4.2. Concentrate properties The concentrate leaving the last effect of the evaporator is liquid. The concentrate may however have different viscosity depending upon the composition, heat sensitivity of the proteins, pre-treatment, temperature and solids content. Whole milk concentrates are generally less viscous than skim milk concentrates, and as a general rule the viscosity should not exceed 60 and 100 cP, respectively, if the atomization should be optimal. Higher viscosities can of course be handled in the dryer, but not without losing capacity (bad atomization - big droplets) and an inferior product will be the result. The composition will influence the viscosity, especially on the protein (P) content in relation to the lactose (L) content. When the ratio P:L is high the concentrate will get a high viscosity. This is especially a problem with jersey cows during the whole year, but other breeds tend to give problems during the beginning and/or the end of the lactation period. The ratio P:L can be adjusted by adding lactose. As a general rule it can be concluded that a higher fat and lactose content will give lower viscosity. Higher protein content will give higher viscosity. When milk is exposed to a high heat treatment, especially in indirect pasteurizing systems, prior to the evaporation, the viscosity of the concentrate will be higher. The concentrate temperature will naturally have a direct influence on the viscosity and higher temperature means lower viscosity. The solids content of the concentrate will have a very significant influence on the viscosity, and the higher the concentration the higher the viscosity. However, the above only states the direct influence of some parameters on the viscosity. One of the main influences on the viscosity is the time, i.e. the viscosity is a function of time, also known as age-thickening. This means that the viscosity will increase if the concentrate is left for some time. The increase is depending on composition, mainly proteins binding to each other, temperature and concentration. The age-thickening is only partly reversible by agitation.

30

2. Evaporation

Fig. 2.26. Age-thickening as a function of temperature (skim milk 48.5% solids) A temperature increase will naturally result in a viscosity drop; but as the age-thickening is more pronounced at higher temperatures, the viscosity will soon increase to the same level and further on as the time passes. See Fig. 2.26.

31

Fig. 2.27. Age-thickening as a function of solids content (skim milk 55°C) The age-thickening will also be influenced by the solids content and will be more pronounced the higher the solids content in the concentrate. See Fig. 2.27. The composition will have same influence on the age-thickening as on the viscosity. If the concentrate should be kept for some length of time, or transported over long distances before further processing, the concentration and temperature should be low. The low temperature will at the same time limit bacterial growth.

32

3. Fundamentals of spray drying

3. Fundamentals of spray drying 3.1. Principle and terms Spray drying is an industrial process for dehydration of a liquid feed containing dissolved and/ or dispersed solids, by transforming that liquid into a spray of small droplets and exposing these droplets to a flow of hot air. The very large surface area of the spray droplets causes evaporation of the water to take place very quickly, converting the droplets into dry powder particles.

3.1.1. Drying air characteristics The drying medium used for drying of milk is atmospheric air, cleaned of dust by filtration and heated to provide the heat necessary for evaporation. Evaporation proceeds initially under adiabatic conditions. In such a system, all sensible heat from the drying air is utilized for evaporation of water, which becomes, as vapour, part of the drying air. The enthalpy of the air remains constant, supposing that the liquid entered the system with a temperature of 0°C (zero enthalpy) and absence of any heat loss. The various terms characterizing the drying air conditions are as follows: - Dry bulb temperature (td) is the temperature of air, which is not saturated with water vapour, as measured by an ordinary thermometer. In practice, the dry bulb is just referred as air temperature and is expressed either in °C (t) or as the absolute temperature in °K (T) whereby T = t + 273.15. - Wet bulb temperature (twb) (or more precisely Adiabatic saturation temperature) is a characteristic of moist air of a given dry bulb temperature, expressing the saturation temperature of that air with the same enthalpy, i.e. obtained by evaporation of 0°C water under adiabatic conditions. The difference between dry and wet bulb temperatures is a measure of drying capability (driving force). It is the temperature to which the air of dry bulb temperature (t) will drop, when evaporating water in an isolated air-water system until saturation condition occur (supposing that the temperature of water to be evaporated is 0°C). The enthalpy of the air during this evaporation remains unchanged, as the heat from the air is utilized for evaporation only. It can be also expressed as the temperature a droplet of water will obtain when exposed to a flow of air of temperature (t). Measuring wet bulb temperature is based on the same principle, i.e. the thermometer bulb is kept wet by a thin film of water and exposed to a flow of air. The relative humidity of the air at wet bulb temperature is 1. - Dew point temperature (tdp) is the temperature where condensation of vapour will commence, if the air is cooled down at constant absolute humidity. The relative humidity of the air at the dew point temperature is 1 and its enthalpy is lower than that of the same air at its dry bulb temperature and wet bulb temperature. - Air absolute humidity (y) is the ratio of the amount of water vapour (mv) to the amount of dry air (ma). Usually it is expressed in kg of water vapour per kg of dry air.

33

Thus:

y=

m m

v a



[3,1]

y = 0.622 *

Pv Pt - Pv



[3,2]

Where: p v is the partial pressure of water vapour p t the total pressure and 0.622 the molecular weight ratio of the water vapour and of air, i.e. 18.015/28.954. - Air relative humidity ( or %RH) is the ratio of partial pressure of water vapour (pv) to the water vapour pressure at the saturation point (ps) at the same temperature.

Φ=

Pv Ps

%RH = ,

Pv ∗ 100 Ps

[3,3]

The Extended Antoine Equation shows the relation between saturated water vapour pressure in Pa and temperature t in °C:

psat = e

 2 7206.7 − 7.1385 ∗ ln (t + 273,15)+ 0.000004046 ∗ (t + 273.15)   72.55 – T + 273,15   [3,4]

- Saturation point is the air temperature at which any further temperature drop will result in condensation. Saturated air has equal dry bulb, wet bulb and dew point temperatures. - Drying air rate (Aa) is usually expressed as the mass flow of ambient air per hour (kg/h) and includes both the amount of dry air (Ad) and water vapour (Av) which can be calculated using equations:

A a = A v + A d [3,5] Ad =

Aa  1+ y

A v = Aa ∗

y 1+ y

[3,6]



[3,7]

- Heat capacity is the amount of heat necessary to heat 1 kg of a substance by 1°C and is a function of the temperature. - Heat capacity of the air (ca) is the amount of heat necessary to heat 1 kg of dry air by 1°C. It is expressed in J/kg/°C and is temperature dependent as shown below, where T is temperature in K:

1.14543  c =1004.68+355.633* e-21900/T a 34

[3,8]

3. Fundamentals of spray drying

To get c a in kcal/kg/°C, equation [3,8] is divided by 4186. For routine technical calculations a constant value 0.245 kcal/kg/°C or 1.026 kJ/kg/°C may be used. The amount of heat (Q) necessary to heat a given amount of dry air (Ad) from t1 to t 2 °C is:

Q = A d ∗ (ca 2 ∗ t 2 − ca1 ∗ t1 ) 

[3,9]

in which ca1 and ca2 are the values calculated from equation [3,8] corresponding to the temperatures t1 and t 2. - Heat capacity of water (cw) is approximately 1.0 kcal/kg/°C or 4.186 kJ/kg/°C. - Heat capacity of water vapour in J/kg/°C is:

c v = 1845.8 + 33336.33 ∗ e-418.99/T

0.7724

[3,10]

To get cv in kcal/kg/°C [3,10] is divided by 4186. For routine technical calculations of water vapour (cv), 0.46 kcal/kg/°C or 1.926 kJ/kg/°C may be used. - Latent heat of evaporation (r) or vaporization is the amount of heat necessary to transform a liquid to vapour at constant temperature. The reverse process i.e. transforming a vapour to liquid requires a release of the same quantity of heat and is called heat of condensation. The latent heat of water is 597.3 kcal/kg or 2500 kJ/kg at the temperature 0°C and barometric pressure 760 mm Hg. - Enthalpy (h) of air is the thermal energy of that air expressed as sum of heat necessary to evaporate its moisture content at 0°C and to heat both the water vapour and dry air to its actual temperature, as expressed by the equation: 

[3,11]

- Density () of air is the weight per unit volume of air and it is a function of air temperature, moisture content and pressure. Usually it is expressed in kg/m3. The density of dry air d at the temperature of 80°C at the barometrical pressure 760 mm Hg is equal to 1, hence at the temperature of t°C it is:

ρd =

353.15  273.15 + t

[3,12]

The density of ambient (moist) air (a) is:

ρ = ρd ∗

1+ y  1 + 1.6 ∗ y

[3,13]

To calculate air density at the actual pressure B (in mm Hg), the above results must be multiplied by B/760.

35

3.1.2. Terms and definitions The following terms are used in spray drying technology: - Ambient air is the atmospheric air supplied to the system from the surroundings of ambient temperature (ta) and ambient humidity (ya) - Inlet air temperature (t1) is the temperature of the air after heating or cooling at the inlet of a processing system having an inlet air absolute humidity (y1) - Outlet air temperature (t2) and outlet air humidity (y2) express the same for air leaving the system - Water content of the feed (milk concentrate) or product (final powder) can be expressed in several ways: - a) Total solids content (TS) or Moisture content (W) expressed in weight percent. - b) Moisture content on dry basis (x) expresses the ratio of the quantity of moisture to the quantity of dry solids. The relationships between these expressions are:

W = 100 − TS and TS = 100 − W 

[3,14]

x=

W  100 − W

[3,15]

W=

100 ∗ x  1+x

[3,16]

- Density or specific gravity of milk products, both concentrates and powders can be calculated using following formula: 20° C

=

m1 1

+

m2 2

100 +

m3 3

+ etc.



[3,17]

Where: m1, m2 etc. are the contents of individual components in percent 1, 2 etc. their densities. The density is usually expressed in g/ml or kg/m3. The densities of some components of milk products are given in Table 3.1.

36

3. Fundamentals of spray drying

Table 3.1. Densities of some components of milk solids.

Component

Density at 20°C,g/ml

Non-fat milk solids

1.52

Milk fat

0.94

Amorphous lactose

1.52

Alpha-lactose monohydrate

1.545

Whey solids

1.58

Milk proteins

1.39

Sugar (sucrose)

1.589

Water

1.00

- Heat capacity of milk solids is also a function of temperature. However, for practical purposes it is sufficient to reckon with constants, as given in Table 3.2.

Table 3.2. Heat capacity of some milk components.

Component

Heat capacity kcal/kg/°C

Non-fat milk solids

0.3

Milk fat

0.5

Water

1.0

Heat capacity of a product containing several components is calculated as a weight sum of heat capacity values of the individual components.

3.1.3. Psychrometric chart. The conditions of drying air throughout the drying process are illustrated by the psychrometric chart (Mollier diagram or h-x, sometimes i-x diagram). The y-axis represents the temperature and the x-axis absolute humidity. The psychrometric chart is constructed so that the isotherm corresponding to 0°C is horizontal. The isotherms for higher temperatures slope gradually more upwards. Lines representing enthalpy, saturation, constant relative humidity and vapour pressure are also shown. The saturation line divides the chart into the zone of unsaturated air and the zone of mist. The psychrometric chart illustrating all these air characteristics is given on fig. 3.1.

37

Fig. 3.1. The principle of h/x diagram. [The ambient air shown by point A has dry bulb temperature TG, enthalpy ha, absolute humidity x A , relative humidity RH, wet bulb temperature TWB and dew point temperature TDP].

3.2. Drying of milk droplets When spray drying milk, very high rates of heat and mass transfer take place in extremely short periods of time. Severe quality defects of the product can occur, if the factors, inducing degradation are permitted to dominate because of lack of knowledge or lack of operation control. The milk concentrate leaves the atomizing device as a thin film at a velocity of 100 - 200 m/s, breaking up into droplets which immediately contact the hot drying air. Evaporation of most of the water in the droplets takes place during the time the droplets decelerate to reach the velocity of the surrounding gas. The smallest droplets lose about 90% of their moisture within a distance of 0.1 m from the atomizing device, whereas the largest droplets need about a 1 m path. The rate of evaporation depends to a great extent on the total surface area of the droplets, which is defined by the droplet size.

3.2.1. Particle size distribution Sprays of droplets as well as the produced powder particles are characterized by mean size and size distribution of the droplets and particles, respectively. The size distribution of a spray of droplets can be measured by laser light scattering techniques. Particle size distribution of a powder can be determined by the same method or by alternative methods such as microscopic counting, sifting or photographic methods with computer evaluation. The results of these methods express the frequency of droplets in a given size ranges or in cumulative numbers (smaller or larger than n microns). The results can also be expressed graphically by a histogram or by frequency. An example of expressing results in tabular form is given in Table 3.3. Its graphical presentation is shown on Figs. 3.2 and 3.3.

38

3. Fundamentals of spray drying

3.2.2. Mean particle size The Mean particle size or droplet size can be expressed in several ways. The most common are: – Most frequent diameter, which can be seen directly from tabularized results or as the highest point of the frequency curve, possibly as an inflection point on the cumulative curve. – Arithmetic mean diameter, defined as the sum of the diameters of separate particles/ droplets, divided by their number. This mean diameter is most significant when the size distribution is not overbalanced by either very large or very small elements. – Geometric mean diameter, defined as the n-th root of the product of the diameters of the n particles analysed. It has the highest frequency in the log-normal distribution. – Median diameter, which is the diameter corresponding to 50% of the number, weight or volume of the droplets /particles. Apart from diameters based on frequency of size occurrence, there exist surface, volume and volume/surface mean diameters. For characterization of the size distribution of a spray of droplets or dried powders, the most common is the geometric mean diameter. The volume/surface also called Sauter mean diameter is most suitable for spray drying operations as it expresses the same surface-tovolume ratio as the whole powder. Table 3.3. Example of expressions for particle size distribution.

39

Fig. 3.2. Cumulative curve of the example in Table 3.3.

Fig. 3.3. Log-normal distribution curve of the example in Table 3.3.

40

3. Fundamentals of spray drying

3.2.3. Droplet temperature and rate of drying The droplet and particle moisture profile during the whole process is often called the particle temperature history and it is of utmost importance not only for the structure of the particle and its surface, but also for potential product heat degradation. The droplet/particle temperature during an ideal drying process is as follows: a) The temperature of the droplet during the whole evaporation process lies between the temperature of the surrounding air and its wet bulb temperature. The droplet moisture determines the water activity of the droplet/particle. This, together with the relative humidity of the surrounding air decides where - between these two limiting points - is the actual droplet/particle temperature. b) Droplets of water (having the water activity aw = 1) will attain the wet bulb temperature regardless of the feed temperature once the first contact is made with the drying air. This temperature will be retained until evaporation is completed. c)  Droplets of milk concentrate at the beginning of the drying process will attain a temperature somewhat higher than the wet bulb temperature because the water activity of the concentrate is somewhat lower than 1 (about 0.85 - 0.90). d) As water evaporates, the water activity (aw) gradually decreases. This results in a gradual rise of particle temperature towards the surrounding air temperature. e) When equilibrium is achieved between the drying air and a particle, the particle water activity is equal to the relative humidity of the surrounding air and consequently the particle temperature is equal to the surrounding air temperature, i.e. aw = and tp = t2. The evaporation of water from the surface of the milk droplets commences under so-called constant or first rate drying period conditions. It does not mean that the rate of drying is strictly constant because, as mentioned above, the water activity is decreasing. The droplet at this stage, however, is still a fluid in which the moisture can migrate easily from the droplet interior to the surface and keep it nearly saturated. The author of this book suggests the following relationship for the droplet temperature within this period:

t p = t 2 − ( t 2 − t wb ) ∗

(aw − Φ) [3,19] (1 − Φ )

At a later stage of the drying process the moisture content achieves a critical value at which the droplet loses the character of a fluid and becomes a wet solid. The critical moisture content of milk products is dependent on many factors and operating circumstances. However, it is in the range of 30 - 15%. It is characterized by a sudden occurrence of a moisture gradient across the droplet diameter. At this stage the factor controlling the rate of drying becomes the rate of diffusion of the moisture through the particle. This period is known as falling or second drying rate period (Fig.3.4). The rate of heat transfer exceeds that of mass transfer and the particle begins to heat up faster, than indicated in equation [3,19]. There are both moisture and temperature gradients in the particle interior, and a hard crust forms on the surface.

41

Fig. 3.4. Rate of drying

3.2.4. Particle volume and incorporation of air During the evaporation of water the droplets decrease in size. The theoretical reduction of diameter, weight, volume and surface area when drying droplets under ideal conditions from 50 to 0% moisture, expressed in percent of initial values is graphically shown on fig. 3.5.

Fig. 3.5. Reduction of droplet size during evaporation

42

3. Fundamentals of spray drying

During the early drying stage the droplet follows closely the ideal weight-volume-diameter relationship and retains its spherical shape. When hard crust formation on the particle surface occurs, the final size is more or less defined. The presence of air in atomized droplets has an important influence on the final shape and structure. There is always some air in the droplets depending on the aeration of the feed prior to spray drying, or during the atomization process. The composition and properties of the feed also play a role. The presence of air in the particles is usually undesirable and should be avoided (nevertheless it may be desirable for some special products and product characteristics). Depending on droplet size, the initial volume of incorporated air and its distribution (i.e. size of air bubbles) and particle temperature history, the air bubbles (and consequently also the particle) may expand, shrink, collapse, form balloons or even disintegrate. Air remaining in the droplets forms so called vacuoles in final dried particles. This is referred to as occluded, entrapped or void air. It increases the bulk volume i.e. decreases the bulk density, affects the reconstitution properties and makes packaging under inert gas more difficult. To avoid heat degradation of the milk concentrate and expansion of air incorporated in the droplets, the constant rate of drying should be retained as long as possible with low surrounding air temperature until the critical point. The efforts to approach such conditions resulted in the development of the two-stage drying process, extended two-stage drying process (as accomplished in dryers with integrated fluid bed or belt) and three-stage drying (spray dryers with both integrated and external fluid beds) methods.

3.3. Single-stage drying In single-stage drying, the total removal of water takes place solely by spray drying, and the heat for evaporation is supplied by the drying air only. In other words, the milk droplets are in the dryer mixed with the hot drying air in such proportion as to achieve the required final moisture content just before particles and air are separated and leave the drying chamber. As discussed in the previous section the rate of drying, especially during the falling rate period, declines. The removal of the last portion of moisture at the end of the drying process proceeds slowly and is costly. For instance, drying of skim milk concentrate of 50% total solids, using air inlet temperature of 200°C to produce a powder with final moisture content 3.6% will in singlestage drying require an outlet temperature of 101°C whereas only 73°C, when drying to 7% moisture in the first stage of a two-stage dryer. The difference between drying to 3.6 and 7% respectively corresponds to 4.1% of total evaporation, however to achieve this evaporation in one stage 33% more heat is required. The last phase of drying may also be harmful to powder quality due to the combination of high outlet temperature and low moisture content causing particles to be heated to relatively high levels. Therefore single-stage dryers must operate under conditions, which keep the particle temperatures reasonably low. This means that relatively low inlet temperatures and feed concentrations have to be used, especially when drying heat sensitive and high quality products. This, of course, affects the drying economy. It is fair to mention, however, that in spite of the important advantages of two-stage drying when compared with single-stage, the application of the latter is sometimes unavoidable. Such is the case with certain thermoplastic and hygroscopic products which are too sticky at higher moisture content, thus making application of two-stage methods more difficult.

43

3.4. Two-stage drying Two-stage drying involves spray drying to a moisture content which is, for normal milk powders, about 2 - 5% higher than the required final moisture content. Subsequent fluid bed drying then removes the excess of moisture. The outlet temperature from the spray dryer is about 15 25°C lower than with a single-stage process. Consequently the surrounding air temperature at the critical drying stage and particle temperature are correspondingly lower as well. Therefore two-stage drying allows an increase of the inlet temperature and/or feed concentration above such values, which would simply be impossible in the single-stage process. This contributes further to economy improvement. The completion of moisture removal is carried out by additional fluid bed drying. In this method, warm drying air is supplied gradually to meet the needs of the rate of diffusion to secure the completion of drying. The temperature of the powder, which in this case is anyhow relatively low, remains low and continues to decrease. It only begins to rise again when moisture content approaches its final value. However, no heat damage takes place under these conditions as the inlet air temperature to the fluid bed is too low to cause this. The second drying stage conducted in the fluid bed requires of course energy input, but in spite of the specific heat consumption being relatively high, after-drying of powder by fluid bed requires only 30 - 50% of that energy, which would have been required if the same drying had been conducted in the first or spray drying stage. Thus, in comparison with single-stage drying, if all other parameters remain the same, the two-stage drying method requires at least 10% less heat. Under certain circumstances considerably more savings are possible by increasing the air inlet temperature and feed concentration. Apart from improved heat economy the plant capacity is also increased. Two-stage drying has its limitations, but it can be applied to such products as skim milk, whole milk, pre-crystallized whey concentrates, caseinates, whey protein concentrates and similar powders. The level of moisture of the powder leaving the first drying stage is limited by the thermoplasticity of the wet powder, i.e. by its stickiness. With increasing moisture content the temperature at which the powder starts to be sticky (so-called sticking temperature) decreases. The sticking temperature is defined as the temperature at which the powder starts to stick to a warm metal surface and forms deposits and lumps. It depends on the powder composition. The components contributing to the stickiness and thereby to lowering of the sticking temperature are amorphous lactose, lactic acid, sucrose and other carbohydrates. For skim milk and whole milk powder the moisture content of the powder leaving the spray dryer should be no higher than 7 - 8%. This is to ensure that the powder is continuously discharged under gravity into the fluid bed without lumps and that the chamber remains reasonably free of wall deposits. Any mechanical treatment of wet powder is undesirable as it will create hard lumps. Therefore the only type of drying chamber which is suitable for application of twostage drying techniques has a reasonably steep cone with a separate outlet for the drying air. The two-stage drying techniques can be applied both for the production of non-agglomerated and agglomerated powders. Agglomeration requires special features which will be discussed later. However, even two-stage dried powder produced without these special means for agglomeration, is always slightly agglomerated and consequently has a lower bulk density. Nevertheless, agglomerates formed due to the high powder moisture content at the chamber outlet are very fragile and are broken down by pneumatic transport or by blow line transport to storage silos. After such treatment the bulk density is usually higher than that obtainable by single-stage drying. 44

3. Fundamentals of spray drying

Two-stage drying is very suitable for production of agglomerated powders by separating the non-agglomerated particles from the agglomerates (i.e. collecting the cyclone fractions and reintroducing these fine fractions, so-called fines, into the wet zone around the atomization device). The agglomeration is in this case much stronger since it takes place when the primary particles have much higher moisture content than they would under the same conditions in single-stage drying. For processing of whey the two-stage drying method is possible only with pre-crystallized whey concentrate. Pre-crystallisation transforms a great part of amorphous lactose (which is a component contributing to stickiness) into -lactose mono-hydrate. Generally, products containing a high amount of amorphous lactose or other carbohydrates are difficult to treat by two-stage processing. It has to be decided on a case to case basis by testing whether two- stage drying is feasible or not.

3.5. Expansion of air bubbles during drying Fig. 3.5 shows the ideal reduction of the dimensions of an air-free droplet during the drying process. This condition, however, never occurs in practice. The presence of air in the feed and in the droplets together with the conditions of the drying process are then decisive as to (a) whether any reduction will take place at all, or (b) at what stage it will cease, or possibly (c) whether an expansion instead of shrinkage will take place. Microphotos on Fig. 3.6 - 3.11., obtained by Scanning Electronic Microscopy (SEM) techniques show skim milk powder particles from various plants operating at various conditions, published by Písecký [51]. Fig. 3.6 - 3.8. illustrate particles from a high capacity single nozzle dryer operating in the single-stage drying mode using an air inlet temperature 195°C (see section 3.3). It can be seen that in spite of relatively low occluded air content in the droplets due to nozzle atomization, the droplets were exposed to expansion due to overheating. Fig. 3.6 is a typical example of a blown-up particle (diameter approximately 100 μm). Expansion of some highly overheated air bubbles that are present close to the particle surface is accompanied by extensive sub-surface evaporation resulting in an explosion-type phenomena causing formation of a balloon of semi-plastic solids. Some of the small satellite particles seen in Fig. 3.6 are in fact such balloons.

45

Fig. 3.6.

Fig. 3.7.

Fig. 3.8.

Fig. 3.9.

Fig. 3.10.

Fig. 3.11.

Fig. 3.6 – 3.11. See description in section 3.5.

46

3. Fundamentals of spray drying

Fig. 3.7 shows such a balloon in higher magnification (diameter approximately 10 μm) having wall thickness of about 1 to a few microns. Such a particle will seldom survive further mechanical handling, and is thus broken down into small fragments, which, as fines, may not be collected in cyclones and therefore leave the dryer with the exhaust air. Sometimes the hard, but crispy crust cannot withstand the pressure and the particle fractures into two or more pieces as shown on Fig. 3.8. Needless to say, the accompanying undesirable effect of overheating is the deterioration of solubility index and overall heat degradation. The microphotos on Fig. 3.9 3.11 show particles from a spray dryer with rotary wheel atomizer operating in the two-stage drying mode (see section 3.4). This process enables much lower surrounding air temperatures than is possible with single-stage drying. Thus, during the critical stage, gentle drying is achieved that results in shrinkage of particles and protects solubility not only with inlet temperature 200°C (Fig. 3.9), but also with 250°C (Fig. 3.10). This effect is achieved in spite of the amount of the incorporated air in the droplets being higher from a wheel atomizer than from nozzles as shown in previous example. Fig. 3.11 shows a particle from a single-stage dryer operating with steam-swept-wheel atomizer. In spite of single-stage drying, shrinkage was achieved also in this case due to the steam creating air-free atomization environments. With no air present, no expansion takes place.

3.6. Extended Two-stage drying The advantages of the two-stage drying techniques regarding product quality and heat economy are obvious and therefore efforts have been made to overcome the limitations mentioned in the previous section. The critical phase of two-stage drying occurs when wet particles contact the surface of the equipment. Spray dryers with an integrated fluid bed as discussed in section 5.3 are better at handling this phase. The basic idea behind this dryer design was to operate the first drying stage at much higher moisture levels, than was previously possible with normal two-stage drying, and at the same time to avoid any contact of the wet powder with the chamber surface by introducing powder directly into the fluidized powder layer of a fluid bed placed at the bottom of the chamber. The powder can be dried in this integrated fluid bed to the required final moisture content. In such a case the two-stage drying process is completed inside the chamber. Alternatively the product can be dried in the integrated fluid bed to a moisture content corresponding to the chamber outlet moisture of a normal two-stage drying process, and dried finally in an external fluid bed to the final moisture specification. Such a process is known as three-stage drying. The expression of extended two-stage drying was here used to emphasize that the process involves two-stage drying, but the feasible limit for moisture of the powder leaving the first stage has been extended or increased from 7 - 8% to 12 - 16% in the case of skim and whole milk powder. Moreover, even products which are difficult to process by the normal two-stage drying techniques as baby food and high-fat products (including not only milk based, but also whey based fat-filled powders), maltodextrins etc. can be successfully produced by this method.

47

3.7. Fluid bed drying The conventional type of fluid bed used for final treatment of milk powders is a vibrating fluid bed. Dry milk products are so-called dead powders and are difficult to fluidize. Vibration is required to overcome this problem, i.e. to avoid channelling effects and to ensure true fluidization. At the beginning of the 1980’s a non-vibrating (so-called static) fluid bed was introduced for milk powder manufacture. This will be discussed later. The first application of the vibrating fluid bed as a component in milk powder plants came about with the introduction of milk powders having high fat content (35 to 50 % and even higher). These powders were to be used as a component for dry mixing of milk replacers for feeding calves. Vibrating fluid beds overcame the difficulties experienced when trying to cool such products in pneumatic conveying systems. After introducing a fluid bed into the spray drying processing line, it was recognized that there was a distinct difference in the structure of fluid bed treated powder compared with powder from a pneumatic conveying system. The fluid bed treated powder was distinctly more coarse and free-flowing. The reason for better flowability was a partial agglomeration. This agglomeration, called primary agglomeration, is always taking place by collisions of droplets and particles of various moisture content in the atomization cloud. It is, however, a loose agglomeration which is easily broken down in an air transport system. The fluid bed treatment, however, is much gentler. The agglomerated structure is retained, resulting in better flowability and appearance. Fluid beds exert a so-called classification effect by blowing off the smallest particles from the powder and collecting them in a cyclone. Recognizing this effect led to the second important application of vibrating fluid beds. This was the manufacture of agglomerated powders by the straight-through process whereby the cyclone fraction was recycled back to the spray dryer and introduced into the wet zone to increase the agglomeration. Initially such a process was introduced with the fluid bed acting as cooling and classifying bed only. In combination with the development of two-stage drying techniques, vibrating fluid beds were further applied for after-drying prior to cooling. Table 3.4. Typical fluidizing velocities for various products.

Product

Fluidizing velocity m/s

Caseinates

0.05-0.15

Skim milk powder

0.15-0.25

Whole milk powder

0.25-0.40

High fat powders

0.40-0.60

48

3. Fundamentals of spray drying

When operating vibrating fluid beds one has to be aware that it is a not too flexible unit for treatment of powders with different properties. An important characteristic of vibrating fluid bed operation is the fluidizing velocity. This is the upwards velocity of the air calculated over the whole plate area. Fluidizing velocities in vibrating fluid bed for various products are given in Table 3.4. Velocities used in integrated (static) fluid beds range between 0.3 - 0.7 m/s for annular bed design and 0.5 - 1.5 m/s for circular design. Fluid beds for cooling operate mostly in two sections. The first section applies the ambient air and the second conditioned air i.e. air which has been cooled down to 11 - 5°C, first by condensation to remove the excess of moisture followed by reheating to reduce the relative humidity to 80% or lower depending on product properties. Vibrating fluid beds for drying operate at the temperature necessary for the required drying duty. The upper limit is normally approximately 110°C. The drying efficiency of a vibrating fluid bed is also a function of the bed depth i.e. the height of the powder layer and product residence time. This is usually 50 - 300 mm. On the other hand effective cooling requires a low bed depth. The operation of a fluid bed must be regularly checked to achieve the optimum performance. If the final moisture is controlled only at the discharge of the fluid bed it may happen that the powder is over-dried in the drying section, followed by picking-up of moisture in the cooling section. Therefore it is useful to check the moisture profile along the whole fluid bed length. As a routine control, it is helpful to check powder temperatures both in the drying and cooling section. This provides an indication of the moisture levels. There is temperature/humidity equilibrium between the air and powder. The water activity of common milk powders is in the range of 0.20 - 0.25. This means that these powders will begin to pick up the moisture from the cooling air when cooled below 30 - 34°C and 30 - 25°C when using cooling air of dew point 8°C and 5°C respectively. Spray dryers with an integrated fluid bed operate with non-vibrating, so-called static fluid beds. This is because the method of drying, applied in these types of dryers, results in coarse powders of larger mean particle size. These powders are easier to fluidize. Static fluid beds operate therefore at much higher fluidizing velocities, e.g. 0.5 - 1.5 m/s and with higher bed depths, e.g. 0.3 to 1 m. The duty of a fluid bed in both external and integrated mode is not only to evaporate the excess of moisture or to cool the powder, but also to classify the powder, i.e. to separate the small from the coarser particles. The usual aim is to blow-off fines from the agglomerates and re-agglomerate to achieve larger mean particle size and thereby better functional, mainly instant powder properties. The amount of particles, which can be separated by fluidization, depends on fluidizing velocity and particle size.

49

Fig. 3.12. Fall velocities of spherical particles of particle density 1400 and 1000 kg/m3 in air at 80°C. Fig. 3.12 shows the free fall velocity which is the reciprocal value of fluidizing velocity or suspension velocity for particles sized between 0.01 and 10 mm (10 - 10000 μm) and having particle density 1400 and 1000 kg/m³. The air reference temperature is 80°C. This range represents roughly the particle density extremes for dried milk products. The calculations were done according to Schlünder [1], who has introduced dimensionless expressions for velocity v* and particle diameter d*, as follows:



[3,20]



[3,21]



where: vf fluidizing velocity m/s d p particle diameter m d* dimensionless particle diameter a density of air kg/m3 p density of particle kg/m3 a air viscosity Pa.s v* dimensionless velocity g gravity constant 9.81 m/s 50

[3,22]

4. Components of a spray drying installation

4. Components of a spray drying installation The main components of a modern spray dryer, powder handling and storage as shown in Fig. 4.1. are:

Drying chamber Hot air supply system •  Supply fan •  Air filters •  Air heater •  Air disperser

Feed supply system •  Feed tank(s) •  Feed pump/supply pump •  Concentrate heater •  Filter •  Homogenizer/high pressure pump •  Feed line

Atomizing device •  Rotary atomizer •  Pressure nozzle atomizer •  Two-fluid nozzle atomizer

Powder/fines recovery system •  Cyclone •  Bag filter •  Wet scrubber •  Combinations of the above

Fines return system Powder after-treatment system •  Pneumatic transport and cooling •  Fluid bed after dryer/cooler •  Lecithin treatment System •  Powder sieve

Final product conveying, storage and bagging off system instrumentation

In the following, the main components of a spray dryer are discussed in details.

51

Feed line Fines return

Drying chamber

Feed tank Concentrate preheater

Filter Supply pump

52

HHP pump

Powder after treatment system

Fines recovery sysem

Hot air supply

Atomizing device

4. Components of a spray drying installation

Product conveying and storage

Inert gas treatment

Start up silo

Bagging off

Fig. 4.1. M  ain components of a modern spray dryer, powder handling and storage © GEA Niro

53

4.1. Drying chamber The shape of the drying chamber, the location of the air disperser, atomizing device, exhaust air outlet, powder discharge and after treatment system determine the air flow pattern, product flow, product structure and quality. Various drying chamber types are applied for drying milk and have the following characteristics (Fig. 4.2.).

Fig. 4.2. Types of spray drying chamber © GEA Niro 54

4. Components of a spray drying installation

A) Chamber shape

a) wide body b) tall form c) horizontal box type

B) Product flow:

a) leaving the drying chamber with the exhaust air b) partially separated from the exhaust air in drying chamber

C) Product discharge:

a) by gravity (conical bottom) b) mechanically (flat bottom)

D) Air flow pattern:

a) rotary downward b) straight downward c) straight horizontal

E) Air/spray mixing:

a) concurrent b) counter-current

F) Powder after-treatment:

a) none b) pneumatic transport system c) external fluid bed d) integrated fluid bed e) integrated belt

Referring to the air flow conventional drying chambers are distinguished by their vertical or horizontal design. The vertical chamber is formed by a vertical cylinder of wide body or tall form shape. The ceiling of a cylindrical tower is usually flat. Recently, however, to comply with the safety requirements on mechanical strength in connection with pressure shock resistance, a conical shaped ceiling, convex or concave, is becoming more and more common. Below the cylindrical part is a cone section of 40 to 60° angle, enabling powder discharge by gravity or a flat bottom (possibly also slightly concave or convex conical), requiring a mechanical device to bring the powder to the discharge opening, placed in the centre. The horizontal chambers are often referred to as box dryers, which is very well descriptive for their shape. The bottom of a box dryer is either flat or trough-formed requiring a mechanical device, a scraper, or a screw conveyor for removing the powder. Drying chambers are equipped with service doors, inspection windows, light sources, air sweep doors, wall sweep ports, hammers, overpressure vents and fire extinguishing water nozzles. The drying chamber is usually insulated with 80-100 mm mineral wool to reduce radiation loss, clad with stainless steel, plastic coated steel or aluminium plates. Today overpressure vents, fire extinguishing and overpressure suppressing equipment, complying with national or international standards are required by the authorities in practically all countries, whereas all unnecessary components such as inspection windows, built-in illumination sources and even service doors etc. and, generally all components affecting the smoothness of the chamber inner wall and creating dead pockets where accumulation of powder or washing water are gradually being eliminated for hygienic reasons. From the very same reason even mineral wool chamber wall insulation is now considered undesirable being a potential danger area for bacterial infection, since cracks in chamber walls eventually occur over the years of operation. A disadvantage of an insulation-free chamber is high heat loss, resulting in about 10% evaporation capacity loss and high temperature in the 55

drying room. Removable air insulation panels are now being introduced and are already being used successfully (see Photo Fig. 4.3.).

Fig. 4.3. Removable insulation panels © GEA Niro

The duct for the exhaust air in the old chamber types is a continuation of the conical base, and in this case the exhaust air carries also all the powder out of the chamber. However, in most modern dryers, the powder is separated as much as possible from the exhaust air already in the chamber. To achieve this, the exhaust air is drawn either from the upper part of the cylinder periphery, sometimes through the ceiling, or by a duct projecting into the cone with a slight downward slope. The tall form dryers are often equipped with an enlarged conical section (bustle) from which the exhaust air is withdrawn. The development of the spray drying technology achieved within the last four decades established the advantages of the two stage drying method. The principle of two stage drying (discussed in chapter 3.4.) requires discharging of moist powder from the chamber. Therefore, modern dryers are based on chamber types enabling powder discharge without mechanical means and partial separation from the exhaust air.

56

4. Components of a spray drying installation

The most up-to-date drying installations i.e. drying chambers with integrated fluid beds or belts accommodate the second drying stage inside the spray drying chamber by means of a static fluid bed such as the GEA Niro MSDTM, or a conveying belt assembly located at the chamber bottom such as the GEA Niro FILTERMAT TM. One of the most important factors when designing a spray drying chamber is that no ducts, air-brooms or the like are placed inside the chamber thus obstructing the air flow as that will give reasons of powder deposits with frequent cleaning and/or burned deposits as a result.

4.2. Hot air supply system In a spray dryer, the hot air supply system consists of •  Air Supply Fan •  Air Filters •  Air Heater •  Air Distribution.

4.2.1. Air supply fan The process air for the main drying chamber is supplied by a centrifugal air supply fan direct driven by a motor the speed of which is controlled by a frequency converter. This way there will be no energy loss due to belt transmission or dampers controlling the air flow.

4.2.2. Air filters The drying air is usually supplied into the system from outside the building. It is normally pre-filtered by a coarse filter. However, it must be realized that dust laden air can cause faster fouling of the filters, and contribute possibly to bacteriological problems or even fire hazard if the filters become too dirty or if they are damaged. When using atmospheric air, the intake filters should be placed on the windward side of the factory, reasonably high above the terrain and far enough from known sources of dust (busy roads, chimneys, exhaust stacks from other dryers etc.). Until a few years ago no special requirements were given as to the filtration of the process air for the spray drying process. Today, however, very strict requirements are presented by local authorities in order to ensure a cleaner operation. Filter standards are referred to below, and it is important to refer to the test method when specifying the filter efficiency in %. Common for the different requirements is that: •  The air should be pre-filtered and supplied by a separate fan to the fan/filter/heater room. This room must be under pressure to avoid unfiltered air to enter. •  Filtration degree and filter position depend on the final temperature of the process air as follows: - For main drying air to be heated above 120°C, only coarse filtration up to 90% is needed. The filter should be placed on the pressure side of the fan. - For secondary air to be heated below 120°C or not heated at all, the filtration must be 90-95%, and the filter must be placed after the heater/cooler. Some countries and customers have even stricter requirements demanding a filtration of up to 99.995%, corresponding to EU13/14 (or H13/14). 57

Current practice is as follows: Dairy-like products, equal to or better than 3A: -Pre-filtration EU4 (or G4) -Main air filtration EU7 (or F7 -Secondary air filtration EU7 (or F7) Baby food products, equal to or better than IDF: -Pre-filtration EU6 (or F6) -Main air filtration EU7 (or F7) -Secondary air filtration EU9 (or F9)

Test method: 35% Dust-spot efficiency 90% Dust-spot efficiency 90% Dust-spot efficiency

70% Dust-spot efficiency 90% Dust-spot efficiency >95% Dust-spot efficiency

4.2.3. Air heater The drying air can be heated in different ways: •  Indirect: Steam / Oil / Gas / Hot oil •  Direct: Gas / Electricity

4.2.3.1. Indirect heater A steam heater is a simple radiator. The temperature to be obtained depends on the steam pressure available. Under normal conditions it is possible to obtain air temperatures 10°C lower than the corresponding saturation enthalpy of the steam. Modern steam heaters are divided in sections, so that the cold air first meets the condensate section, then a section with low steam pressure (which is usually the biggest one in order to utilize as much low-pressure steam as possible), and then the air finally enters the highpressure steam section. The air heater consists of rows of finned tubes housed in an insulated metal case. The heat load is calculated from the quantity and specific heat of the air. The heater size depends upon the heat transfer properties of the tubes and fins and is usually about 50 Kcal/°C / h / m3 for an air velocity of 5 m/sec. Steam-heated air heaters will usually have an efficiency of 98-99%. As the steam boiler is usually placed at some distance from the air heater, 2-3 bar g extra pressure on the boiler should be anticipated due to pressure loss in the steam pipe and over the regulating valve. To avoid corrosion of the tubes in the air heater it is recommended to use stainless steel. In indirect oil and gas heaters drying air and combustion gases have separate flow passage. The combustion gasses pass through galvanized tubes that act as heat transfer surface for the drying air. The combustion chamber is made of heat-resistant steel. The end cover of the heater should be removable for cleaning of tubes. Heaters of this type will in the range of 175250°C have an efficiency of about 85%. See Fig. 4.4.

58

4. Components of a spray drying installation

Fig. 4.4. Indirect oil-fired air heater © GEA Niro

Hot oil liquid phase air heaters are used both alone, or when high inlet drying air temperatures are required, and the steam pressure is not high enough. The heater system consists of a heater, which can be gas- or oil-fired, and an air heat exchanger. Between these two components a special food-grade oil or heat transfer fluid, which does not crack at high temperatures, is circulated at high speed. The main advantage of hot oil liquid phase is the open pressure-less system.

4.2.3.2. Direct heater Direct gas heaters are only used when the combustion gas can be allowed to come into contact with the product. They are therefore not common in the food and dairy industries. The direct gas heater is cheap, it has a high efficiency, and the obtainable temperature can be as high as 1650°C. When a plant is designed with an air heater with direct combustion, it is necessary to calculate the amount of vapour resulting from the combustion (44 mg/kg dry air/°C), as this will increase the humidity in the drying air. The outlet temperature has therefore to be increased in order to compensate for this increase in the humidity and to maintain the relative humidity. Combustion of natural gas (methane) takes place according to the following stoichiometric reaction formula: CH4 + 2 O2  2 H2O + CO2 + Heat The oxygen for the combustion originates from the atmospheric air with 21% O2 and 79% N2. All combustion yields small quantities of oxides of nitrogen as a result of the reaction of nitrogen and oxygen at elevated temperatures. Subsequent nitrogen oxide NO and nitrogen dioxide NO2 formation occurs and is referred to as the sum (NOx) of the two. It should be noted that high combustion temperatures, high heat transfer rates, high excess air, and low residence time in the combustion chamber are all factors increasing the formation of NO x.

59

For comparison the following approximate NOx concentration prevails:

Cigarette smoke: Exhaust gas from a car: Heavy traffic intersection: Natural gas boiler stack: WHO food limits for infants: Spray drying chamber: Normal fresh milk: Normal water supply:

4000 2000 900 75 45 2-5 <1 0.1

ppm ppm ppm ppm ppm ppm ppm ppm

The level of NOx in the process air after the direct fired natural gas air heater will depend on many variable factors; however, with a well-adjusted air heater it should be limited to the above. Only about 2% of the NOx formed will be absorbed in the milk powder. The level of NOx in milk powder depends not only on the method used for heating the process air, but also on the type of food used for the cows, as well as on the type of fertilizer and soil used for producing the food. The NOx level in milk powder is: Indirect heating: Direct heating:

Traces - 2 ppm 1 - 3.5 ppm

The level of nitrates (NO3) is in the order of 5-10 times the level of nitrites (NO2). Electric air heaters are common on laboratory and pilot plant spray dryers. The heater has low investment costs, but is expensive in operation and therefore not used in industrial size plants.

4.2.4. Air dispersers A good mixing of the hot drying air with the spray of droplets and the control of the air and particle flow are essential for the whole process and has a decisive influence on the endproduct quality and trouble free operation. This is ensured by the air disperser which is a vital part of the whole system. There are two basic types of air dispersers: a) Air disperser creating rotary air flow see Fig 4.5. and used in vertical wide body chambers. This type of air disperser operates usually with a rotary atomizer but is also suitable for pressure nozzles.

60

4. Components of a spray drying installation Fig. 4.5.

Fig. 4.6.

Fig. 4.5. Air disperser for rotary air flow © GEA Niro

Fig. 4.6. Air disperser for straight downward air flow © GEA Niro

b) Air disperser creating straight air flow see Fig 4.6. and used exclusively with pressure nozzles for vertical downward air flow (for instance in the tall form dryer, multi stage dryer and the integrated filter dryer). Depending on the type of dryer one chamber can accommodate either just one or several air dispersers of this type (arranged symmetrically in the ceiling). The common goal is to have an air distribution and nozzle assembly that minimizes powder deposits in the drying chamber, and that the nozzles are interchangeable during the production to allow for continuous operation for weeks without stop. To secure a straight downward air flow, this type of air disperser is typically equipped with a number of perforated plates through which the long nozzle lances protrude. This results in a high pressure drop of the drying air (high energy consumption) and a difficult nozzle adjustment to obtain an optional agglomeration. c) Today a new type of air disperser has therefore been developed. See Fig. 4.7. It operates still with a straight downward airflow but without perforated plates i.e. the pressure loss is low. It is even possible to obtain a rotation of the drying air to utilize the drying air best possible. The nozzle lances are short and operator friendly, and it is easy to adjust the nozzle position - also during operation - to obtain the degree and type of agglomeration as wanted.  

  



The nozzle lances can of course also be changed during operation, so the plant can operate continuously for weeks so only the feed system needs to be cleaned every 20 hours, all depending on product composition.

4.3. Feed supply system


  



The duty of the feed supply system is to deliver feed to the spray dryer via the atomizing device. It is actually a link between the evaporator and the spray dryer, and must compensate also for the capacity fluctuations of both units. The components of the feed supply system, shown on Fig. 4.1. are:

Fig. 4.7. Air disperser DDD for downward and rotating air flow © GEA Niro 61

4.3.1. Feed tank Feed tanks act as a buffer compensating capacity variations. Usually, two feed tanks are installed to enable change-over from one tank to the other after several hours’ operation for intermediate washing of the emptied tank. This is important for bacteriological reasons as the temperature of 45-50°C with which the concentrate leaves the evaporator is ideal for the growth of thermophile bacteria. The size of a feed tank must be in relation to the plant capacity corresponding to about 5-10 minutes operation. In the case of an emergency, i.e. if the dryer has to be stopped suddenly, the surplus of the concentrate has to be transferred into a so-called escape tank, which is a part of the evaporator. Closed feed tanks with inspection covers, maxi­ mum and minimum level transmitters, water connection(s) and CIPnozzles are today common requirements.

4.3.2. Feed pump Supplies the concentrate to the atomizing device and therefore the type of pump depends on atomizer type. For low pressure systems, as in case of wheel atomizers, almost any type of pump can be used, however preferably a positive pump as e.g. a mono-pump. For high pressure nozzles, a high pressure pump has to be used. If the pump has to process whole milk or other fat Feed tanks © GEA Niro containing products, it can be combined with a homogenizer, preferably two stages. In such cases, it must be designed for a total pressure corresponding to the required homogenizing pressure over both stages plus atomizing pressure, including a safety factor. The homogenizer can be used as a feed pump also for wheel atomizer. It is then advantageous to install a second feed pump for non-fat products e.g. a mono-pump. A nonpositive displacement pump delivering the feed to the inlet side of the high-pressure pump with a pressure of a few bars has in any case to be incorporated into the system. The feed pump is in fact a dosing pump, supplying the required amount of concentrate to the dryer. The amount is controlled by the outlet air temperature of the dryer by means of variable speed drive. For low pressure positive pumps and for high pressure pump frequency converters are used to control the feed flow to the atomizer.

62

4. Components of a spray drying installation

4.3.3. Concentrate heater Is essential for high pressure nozzle operation and is highly recommended also for other systems. From various heat exchanger types available, a plate heat exchanger is less suitable due to fouling. This gives a limited time for continuous operation with liquids having properties and viscosities as normal milk concentrates. The most common concentrate heater is today a counter current corrugated tubular “tube in tube” system, where warm water is used as heating medium. With a maximum temperature difference of 5oC between the product and water, this type of pre-heater can operate 20 hours at 80 oC product temperature. A filter is placed after the preheater to protect against mechanical impurities and possibly also some lumps created in the evaporator or heater. Usually a twin filter arrangement is used with changes over at regular intervals. The concentrate heater and filter is usually placed prior to the high pressure/homogenizer feed pump.

Concentrate heater type TCM © GEA Niro

However, spiral tube heaters can be made also in a high pressure execution and be placed in the feed line close to the nozzles. It has the advantage that the residence time of the concentrate after it has been heated is short thus preventing increase of viscosity or agethickening. The pressure drop over a spiral heater is often 20-50 bars, which has to be taken into account for specifying the pump. A low cost method of heating concentrate is direct steam injection. The injector is installed on the feed line with low pressure systems or prior to high pressure pump with pressure nozzles. Food grade steam should be used as the heating medium. The condensing steam dilutes somewhat the concentrate. However it does not cause any reduction of capacity. The diluting medium is supplied into the system as steam (i.e. inclusive the heat of evaporation) and thus it does not consume any heat from the drying air. On the contrary there is some capacity increase due to the reduction of viscosity of the concentrate allowing a reduction of the outlet temperature while retaining the same powder moisture content without heating.

63

4.3.4. Filter An in-line double filter is always incorporated after the pre-heating system to avoid lumps etc. to pass through to the atomizing system.

4.3.5. Homogenizer/High-pressure pump If whole milk powder or other fat-containing products should be produced, it is recommended to incorporate a homogenizer in order to reduce the free fat content in the final powder. A two-stage homogenizer is preferred. First stage is operated at 70-100 bar g, and the second stage at 25-50 bar g, usually the homogenizer and feed pump are combined in one unit. If nozzle atomization is used then a higher pressure (up to 250 bar g for the nozzles + 150 bar g for homogenizing) is required, and a combined homogenizer/high-pressure pump is chosen to save cost. A variable speed drive for controlling the feed flow and thereby the outlet temperature is preferred, as a return valve tends to give uncontrollable holding time resulting in viscosity problems.

Homogenizer/High pressure pump © GEA Niro Soavi

4.3.6. Feed line The feed line should naturally be of stainless steel and of course of the high-pressure type, if the atomization is to be carried out by means of nozzles. The dimension should be so that the feed velocity is approx. 1.5 m/sec. In a feed system a return pipe and a device to clean the rotary atomizer, incl. the wheel, as well as the nozzle lances, should also be included for the cleaning solution, so that the entire equipment can be thoroughly cleaned.

4.4. Atomizing device The purpose of the atomizing device is to transform the feed into a large number of droplets of well-defined size distribution. The atomization increases tremendously the surface area of the milk concentrate which is then exposed to the hot drying air. The rate of evaporation is then directly proportional to the surface area and thus a fine atomization has positive influence on 64

4. Components of a spray drying installation

many properties. The effect of droplet size on the number of droplets and their total surface area when atomizing one litre of concentrate, supposing totally homogeneous sprays is shown in Table 4.1. Table 4.1. Characteristics of homogenous sprays © GEA Niro

The spray of droplets is characterized by a mean droplet size and droplet size distribution. Both depend on the type of atomizing device, operating conditions and the properties of the atomized liquid (concentrate viscosity, surface tension and density). It is common for all types of atomizers that increasing the amount of energy available for atomization results in smaller droplet size. With the same device and same amount of energy, the viscosity of the atomized liquid appears to influence the mean droplet size by 0.2 direct power relationship whereas surface tension has much less significant influence. The particle size distribution of the dried product is to a great extent influenced by the droplet size distribution of the spray, but not necessarily directly proportional. There are many additional factors influencing particle size distribution of the powder. Incorporation of air during the formation of droplets and their expansion or deflation during drying has greater influence on large droplets than on smaller ones. The amount of feed introduced to a single atomizing device or location of atomizing devices close to each other, increases the particle size of the powder through collisions of droplets with primary particles resulting in agglomerates. Such non-intentional agglomeration which takes place in a spray cloud without forced introduction of already dry powder is called primary agglomeration. The selection of the atomization device for a given duty depends mainly on the desired characteristics of the final product. There are three types of atomizers as described below. A photo of a rotary wheel atomizer is shown on Fig. 4.8.

4.4.1. Rotary wheel atomizer From the design point of view, the wheel atomizer consists of circular horizontal top and bottom plates with radial vanes or bushings between them. The feed enters close to the centre, accelerates across the wheel surface (vanes) and achieves the wheel’s peripheral speed. In a wheel atomizer centrifugal energy is utilized for atomization. A thin film of liquid is formed, as the liquid moves across the vanes or through the bushings, and this film readily disintegrates into droplets when thrown off the wheel edge. Wheel atomizers applied for milk drying operate with peripheral speeds in the range 100-200 m/s, however mostly in the higher speed range. Some types of wheel are shown on Fig.4.9.

Fig. 4.8. Rotary atomizer © GEA Niro 65

Fig. 4.9. Some types of atomizing wheels: a) with straight vanes, b) with curved vanes, c) with bushings © GEA Niro

The atomization effect depends mainly on the peripheral speed. However it was found, that at a given peripheral speed, a wheel of smaller diameter produces a finer spray than a larger wheel. Further influencing factors include the liquid loading and the number, height and design of vanes. Efforts have been made to predict the mean droplet diameter by mathematical expressions. However, it is still difficult to do it with reasonably confidence and universal validity. The various factors and their influence on mean droplet size, reported by Masters [2], are shown in Table 4.2. Tabel. 4.2. The Influence of various factors on mean particle size.

The wheel atomizer has important advantages in comparison with other atomizing devices. Under practical circumstances, i.e. when considering a given installation, the wheel can operate with all types of dairy products and handle a wide range of capacities. Droplet size is insensitive to the feed rate fluctuations.

66

4. Components of a spray drying installation

In comparison with pressure nozzles (see 4.4.2.) the wheel is also insensitive to fluctuations in concentration (resulting in a change of viscosity and of feed rate) and it is able to operate with higher feed viscosities, i.e. also concentrations. Furthermore, as it operates with low pressure feed system, there is a minimum risk of blocking, and it is therefore more suitable to handle feeds containing dispersed, possibly abrasive solids (for instance lactose crystals). Finally, much higher capacities can be handled in a single atomizing device. Generally, wheel atomizers offer a more flexible operation. One disadvantage of wheel atomizer is that it works also as a fan pumping air through the vanes during wheel rotation. Due to this, an amount of air is incorporated into the droplets as bubbles, and these result in occluded air in the final dried particles. This effect depends upon the properties of the feed. It is stronger with liquids forming stable foams, i.e. products of high protein content. The presence of air in the dried particles increases the powder volume. This is undesirable if high bulk density powder is required. A special wheel developed for milk products has curve formed vanes accomplishing the removal of air bubbles from the milk film by centrifugal force as it flows over the vanes. Powders produced by this wheel, (see Fig. 4.10.) other conditions being the same, have lower contents of occluded air and higher bulk densities than powders from radial vane wheels. More about this phenomenon is discussed in chapter 10. Bulk Density.

Fig. 4.10. Rotary atomizer wheel with curved vanes © GEA Niro

Fig. 4.11. Screw distributor © GEA Niro

An important part of the rotary atomizer is the liquid distributor, which ensures a uniform distribution of the feed into the vanes in the wheel. If the feed distribution is not uniform it will result in a non-optimal drying process with deposits in one side of the drying chamber and excessive hot air in the opposite side. Also heavy vibrations on the spindle and wheel with bend and/ or cracked spindles are the result of a non-optimal liquid distribution. The liquid distributor is positioned above the atomizer wheel. The feed is led through the atomizer via the feed pipe down into the distributor.

Fig. 4.12. VOLUTE™ distributor © GEA Niro 67

Several types of liquid distributors have been used through the time. First the hole-distributor was used. The hole-distributor consists of a ring with holes close to each other, located just above the wheel and placed around the drive shaft on which the wheel is mounted. To ensure good distribution a slight positive pressure (to compensate for the suction effect of the wheel and hydrostatic pressure of the liquid) must be kept in the feed line. This is achieved by adjusting the total surface area of the holes to the liquid flow rate. The flow velocity through the holes should be about 5 m/s. Then the screw liquid distributor, see Fig. 4.11., was introduced in the late 70’s. It operates in fact on the same principle as the grooved core insert in a pressure nozzle (see Fig. 4.13.A), where the feed is brought into rotation to form a conically shaped film. In the 90´s the volute distributor, see Fig. 4.12, became the standard distributor due to deposit and blocking problems in the screw distributor. However the volute distributor has turned out to be less functional on sticky and viscous liquids and it has led to a construction of a combined “volute-hole” distributor. Today a new rotary atomizer may be supplied with more than one type of liquid distributor, each designed to the product to be dried. Wheels have also been developed which can simultaneously atomize two liquids. A special liquid distributor ensures the feeding of two different feed flows separately into the two-tier wheel.

4.4.2. Pressure nozzle atomizer In pressure nozzles, the pressure energy applied by the high pressure pump to the liquid is converted into kinetic energy of thin liquid films, which are at the same time brought into rotary motion. The liquid film has the shape of a hollow cone. The film thickness decreases with the distance from the nozzle orifice. The disintegration of the film into droplets depends on the physical properties of the liquid and is assisted by the frictional effects of the surrounding air.

Fig. 4.13. Types of pressure nozzles: a) with grooved core, b) with swirl chamber © GEA Niro 68

4. Components of a spray drying installation

Pressure nozzles used in the dairy industry are of two types. See Fig. 4.13.: - Pressure nozzle with grooved core insert, - Pressure nozzle with swirl chamber. Both types operate on the same principle and are often called centrifugal pressure nozzles. It is the grooved core and the swirl chamber which brings the liquid into rotary motion. For assembling a nozzle for a given duty, there are a number of grooved cores, swirl chambers and orifices available. The combinations of grooved core or swirl chamber with orifices (nozzle internals), define the two main characteristics of that combination, i.e. the nozzle capacity and the spraying angle at given pressure. The liquid flow rate is directly proportional to the square root of the pressure. Increase of the pressure increases the capacity but decreases slightly the spraying angle. The influence of the liquid physical properties on the droplet size is similar to that in wheel atomization. Increase of viscosity decreases also the spraying angle. The droplet size depends mainly on the atomization pressure and spraying angle. The higher the pressure and the wider the angle, the smaller is the mean droplet size. However the effect of spraying angle on the droplet size decreases with increasing pressure. Due to the influence of the operational variables on pressure nozzles performance it is essential to choose carefully the combination of grooved core/swirl chamber and orifice and to keep the operational variables as constant as possible. Nozzles for milk drying operate usually in the range of 150-250 bar (80-500 bar in exceptional cases). The correct choice of nozzle internals is normally based on information supplied by the nozzle manufacturer. The following equation applies:

K=

Q  ρf ∗ F ∗ n ∗ P

[4,1]

where: Q  = the flow rate in kg/h f= the density of the feed in g/ml F = a factor based on the feed properties, 0.8-1.0, for milk close to 0.9 n = number of nozzles installed P = the pressure in bar K = capacity constant (the flow rate of water through one nozzle at 1 bar pressure) The most important advantage of the pressure nozzle in comparison with wheel atomizers is the formation of almost air-free droplets thus achieving powders of low content of occluded air and of high bulk densities. Another advantage is good flowability of the final powder. Furthermore it is easy to direct the spray cloud in any direction. This means that in multinozzle dryers, one can combine the individual nozzles close to or apart from each other and let them spray towards or away from each other depending on whether primary agglomeration is required or should be avoided. The disadvantages have been more or less expressed by the advantages of wheel atomizers as discussed in the previous section.

69

4.4.3. Two-fluid nozzle atomizer In this type of atomizer, also called pneumatic nozzle the feed is atomized by high velocity air. There are two types, i.e. with internal or external mixing. See Fig. 4.14. However it is seldom used in the dairy industry. Otherwise, two-fluid nozzles with internal mixing are used only in small pilot dryers and for special duties, where small amounts of liquid must be atomized into very fine sprays (for instance spraying lecithin/oil mixture on powder to produce an instant whole milk powder).

Fig. 4.14. Two-fluid nozzles a) with internal mixing, b) with external mixing © GEA Niro

4.5. Powder recovery system The exhaust air from some old types of single stage spray dryers carries the total powder production to the cyclones. However in more modern systems partial separation of the powder from the drying air takes place already in the drying chamber so that the exhaust air contains much less powder. The actual amount depends on the product; spray dryer type and operating conditions. Factors contributing to low powder carry-over to cyclones are high feed total solids content, high fat content, high moisture content of the powder leaving the chamber, degree of agglomeration and low protein content. Under such conditions carry-over is usually less than 10 %. On the other hand the carry-over can be more than 50% especially for high-protein low-lactose products dried from low total solids content feeds. The fraction of the powder carried over by the exhaust air is referred to as fines, as it consists of the smallest particles. Generally, a milk spray dryer utilizes - depending on operating conditions - 15-30 Nm3 of drying air per kg dry product. The air leaving the chamber contains - depending on product type - higher (protein rich powders) or lower (high-fat powders) amounts of powder. Before the drying air is discharged to atmosphere, the powder must be separated from the exhaust air as completely as possible, because even a small amount of powder will represent a noticeable economical loss and cause environmental problems. In EU and most other countries, the general requirement for maximum powder emission is 10 mg/Nm3. 70

4. Components of a spray drying installation

There are various types and constructions of powder separation equipment, but due to hygienic and safety requirements, only three types are acceptable for milk powder production, i.e.: Cyclone separator Bag filter Wet scrubber Combinations of the above.

4.5.1. Cyclone separator The cyclone has some obvious advantages if it is constructed properly, it is easily maintained as there are no moving parts, and, furthermore, it is easy to clean, if it is a fully welded construction. But it does not live up to today’s strict emission requirements as it - depending on the product and operation of the dryer - may reach 250 - 400 mg/Nm3 The operation theory is based on a vortex motion where the centrifugal force is acting on each particle and therefore causes the particle to move away from the cyclone axis towards the inner cyclone wall. However, the movement in the radial direction is the result of two opposing forces where the centrifugal force acts to move the particle to the wall, while the drag force of the air acts to carry the particles into the axis. As the centrifugal force is predominant, a separation takes place. Powder and air pass tangentially into the cyclone at equal velocities. Powder and air swirl in a spiral form down to the base of the cyclone separating the powder out to the cyclone wall. Powder leaves the bottom of the cyclone via a locking device. The clean air spirals upwards along the centre axis of the cyclone and passes out at the top. See Fig. 4.15. The centrifugal force each particle is exposed to can be seen in this equation:

Fig. 4.15. Cyclone separator © GEA Niro

C=



m x Vt r

2



[4,2]

Where: C = centrifugal force m = mass of particle

Vt = tangential air velocity r = radial distance to the wall from any given point

71

From this equation it can be concluded that the higher particle mass, the better efficiency. The shorter way the particle has to travel the better efficiency, and the closer the particle is to the wall the better efficiency, because the velocity is highest and the radial distance is short. However, time is required for the particles to travel to the cyclone wall, so a sufficient air residence time should be taken into consideration when designing a cyclone. From above equation it is evident that small cyclones (diameter less than 1 m) will have the highest efficiency, a fact generally accepted. However, as the cyclone(s) alone - irregardless of the diameter - cannot clean the exhaust air sufficiently, the spray dryers - especially in the Baby-Food industry - today are equipped with cyclones of up to 4 m diameter followed by a bag filter (a “police filter”) to fulfil requirements from the authorities. Each cyclone has an outlet for powder in form of a rotary valve see Fig. 4.16., or directly into a “blow-through” valve connected to a fines return system.

Fig. 4.16. A rotary valve with conical rotor © GEA Niro

4.5.2. Bag filter Since 2007 the EU has required “best available technology” to minimize powder loss from spray drying plants being 10 - 15 mg/Nm3 in the exhaust air. Final cleaning of the air is therefore necessary if cyclones are used as the primary separators. Bag filters consist of numerous bags or filters arranged in a filter house so that each bag receives equal quantities of air. The dust loaded air should enter the filter house tangentially to minimize wear and tear of the bags. The direction of the air to the filter bags is from outside in through the filter material to the inner part of the bag from where the cleaned air enters a clean air plenum, from where it is led to the atmosphere via an exhaust fan. With a correct selection of filter material high efficiencies can be obtained and collection of 1 micron particles is reported from the manufacturers. The collected powder is automatically pulsed off by blowing compressed air into each bag at predecided intervals depending on the product. This is done via a specially designed reverse air 72

4. Components of a spray drying installation

nozzle positioned above each bag. The powder is collected at the bottom via a rotary valve. If the bag filter is placed after the cyclone, the amount of collected powder is minimal, and it consists of very fine powder particles that flow only with difficulty. The powder may therefore stick to the conical part of the filter house and become discoloured due to the exposure to high temperatures. The filter fraction is therefore considered as a second class powder as it may also have high mould content. Clean air Compressed air Powder loaded air

CIP-able bag filter SANICIP™ © GEA Niro

Fig. 4.17. CIP-able bag filter SANICIP™ © GEA Niro

The bag filter may also replace the cyclones, a solution often chosen for one-stage dryers for whey protein powder or egg white. To prevent condensation, especially on the conical part of the filter housing, warm air or heat tracing is established. When a bag filter is installed after a cyclone, the total pressure loss over the exhaust system – including air ducts – will be 300 to 400 mm WG, equal to a high energy consumption. Designers of spray dryers – including GEA Niro – have therefore developed bag filters designed for CIP. The GEA Niro SANICIP™ CIP-able bag filter is replacing the cyclones and is of the reverse jet type. It consists of cylindrical bag housing with top air inlet, clean air plenum on top, and a conical bottom with fluidized powder discharge. During operation the product collected on the outside of the filter material is removed by a compressed air jet stream blown into each bag via a special reverse jet air nozzle positioned above each bag, see Fig.4.17. A jet is formed which draws air from the clean air plenum into the bag as well, thereby saving compressed air. This is an efficient and sanitary solution. The reverse air jet nozzle has furthermore a dualpurpose during CIP as described below.

73

The bags are clean-blown individually or 4 together, resulting in a very even discharge of powder and using higher air-to-cloth ratios. The frequency and duration of the cleaning sequence can be adjusted to suit actual running conditions. A supply air system for the fluidizing bottom and heating of the cone secures that the fine powder is easily discharged from the filter and that the conical section is kept warm to avoid powder deposits. During standstill the cone heating system is used for heating of the cone alone. Condensation and risk of mould growth is therefore avoided. The filter bags are made from an FDA approved polyester material. This material is fully CIPable with NaOH and HNO3 in 1 to 2% solutions at 75°C and 60°C, respectively. It is heattreated to give a special dust-releasing surface. Each bag is supported on a stainless steel cage and is easy to dismantle. The CIP of the bag filter is divided into the following main sequences: 1. The internal bag CIP cleans the bag from the inside towards the dirty side (outside). CIP liquid running on the inside of the bags is forced out through the filter material by the compressed air pulse by the reversed jet nozzle. Powder that has penetrated into the bag material is thus forced out towards the dirty side. 2. The clean air plenum CIP cleans the clean air plenum of the bag filter above the whole plate. No recirculation of CIP liquid in this area. 3. The hole plate CIP cleans the bottom side of the hole plate and the snap ring area of the bag using a specially designed nozzle. This nozzle is positioned on the bottom part of the hole plate between the bags, and it also cleans the outside of the filter bags. The nozzle has a dual purpose as well, as it during the drying process is purged with warm air to keep the hole plate free of deposits. Discolouring/denaturation are thereby minimized. The CIP liquid is recirculated. 4. The shell CIP is performed by means of standard retractable CIP nozzles. The CIP water is recirculated.

Advantages of the SANICIP™ filter: •  Low pressure loss across the bag filter and thus the entire exhaust system i.e. reduced energy consumption and noise emission •  Designed for optimum air-to-cloth ratio and powder load (due to one to four bag(s) being blown at the time) •  Better utilization of raw materials due to no second grade products •  Design with 4 or 6m bags to suit specific building requirements.

74

4. Components of a spray drying installation

4.5.3. Wet scrubber In a wet scrubber powder particles are collected by a washing liquid which is sprayed into the powder-laden air. It is essential to bring the laden air into intimate contact with the spray. This is carried out in a venturi type tube (Fig. 4.18.). The washing liquid is separated from the clean air in a cyclonic type separator. Wet scrubbers can operate with liquid milk or whey as washing liquid whereby the evaporator feed, before entering the evaporator is used as washing medium in a single pass operation. The advantage of this system is complete recovery of the powder into product and some pre-concentration of the milk. Due to evaporation, both the temperature of the washing liquid and of the air closely approaches the wet bulb temperature. This is at usual milk drying conditions about 40-45°C which is a temperature range favourable for bacterial growth. To minimize such growth the whole unit is designed for minimum liquid holding volume to ensure a sharp border-line between the dry and wet zones. Good bacteriological quality of incoming milk is of utmost importance for long continuous operation without jeopardizing the bacterial quality of the final powder. Besides, it might be necessary to apply an Scrubbling liquid intermediate CIP of the scrubber in frequent intervals, about 8 hours. Another problem encountered may be excessive Fig. 4.18. Wet scrubber foaming. For all these reasons wet scrubbers, using © GEA Niro evaporator feed as washing liquid, are usually applied for products for non-human consumption only and exclusively as secondary separators after cyclones. The separation efficiency of a wet scrubber is just as high as a bag filter. Powder loaded air

Clean air

If a wet scrubber is used it is more common to operate it with water in recirculation. Washing water has to be replaced in regular intervals so that the total solids content remains about 10%. Often it is discharged as effluent. Some factories, however, have full control of the bacteriological problems. The washing water is heat treated, cooled down and either spray dried or more often delivered in liquid form for animal feeding. The above obvious drawbacks of the wet scrubber mean that it is practically not used any longer in modern spray dryer installations.

4.5.4. Combinations In the below Table 4.3., a comparison of the different combinations of powder separators is given. Which one to select depends entirely of the product produced and how the collected product – if a bag filter or wet scrubber is used in combination with a cyclone – can be disposed of.

75

Table 4.3. Comparison of powder separation systems © GEA Niro

Cyclone Emission

Cyclone + Bag Cyclone + Wet Filter Scrubber

SANICIP™

20-400 mg/Nm3

5-20 mg/Nm3

max. 20 mg/Nm3

5-20 mg/Nm3

280 mm WG

340 mm WG

340 mm WG

170 mm WG

Auxiliaries

none

compressed air

liquid circulating system

compressed air

Cleaning

suitable for CIP

difficult

suitable for CIP

suitable for CIP

Hygroscopic products

insensitive

sensitive

insensitive

insensitive *)

Use of separated product

first grade

first and second grade

not recommended

first grade

minimal

service of compressed air system and change of bags

minimal

service of compressed air system and change of bags

good

relatively good

less good

good

Pressure loss Exhaust system (incl. ducts etc.)

Maintenance Sanitary conditions

*)watch out for permeate, if the humidity in the outgoing air is too high

4.6. Fines return system The fines collected from dry powder separators - be it cyclones and/or bag filters - have to be collected at one point and returned to the process. This can be done in several ways, all depending on the wanted final powder structure. Most common today is a blow-through valve fit directly to a cyclone(s) and/or the CIP-able bag filter(s) product discharge point. The collected powder is then fed into a blow line system and can now be directed back into the process at any given point. For high fat powders gravity fall tube is used leading the fines directly into a cooling bed. A blow line usually has several branches connected to the main line by flow diversion valves. Fines can be directed: a) into the atomizing zone to form agglomerates, b) into the rear section of a vibrating fluid bed to produce non-agglomerated powders, c) into the end of vibrating fluid bed when emptying the system at the end of the production run.

Fines powder return arrangement © GEA Nu-Con

76

In order to achieve good agglomeration the fines must be brought into the wet atomizing zone to achieve intimate contact with the spray of droplets and fines. There are numerous methods of reintroducing the fines depending on the type of dryer and atomizing device.

4. Components of a spray drying installation

Generally speaking it is easier to bring the fines to a pressure nozzle assembly than to a wheel atomizer. Several methods have been developed and used:

4.6.1. For wheel atomizer a) through the atomizer above the wheel, b) below the wheel (Fig. 4.19.), c) through the air disperser above the wheel (Fig. 4.20.)

Fig. 4.19. Fines return from below  the wheel © GEA Niro

Fig. 4.20. Fines return through the air disperser © GEA Niro

4.6.2. For pressure nozzles

Fig. 4.21. Fines return to pressure nozzles © GEA Niro 77

a) around one nozzle, b) in the centre of three or more nozzles, c) into a rotating air stream between several nozzles. Returning of fines into the atomizer cloud is an important step when producing agglomerated powders. There are, however often vital factors decisive for agglomeration, and these will be discussed in chapter 10. The fines return system can also be used with advantage for introducing additional components (as sugar, cocoa, vitamins etc.) to the dried powder. In such case, a supply silo with dosing equipment (for instance a screw feeder) is connected by a separate blow-through valve to the transport blow line. In the Baby-Food industry - where the powders have high carbohydrate content, the powder agglomerates very easily, and sometimes too much - it has been a common practice to return only a fraction of the fines to the atomizer and the rest to the fluid bed. The degree of agglomeration and also the bulk density can thus be controlled.

4.7. Powder after-treatment system The drying conditions together with the method of the treatment of the product leaving the chamber determine the structure and overall quality of the produced powder. Powder after-treatment method can be either a pneumatic conveying system or fluid bed system. The latter may be further combined with fines return system and lecithin treatment. Pressure Nozzle assembly with fines return © GEA Niro

4.7.1. Pneumatic conveying system Powder is discharged from the spray drying installation usually at two points, i.e. from the bottom of the spray drying chamber and from the cyclone(s) and or bag filter(s). A pneumatic conveying system is the cheapest way to collect the powder at these points, to cool it down and to transport it to a bagging-off point. The air velocity for pneumatic transport is in the order 20 m/s and the air/powder ratio has to be at least 5:1. The powder is finally separated from the transport air in a cyclone (bagging-off cyclone). Combining a drying chamber with a pneumatic conveying system forms the most simple of spray drying installations, is inexpensive in investment and easy to operate. On the other hand it permits production of only so-called regular or ordinary powders (which mean nonagglomerated powders consisting of single particles). Consequently these powders have relatively high bulk density, are dusty, have poor flowability and are difficult to reconstitute in water. The drying process is single stage drying (see chapter 3) requiring relatively high exhaust air temperature to complete the drying. It is therefore not too gentle towards the powder and it has low thermal efficiency. It cannot be used for products with high fat content.

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4. Components of a spray drying installation

4.7.2. Fluid bed system Fluid bed after-treatment can be used just for cooling or for after-drying with subsequent cooling. Fluid beds can be either stationary or integrated at the bottom of the drying chamber or they can be vibrating. Typically a combination of the two is used in modern dryers. A vibrating fluid bed (Fig. 4.22.) consists of an oblong housing which features a perforated horizontal plate that separates the lower air plenum section from the upper powder plenum section. The process air is supplied below the perforated plate and exhausted above. The powder enters the fluid bed at the inlet, moves across the surface of the perforated plate and leaves at the discharge. Simultaneously the air passes through the holes of the perforated plate and upwards through the moving powder layer. Powder is brought into fluidization which is a turbulent movement resembled boiling. Powder moves under plug-flow conditions Fig. 4.22. Vibrating fluid bed VIBRO- coming all the time into contact with air, i.e. FLUIDIZERTM © GEA Niro powder receives successive treatment. In order to achieve the desired drying or cooling effect, a relatively high quantity of air has to be used. The air passes through the holes at the velocity of about 20 m/s. This is necessary to avoid powder particles falling through the holes. However the average air velocity above the plate, called fluidizing velocity must be considerably lower to avoid blowing-off the powder with exhaust. The fluidizing velocity is therefore a very important technological parameter which must be adjusted according to the type of product processed. This velocity is typically 0.1-0.2 m/s for caseinates and similar powders, 0.2-0.3 m/s for skim milk, 0.25-0.4 m/s for whole milk and 0.3-0.5 m/s for high fat milk products and for agglomerated whey products. The degree of perforation of the perforated plate expressed in percent of the total area is between 0.5 to 2 %. This enables the required range of fluidization velocities to be achieved. A photo of a modern vibrating fluid bed is shown on Fig. 4.23. When fluidizing with high fluidizing velocities and high powder layers powders behave as liquids. Such conditions are difficult to obtain with dry milk products. These so-called dead powders are not easy to fluidize and tend to create channelling effects. To overcome this problem, the whole unit must be vibrated or shaken. The vibration helps to achieve good fluidization even below the critical fluidizing velocity and powder layer.

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Fig. 4.23. A modern VIBRO-FLUIDIZERTM © GEA Niro Perforated plates exist in a number of designs allowing horizontal movement. Such movement, resulting in a fast transport of the powder from the inlet to the outlet, is advantageous for emptying of the unit at the end of the operation, but it inhibits the build-up of a powder layer and reduces residence time. Therefore the fluid beds with such a type of plate are provided by an overflow weir at the outlet end. Such an overflow weir secures the desired bed depth and residence time.

Weir regulator in VIBRO-FLUIDIZERTM © GEA Niro

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Various types of perforated plates are shown on Fig. 4.24. The perforated plate can be mounted between flanges. This is very seldom done today, but it has the advantage that different plates can be used for different products. Besides the plate can be taken out for washing outside the spray dryer hall if for sanitary reasons wet cleaning is undesirable. Integrated fluid beds are of the back mix type where the residence time is undefined. To improve on this, GEA Niro´s Multi Stage Dryer, MSD™ is equipped with a perforated plate - with directional air flow - welded together in sections to direct the powder onto the powder outlet. Modern sanitary fluid beds have their perforated plate fully welded. More details on fluid bed operation can be found in chapter 5. Fig. 4.24. Various types of perforated plates © GEA Niro

4.7.3. Lecithin treatment system The background for the philosophy of lecithin treatment of fat containing powders is explained in section 10.4. The lecithin treatment or lecithination refers to the coating of powders by a wetting agent consisting of lecithin dissolved in butter oil or another low melting oil. The concentration of lecithin in fat is between 10 to 60% and the wetting agent is added in such amounts as to achieve lecithin content in the powder of 0.2-0.5%. The wetting agent is sprayed by means of a two fluid nozzle either directly on the powder under vigorous fluidization in a vibrating or static fluid bed or it falls through a funnel feeding the inlet of a fluid bed. The lecithination equipment (Fig. 4.25.) includes:

Fig. 4.25. Lecithination equipment © GEA Niro

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a) b) c) d) e) f) g) h) i)

preparation tank with agitator and heating jacket for dissolving powdered lecithin in fat or oil, transfer pump, which is a centrifugal pump, for pumping the wetting agent from the preparation tank to the supply tank, supply tank with agitator and heating jacket, dosing pump with variable speed control, electric heater for compressed air, two fluid nozzle placed in a powder flow, heat traced pipes for both lecithin and compressed air, special 4-way valve allowing combinations of lecithin and compressed air flow: - both lecithin and compressed air to the nozzle, - lecithin to recirculation to the supply tank and compressed air to both passages of two fluid nozzle. Vegetable oil tank for rinsing the supply pump and pipeline.

For effective lecithination, the temperature of wetting agent must be 50-60°C and compressed air 60-80°C. The spraying angle must be adjusted to 70-90° and the powder kept in vigorous fluidization by warm air also after applying lecithin so as to maintain the final powder temperature at the powder outlet at least 45°C. The lecithination nowadays is provided as an integrated part of the spray drying installation. It can however be an independent unit, consisting of two fluid beds with lecithination spraying conducted in between. The duty of the first fluid bed is to heat the powder prior to lecithination to about 45°C; the second ensures good mixing for uniform distribution of lecithin within the treated powder. Powder is usually fed from a silo by means of dosing equipment. Separate lecithination is advantageous especially when shipping non-lecithinated powder in bulk from the area of production to the point of market distribution. However, for obtaining the best powder quality, filling the lecithinated powder into tins should be linked directly with lecithination process.

4.7.4. Powder sieve The last component of a spray drying installation is usually a sifter which is a shaking or vibrating mesh or a cylindrical static mesh with rotating arms. The aim of sifting is to separate the oversize agglomerates or powder lumps, which can occasionally occur. Sometimes the sifter contains a second smaller mesh for separating out the finest fraction which can then via the fines return system be returned to the process.

4.8. Final product conveying, storage and bagging-off system After the sifting, the powder is either (a) directly bagged-off into paper bags, (b) filled into transport containers (tote-bins, big bags, etc.) or (c) blown into storage silos before final bagging-off or filling into tins or other retail packages. See various photos Fig. 4.26.

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4. Components of a spray drying installation

Top of silos

Powder conveying from dryer

Inlet gas treatment

Bagging off

Bottom of silos

Fig. 4.26. Various photos of powder handling and storage equipment © GEA Colby / GEA Avapac Agglomerated products can also be transported by air, in modern dense phase transport systems to silos - using only a small amount of air and low transport velocity. Vacuum systems are usually used from the silos to the bagging off line combined with pre-gassing of the powder before it is bagged off in 25 kg bags. Residual oxygen of 1% or less in the bag is reported from the suppliers. On modern spray dryer installations a start-up silo may be installed. The very first powder leaving the dryer is often “out of spec.”, and this powder is then led to a separate silo from where it is slowly fed back to the dryer via the fines return system when it is in full balance. There are many kinds of silo storage systems, bagging-off systems and retail packaging systems available ranging from the most simple to fully automated systems requiring little or no manpower.

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4.9. Instrumentation and automation In order to control the drying process and at any time to be able to record the drying parameters the installation should include instrumentation and control equipment. This is done using field instruments and a PLC with monitor, which is placed in a separate control room, (see Fig. 4.27.) partly to keep it dry, but also because the operators here can be in a place with reduced noise level.

Fig. 4.27. Control room © GEA Niro Instrumentation, (see Fig. 4.28.), for a modern spray dryer should include all relevant processing parameters, incl. inlet drying air temperature for the main chamber and fluid beds, as well as outlet air temperature. All temperatures are recorded enabling the operator to see the trend of the temperature development, and also to go back and find the reason why a powder has been downgraded in the laboratory. An hour counter for the atomizer or high-pressure pump is also necessary, as it tells when oil should be changed. A feed pressure gauge should also be included, if the atomization is carried out with nozzles. In order to check the pressure in the chamber which is usually operated under a vacuum of 5-10 mm WG frequency converters on the inlet and outlet air fans should be provided. These can of course be operated manually, but in most cases they are automatically controlled. Automatic start/stop of the plant is therefore possible. The inlet temperature can be automatically controlled by regulating either the steam pressure or the amount of oil or gas to the air heater. The outlet temperature should always be automatically controlled to ensure a powder with constant residual moisture content. If the atomization takes place by means of a rotary atomizer the regulation of the outlet temperature is done by changing the revolution of the feed pump. Another system, which, however, is not very often used - and then only for nozzle atomization - is with a constant supply of feed to the atomizer and then keep the outlet temperature constant by changing the inlet temperature. If the atomization is done by means of nozzles the outlet temperature may be kept constant by changing the revolution of the high-pressure pump. This will naturally have an influence on the nozzle pressure which again will have an influence on the mean particle size and the particle distribution. However, once the right nozzle combination has been found, only marginal changes are seen.

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4. Components of a spray drying installation

S C ADA-C lients

S C ADA-C lients Production network

P rinter

Process network

*

S witch 1

Field instrumentation R emote IO-panels

IO-S erver 2

IO-S erver 1

S witch 2 MCC PLC

PLC

*

M

*

PLC

M

MCC

MCC

MCC

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PLC

M

*

M

• MILK • E VAP OR ATION • S P R AY • P OWDE R HANDLING AND P AC KING DR YING R E C E P TION Fig. 4.28. Instrumentation diagram © GEA Niro A drying installation, however, is not only the spray dryer. There is also the evaporator. As the raw milk solids can vary from tank to tank and fouling (micro-thin deposits in the tubes which will alter the K value) may occur after a certain running time, the evaporating capacity and therefore the amount of concentrate will not be constant. It is of course possible to counteract this by manual regulation on the evaporator or spray dryer, but it is also possible to do the regulation automatically. The most common system is to delete the feed tanks and let the last stage of the evaporator or a special vacuum tank take over this function. Level transmitters are then built into the last effect calandria in the evaporator. The level in the evaporator is now controlled by the feed flow and/or steam pressure to the thermo-compressor. During the last years, the development of PLC’s has resulted in equipment for process control, which is attractive both with regard to price and intelligence. The PLC has many advantages, also when we are talking about traditional and uncomplicated controls consisting of simple isolated loops trying to maintain a given parameter at the set point disregarding other parameters, which might well have an influence on the selected set point. This means that the operator’s knowledge of the process has less influence on the plant operation and therefore on the product quality, which can then be closer to the specifications. The PLC also offers a perfect tool for start-up or shut-down of the entire plant. This means that non-productive running time can be avoided. Also sequence control of valves and pumps during CIP of the plant is controlled by the PLC. Data-logging is possible by computing mean values of any selected parameter, and trend curves over for example one hour can be monitored and printed as a hard copy. 85

5. Types of spray drying installations The capability of spray drying installations is determined by the type of chamber and by the type and combination of the components incorporated. Any modern spray drying installation must have the components as given in chapter 4. What makes the difference between various installations as to their ability to fulfil the qualitative and economical requirements is the type of after treatment system utilized and mutual compatibility of the components selected. From this point of view, spray drying installations can be divided into single-stage, two-stage, three-stage (like GEA Niro´s MSD™ plant) and special systems. Furthermore, depending again on the type of individual components, installations can be distinguished by their ability to produce either regular (non-agglomerated) powders or agglomerated products. The flow sheets of the individual installations in this section are presented with the main components drawn only by symbols, which are self-explanatory. Obviously all installations can have additional components such as bag filters, heat recuperation etc.

5.1. Single stage systems These are the simplest types of installations. Any type of chamber described in chapter 4 and shown on the first two rows of Fig. 4.2., can be used for drying milk products, usually in combination with a pneumatic conveying system or external fluid bed. However, some designs operate without any form of such after treatment.

5.1.1. Spray dryers without any after-treatment system Spray drying chambers where powder leaves the chamber together with the drying air do not need any after-treatment system, if the powder can be bagged-off without cooling. These types of drying chambers represent standard equipment used in the early days of drying milk. This goes also for the box dryer which has a bag filter built into the drying chamber discharging the fines fraction back into the chamber. Space requirements are small, and building costs are low. Generally, installations without any after treatment system are only suitable for nonagglomerated powders not requiring cooling (cooling is often necessary to avoid powder caking in bags). The flow sheet of such system is shown on Fig. 5.1. This system can also feature a chamber with flat bottom with either a rotating mechanical powder scraper or rotating suction arm (see Fig. 4.2.) to convey the powder into the exhaust duct.

Fig. 5.1. Spray dryer without any after treatment © GEA Niro 86

5. Types of spray drying installations

5.1.2. Spray dryers with pneumatic conveying system The aim of the pneumatic conveying system is to cool the powder while transporting the chamber fraction and cyclone fraction to a single discharge point. This type first appeared in the 1950’s and dominated the milk powder industry up to the middle of the 1960’s. A typical flow-sheet is shown on Fig. 5.2. It is suitable for production of non-agglomerated powders. The fat content of the powder must not be higher than about 35%.

Fig. 5.2. Spray dryer with pneumatic conveying system © GEA Niro

5.1.3. Spray dryers with cooling bed system The primary reasons for introducing fluid beds into dairy drying installations were (a) the disadvantage of pneumatic conveying systems not being able to handle milk powders of high fat content, (b) the introduction of milk replacers with high fat contents for feeding calves and (c) producing in winter high fat powders which were standardized to normal fat content by dry mixing with skim milk powder from the peak season. It was also very convenient for the fines from main cyclone and fluid bed cyclone to be transported via the rotary valves below the cyclones by gravity or screw conveyor into the fluid bed. This plant configuration is today considered obsolete and not further discussed in this second edition. It was however later found that the fluid bed had another advantage over pneumatic conveying. It not only collects and cools the high-fat powder, but it was observed that when using the same system for skim milk the powder was more coarse, better free flowing and more easy to reconstitute. This was because the fluid bed provides more gentle treatment than pneumatic conveying system by not breaking down the primary agglomerates. Furthermore it also had the ability to classify the powder i.e. to blow-off the finest size fractions. The possibility of manufacturing instant powder and the importance of agglomeration for instant properties was already known from the rewet process which was introduced at the beginning of the 1950’s. Therefore the next logical step was to introduce the fines from the powder separators back to the wet zone of the drying chamber to support the agglomeration. This resulted in the successful development of the so-called cooling bed process for producing a sort of instant powders, even when the fluid bed was supplied with only cold air. This installation which can operate alternatively with high fat powders (introducing the fines back to the fluid bed) or agglomerated powders (with fines to the wet zone) is shown on Fig. 5.3. 87

The spray drying installations with cooling bed are still used for products which cannot be processed successfully by two stage drying such as high fat milk powders etc.

5.2. Two stage drying systems The principles and advantages of two stage drying were described in chapter 3.

Fig. 5.3. Spray dryer with a VIBROFLUIDIZERTM as cooling bed © GEA Niro

The second stage of drying can be conducted in either an external or an integrated fluid bed. Two stage drying installations can also be operated as single stage installations, if the product characteristics require single stage processing. In such case there is no heating of the fluid bed air.

5.2.1. Spray dryers with fluid bed after-drying systems Referring again to the development of cooling bed systems discussed in previous section, it was soon recognized that when operating the dryer with lower outlet temperature and producing powder of high moisture content, it was better agglomerated and consequently had better instant properties. Introducing heated air in the first section of the fluid bed removed the excess moisture while maintaining the instant properties. This process became known as straight-through process and dominated production of instant milk powders especially instant whole milk powder from 1970 to about the middle of the 1980’s. It is still extensively in use in spite of further developments discussed below. Originally this system was provided with fines return system below the wheel (Fig. 4.19.). Today’s modern installations have the fines return from above the wheel (Fig. 4.20.). This system can operate also with pressure nozzles. The flow sheet is shown on Fig. 5.4. Obviously any type of chamber from which the powder leaves under gravity can be used for two stage drying with a fluid bed after drying system. Soon after the development of the straightthrough process, which is characterized by introducing fines into the wet zone around the atomizer and by fluid bed after drying, it was found that the same installation operating in principle with the same conditions but with introduction of fines into the fluid bed, can produce also non-agglomerated powders while still utilizing the advantages of two stage drying.

Fig. 5.4. Spray dryer with a VIBROFLUIDIZERTM as after dryer/cooler and fines return © GEA Niro 88

5. Types of spray drying installations

Products leaving the fluid bed are still somewhat agglomerated. However with a simple blow line, transporting the powder to silos, which was anyhow necessary taking into account the yearly capacity growth, sufficient break-down of agglomerates occurred. The resulting powder, utilizing the benefits of two stage drying, was heavier than from a single stage process. The overall powder quality was also better and the overall energy consumption lower.

5.2.2. TALL FORM DRYER™ The TALL FORM DRYER™ is a spray dryer with a tall slim drying chamber with top mounted nozzle assembly featuring a fines return capability. See Fig. 5.5. The resulting powder is discharged into a fluid bed for final drying and cooling, while the exhaust air is discharged through the enlarged lower cone section called a ’bustle’. This type of dryer is suited for both non-fat and fat-containing products, producing non-agglomerated and agglomerated freeflowing powders including baby food. The special air outlet further means that there is nothing inside the drying chamber that might obstruct the air flow, such as ducts etc. where powder can stick. The atomization of the feed material is always done by high pressure nozzles, which form an integrated part of the air disperser. The air disperser is designed to assure a high velocity drying air stream necessary to obtain a final product with good re-solubility properties.

Fig. 5.5. TALL FORM DRYERTM © GEA Niro

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All process air is passed through cyclones and/or bag filters for separation of the entrained fines powder particles. These fines are then returned to the fluid bed for regular non-agglomerated powders and/ or the atomization device for production of agglomerated powders. The advantage of the TALL FORM DRYER™ (TFD) is the downward drying air stream in the drying chamber, since this reduces the tendency of powder deposits on the chamber wall. The specially designed chamber cone with a “bustle” air outlet reduces the amount of fines powder particles carried along with the air into the fines collector.

TALL FORM DRYERTM © GEA Niro

5.2.3. Spray dryers with Integrated Fluid Bed The background and principles of the integrated fluid bed are described in chapter 3. It was introduced in 1980 and became very soon a dominating design of spray dryer not only in the milk powder industry, for which it was originally developed, but also in other industries (used for chemicals, pharmaceuticals, coffee, sorbitol etc.). The integrated fluid bed is non-vibrating (so-called static) and can either be circular (used in MSD™ chambers) or annular (used in COMPACT DRYER™ chambers). Both chambers are shown on Fig. 4.2. Typical ratios of primary drying air through air disperser to secondary drying air (static fluid bed) are in the range 3-4 to 1. Both of these chamber types operate as two stage drying systems but only for the production of non-agglomerated powders. This is because the integrated fluid bed powder fraction is collected together with the cyclone fraction and during pneumatic conveying, agglomerates are broken down.

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Fig. 5.6. COMPACT DRYER™ Type CDP © GEA Niro

The combination of integrated fluid bed dryer with pneumatic conveying system is applied in the milk powder industry in connection with GEA Niro COMPACT DRYER™ chambers for the production of non-agglomerated powders and the installation is called CDP (Compact Dryer Pneumatic) – see Fig. 5.6. The combination of the chamber of a Multi Stage Dryer with pneumatic conveying system is used extensively in other industries and known as a FSD™ (Fluidized Spray Dryer). Fig. 5.7. shows a GEA Niro FSD™ dryer with pneumatic cooling system.

Fig. 5.7. Fluidized spray dryer with integrated fluid bed and pneumatic conveying system © GEA Niro

5.3. Three stage drying systems These installations with a static fluid bed as a second stage dryer in combination with external vibrating fluid bed as a third stage dryer appeared for the first time at the beginning of the 1980’s and were called COMPACT DRYER™ (GEA Niro) type CDI (I for Instantization) and . They dominate today the milk powder industry.

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COMPACT DRYER™ © GEA Niro

Multi Stage Dryer MSD™ © GEA Niro

Three stage systems combine all the advantages of extended two stage drying using spray drying as the primary stage, fluid bed drying on a static fluid bed as second drying stage, external vibrating fluid bed as the third. The final drying stage terminates with cooling. Evaporation conducted in each stage can be optimised to achieve both gentle drying conditions and good thermal economy.

5.3.1. COMPACT DRYER™ type CDI (GEA Niro) The COMPACT DRYER™, as shown on Fig. 5.8. is suitable for producing both nonagglomerated and agglomerated powders of practically any kind of dried dairy product. It can cope successfully also with whey powders, fat-filled milk and whey products as well as caseinates, both non-agglomerated and agglomerated. It has a limitation as to fat content, which is about 50% fat in total solids. The powder quality and appearance is comparable with products from two stage drying systems, but they have considerably better flowability and the process is more economical.

Fig. 5.8. COMPACT DRYER™ with VIBRO-FLUIDIZERTM and fines return © GEA Niro 92

5. Types of spray drying installations

Installations with capacities higher than approximately 1500 kg/h powder production have better performance than smaller units in all respects and especially powder quality.

5.3.2. Multi Stage Dryer MSD™ type The Multi Stage Dryer, MSD™ Fig. 5.9. in comparison with the COMPACT DRYER™ can process an even wider range of products and handle even higher fat contents. The structure and appearance of MSD™-dried powders differ from the products obtained in two stage drying installations and that from a COMPACT DRYER™. In fact it is different from all previously produced powders. The main characteristics of MSD™-powders relate to very good agglomeration and mechanical stability, low fractions of particles of size below 125μm and superb flowability. The mean particle size of, for instance, agglomerated whole milk powder may be adjusted between 180-300μm but the most typical and best functional properties are obtained at 250μm with bulk densities in the range of 420-480 kg/m3. MSD™-type spray dryers do not need any fines return system to obtain such agglomeration, but may be supplemented with such a system which then produces skim milk powder of mean particle size 500-1000μm and bulk density as low as 300 kg/m3. It is also suitable for high fat powders up to 80% fat in total solids. However, high bulk density powders are difficult to produce in the MSDTM-type plant due to the spontaneous secondary agglomeration taking place in the atomization area as a result of the high drying air inlet velocity. By adjusting the operating conditions (higher exhaust air temperatures = lower capacity) it is possible to produce whole milk powder and skim milk powder having bulk densities up to 580-630 kg/m3 650-720 kg/m3 respectively after milling and a blow line conveying system to powder silos.

Fig. 5.9. Multi stage dryer MSD™ © GEA Niro

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5.3.3. Spray drying plant with Integrated Filters and Fluid Beds - IFD™ The Integrated Filter Dryer design, see Fig.5.10., is based on proven spray dryer unit operations from the COMPACT DRYER™ and the MSD™ dryer and the development of the SANICIP™ bag filter such as: Integrated filter dryer IFD™ with integrated filters and fluid beds © GEA Niro

Fig. 5.10. Integrated filter dryer IFD™ with integrated filters and fluid beds © GEA Niro

•  Feed system with concentrate preheating, filtration, homogenization, and highpressure pumps. All equipment as used in all other types of spray dryers •  Atomization using pressure nozzle atomization •  Drying air filtration, heating, and distribution using an air disperser suitable for vertical air streams •  Drying chamber designed to ensure hygienic operation conditions and to maintain lowest possible heat loss by means of e.g. dismountable insulation panels featuring airfilled sandwich panels, see chapter 4 •  Integrated fluid bed designed as a combined back-mix bed for the drying and a plug-flow bed for the final drying and cooling. Between the back-mix bed and the surrounding plug-flow bed there is an air gap to avoid heat transmission •  The dryer exhaust air system is new and though the idea is revolutionary, it is still based on the same principles as applied in GEA Niro’s SANICIP™ CIP-able Bag Filter. The fines collection system operates with particulate filters integrated in the drying chamber. The filter bags are supported on stainless steel cages mounted in the ceiling around the circumference of the drying chamber. These filter elements operate with blow-back air cleaning systems and CIP operation similar to those used in the SANICIP™. See chapter 4 •  The advantage of this dryer is the reduced building height/volume as there are no external fluid bed(s) for after drying and cooling and no external bag filter(s). Further a full CIP turnaround time - including also the dry out time - is only 6-8 hours. As the pressure drops over the air disperser and exhaust system is low, it results in low energy consumption and a low noise level. 94

5. Types of spray drying installations

5.3.4. Multi Stage Dryer MSD™-PF Based on the knowledge from operating the Multi Stage Dryer MSD™ and the Integrated Filter Dryer IFD™, a new dryer design was evolved. See Fig. 5.11. In a traditional MSD™ dryer with an external bag filter, the external fluid bed, the VIBRO- FLUIDIZER™ has been replaced by a circular plug flow fluid bed similar to that used in the Integrated Filter Dryer IFD™. The advantage of this design is a reduced building height.

Multi stage dryer MSD™-PF © GEA Niro

Fig. 5.11. Multi stage dryer MSD™-PF with integrated fluid beds and external CIP-able bag filter SANICIP™ © GEA Niro

5.3.5. FILTERMAT™ (FMD) integrated belt dryer This type of dryer differs substantially from the installations discussed so far. It has all the drying stages conducted in separate compartments of one box-type chamber.

FILTERMAT™ (FMD) © GEA Niro

Fig. 5.12. FILTERMAT™ (FMD) integrated belt dryer © GEA Niro The air flow in the spray drying section and also the after-drying, conditioning and cooling sections is streamline downward. The bottom of the chamber is formed by a conveyor belt, which is a mesh made of polyester material. Atomization is by multi-nozzles type and each nozzle is placed in a separate air inlet. Moist powder lands on the belt forming a layer through 95

which the drying air is passing downwards (see Fig. 5.12.). Thus the moist powder layer acts as a filter resulting in negligible amounts of fines passing with the exhaust air through the belt and to the cyclone. This effect is the same in all sections. In chapter 3 limitations were mentioned as to powder moisture levels from the primary stage in two stage drying. Milk powder of high moisture cannot withstand any mechanical handling, and this includes just the rolling down of the powder over the cone of conventional chamber design. This is especially the case with highly thermoplastic and sticky powders of high carbohydrate and fat content. The extended two stage drying, as conducted in spray dryers with integrated fluid beds, has shifted these limits, although there is some mechanical treatment and contact with the walls. The FILTERMAT™ Dryer (having just a horizontal base formed by a porous belt on which the powder can settle and be after-dried) creates no mechanical treatment and thus is suitable for highly thermoplastic and sticky materials such as whey products, fat filled whey powders and especially for whey and milk products with hydrolysed lactose, fruit juices and tomatoes etc. This type of dryer is not particularly suited for high protein containing products like skim milk and WPC.

5.4. Spray dryer with after-crystallization belt This type of dryer is in principle a two stage drying installation but has rather special and many original features that require separate description. The flow sheet is shown on Fig. 5.13. It is a single duty spray dryer for non-caking whey and permeate powder. The production technology involved is described in chapter 8. However, it provides a most economical production method being able to operate at low air outlet temperature and high feed concentration securing at the same time a high quality non-caking product. Special features of this dryer, which can be seen on the flow sheet, is the after crystallization belt ensuring that the moist powder (8-10%) has a residence time of 6-8 minutes. This means that the remaining amorphous lactose has time to crystallize as there is enough “mobile water” and sufficient time prior to entering the fluid bed for final drying and cooling. Typical operating conditions are: inlet temperature 150-160°C, outlet temperature 55-58°C. The feed should be pre-crystallized and have a concentration of 55-65%. The resulting powder can be termed 100% non-caking and non-hygroscopic as all the lactose has been crystallized. And that means that the powder can be exported and used also in countries where high air humidity is prevailing, and that without the powder will start to cake and lump.

5.5. TIXOTHERM™

Fig. 5.13. TALL FORM DRYER™ (TFD) with after crystallization belt © GEA Niro

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Whey and permeate from ultra-filtration of whey and milk is considered low-price byproducts, if processed into a powder or expensive if disposed of into sewage plants. With the increasing cheese production there will be more and more whey and/or

5. Types of spray drying installations

permeate available. A new process – the TIXOTHERM™ process has been developed. It will convert this by-product into a first-class 100% non-caking powder product using less energy and a reduced building volume means lower total investment. Pre-crystallized whey and permeate concentrates are very thixotropic, i.e. if the concentrate is not agitated continuously in the crystallization tank in the traditional process, it will solidify, and the higher total solids in the concentrate, the more solid becomes the concentrate. This phenomenon is used advantageously in the TIXOTHERM™ process. See Fig. 5.14.

Fig. 5.14. TIXOTHERM™ dryer © GEA Niro

The TIXOTHERM™ process is a four-step process, where the whey or permeate is evaporated in a falling film evaporator to 60 % TS. From the evaporator the concentrate is pumped to a vertical agitated film high concentrator. Here the solids is increased to 85% TS. The high shear rate in the agitated film, however, keeps the viscosity relatively low. The combination of high solids and low temperature as a result of the evaporation initiates the lactose crystallization, and many nuclei are formed.

In the subsequent process – the mixing crystallizer – a phase-conversion of the product takes place enhanced by addition of fines particles/recycled cold product from the subsequent fluid bed drying/cooling. This encourages rich lactose crystallization. At the discharge of the mixing crystallizer, the product is now like a semi-solid mass with a friable texture ideal for the following fluid bed drying and cooling. In this last step of the process the remaining moisture is evaporated in an agitated back-mix fluid bed followed by final drying and cooling in a plug flow fluid bed. The drying air for the bag mix and plug flow fluid bed is exhausted through a bag filter, from where the collected fines particles are returned back to the mixing crystallizer. The final powder has almost 100% of the lactose crystallized, i.e. the total moisture content is 5% or more. The powder is non-hygroscopic and non-caking. Energy consumption is reduced by more than 30%, and building requirements are reduced by more than 50 %. No crystallization tanks are needed, and no spray drying plant is involved to produce whey or permeate powder. The capacity of a given TIXOTHERM™ plant is a function of the content of protein, galactose and lactic acid in the product. That is why a TIXOTHERM™ plant for whey/permeate from mozzarella cheese has a lower capacity than for instance whey/permeate from Swiss cheese. The disadvantage of this plant is that it can only be used for whey and permeate.

97

Choosing a spray drying installation The decision as to the choice of a spray drying installation for a powder plant - regardless whether it is for a new green field site or additional equipment for an old plant - requires an extensive preliminary preparation and study. First of all it is necessary to make an analysis, which must be based on market conditions, the range of products which will be produced and their quality specification. Secondly the installation capacity must be decided, based on prognosis of milk availability in the area in question. The decision will influence the performance and economy of the whole powder plant for the next many years. The easiest task, which is giving the possibility to choose a plant with optimum performance both as to product quality and economy, is when it has to produce one product only. However, a single duty installation is very seldom in question. A choice of an installation for two or more duties might often be a compromise with a consequence that some of the duties will not be fulfilled to the optimum. The capabilities of various spray drying installations as to various products and qualities are very different and there is no installation which can cover the whole range to the optimum. Besides one has to be aware that nowadays the dairy drying installations are becoming more and more food drying plants which have to process not only pure dairy products, but also milk based products with other food additives and even non-milk compositions. The main aspect to take into account is product quality, length of continuous production (20 hours or more) and production economy. The general rule for installations with a choice between rotary wheel and pressure nozzle atomization is that pressure nozzles can comply better with high bulk density powder specifications while a rotary wheel gives the possibility of handling feeds of higher total solids content. Similarly, with a choice between a conventional chamber and a TALL FORM DRYER™, the latter allows longer periods of operation, and tendencies for sticky powders, i.e. high carbohydrate and/or high fat products to deposit on the walls are smaller. Generally it can be stated that the FMD™ is first preference for sticky powders, the MSD™ leads for agglomerated instant powders including baby food, and the TALL FORM DRYER™ and the COMPACT DRYER™ are ideal for fat-filled powders and baby food.

98

6. Technical calculations

6. Technical calculations Industrial production requires a daily check of the plant capacity, product output yield, consumption of energy etc. by collection of various production data and calculations. Examples of some useful technical calculations are given in the following sections.

6.1. Evaporation and product output The symbols used in the examples consist of three letters, the meaning of which is as follows:

First letter

Second letter

Third letter

E = evaporator

F = feed

R = rate (kg/h)

D = spray dryer

E= evaporation

S = total solids (%)

B = fluid bed

P = product

a) Evaporation and product rate from the evaporator: EER = EFR – EPR EPR = EFR – EER EPR = EFR × EFS / EPS EEF = EFR × [1 – EFS / EPS] EPS = EFR × EFS / EPR

[6,1] [6,2] [6,3] [6,4] [6,5]

b) Evaporation and product rate from the spray dryer: DRF = EPR DER = DRF – DPR DPR = DFR – DER DPR = DFR × DFS / DPS  DER = DFR × [1 – DFS / DPS] DPS = DFR × DFS / DPR

[6,6] [6,7] [6,8] [6,9] [6,10] [6,11]

c) Evaporation and product rate from the fluid bed: BFR = DPR BER = BFR – BPR BPR = BFR – BER BPR = BFR × BFS / BPS BER = BFR × [1 – BFS / BPS] BPS = BFR × BFS / BPR 

[6,12] [6,13] [6,14] [6,15] [6,16] [6,17]

99

d) Example: Evaporator feed rate: Total solids of evaporator feed: Total solids of the concentrate: Total solids of powder from spray dryer: Total solids of powder from fluid bed:

EFR = 50000 kg/h EFS = 12.2 % EPS = 48.0 % DPS = 94.2 % BPS = 97.2 %

Notice that DFR = EPR, BFR = DPR, DFS = EPS and BFS = DPS. The results are given in Tab. 6.1. Tab. 6.1. The Results of calculation of the example d)

There are examples of calculations in all the following sections and the results are rounded up to the reasonable decimals. However, the calculations have been made using the exact figures.

6.2. Heating of atmospheric air If an amount Aa of atmospheric air with humidity ya is to be heated from a temperature t1 to a temperature t 2, the amount of heat Q necessary is calculated by the equation:

Q=

Aa ∗  t ∗ ca 2 − t1 ∗ ca1 + ya (cv 2 ∗ t 2 − cv1 ∗ t1 ) 1 + ya  2

[6,18]

If Aa =30000 kg/h, ya =0.009 kg/kg, t1=15°C and t 2=200°C then from equation [3,8] ca1=0.240, ca2=0.245 and from equation [3,10] c v1=0.445 and c v2=0.463 kcal/kg/°C. The result is: 30000 Q= ∗ [200 ∗ 0.245 − 15 ∗ 0.24 + 0.009 ∗ (0.463 ∗ 200 − 0.444 ∗ 15 )]  1 + 0.009

100

= 1,372,848 kcal/h

6. Technical calculations

6.3. Mixing of two air stream Two quantities of air A1 and A 2, with humidities y1 and y2 and temperatures t1 and t 2 are to be mixed, the temperature t3 of the mixed air calculated by a simplified method is as follows:

t3 =

A1 ∗ t1 + A 2 ∗ t 2 [6,19] A1 + A 2

Example: A1 = 50000 kg/h, A 2 = 10000 kg/h, t1 = 90°C, t 2 = 20°C, y1 = 0.0442 kg/kg and y2 = 0.007 kg/kg:

t3 =

50000 ∗ 90 + 10000 ∗ 20 = 78.33°C 50000 + 10000

For a more precise calculation, heat capacities of air must be considered. From equation [3,8] values are ca1 = 0.240 kcal/kg/°C at 90°C and c a2 = 0.241 kcal/kg/°C at 20°C and calculation of the resulting air humidity y3 and enthalpy h3 is by:

A1 A2 * y1 + * y2 1 + y1 1 + y2 y3 =  A1 A2 + 1 + y1 1 + y 2

[6,20]

50000 10000 * 0.0442+ * 0.007 1+0.0442 1+0.007 y3 = = 0.0378 kg/kg 50000 10000 + 1.0442 1.007 The enthalpy of both components using heat capacities of water vapour from equation [3,10] at 20°C c v1 = 0.445,at 90°C c v2 = 0.451 and using equation [3,11]:

h1 = ca1 ∗ t1 + y1 ∗ ( r0 + cv1 ∗ t1 ) =

0.241 ∗ 90 + 0.0442 ∗ ( 597.3 + 0.445 ∗ 90 ) = 49861 kCal / kg

h 2 = ca 2 * t 2 + y 2 * (r0 + cv 2 ∗ t 2 ) =

0.240*20 + 0.007* (597.3 + 0.451*20 ) = 9,044 kCal / kg The enthalpy of the resulting air mixture is then: A ∗ h1 + A 2 ∗ h 2  h3 = 1

A1 + A 2

h3 =

50000 * 49.861 + 10000 * 9.044 50000 + 10000

[6,21]

= 43.058 kCal/kg

Using equation [6,22] which is another form of equation [3,11] the resulting temperature t3 is: 101

t3 =

h 3 ∗ y3 ∗ r0  ca 3 + y3 ∗ cv3

[6,22]

The values ca3 and c v3 are calculated from equation [3,8] and equation [3,10] using the approximate mixing temperature calculated by [6,19], i.e. 78°C which are 0.241 and 0.450 kcal/ kg/°C respectively.

t3 =

43.058 − 0.0378 ∗ 597.3 = 79.38°C 0.241 + 0.0378 ∗ 0.45

In comparison with the simplified calculation there is a difference of 1.04°C.

6.4. Dry air rate, water vapour rate and air density Using figures from the previous example and equations [3,5] through [3,7] and [3,13], the dry air rates Adn, water vapour rates Avn and air densities n are: Ad1 Av1 1 Ad2 Av2 2 Ad3 Av3 3

= A1 / (1 + y1) = 50000/1.0442 = 47883.5 kg/h = Ad1 * y1 = 47883.5 * 0.0442 = 2116.5 kg/h = 353.15 / (273.15 + t1) * (1 + y1)/(1 + 1.6 * y1) = 353.15/(273.15+90) * 1.0442/(1 + 1.6 * 0.0442) = 0.9484 kg/m3 = A 2 / (1 + y2) = 10000/1.007 = 9930.5 kg/h = Ad2 * y2 = 9930.5 * 0.007= 69.5 kg/h = 353.15 / (273.15 + t 2) * (1 + y2) / (1 + 1.6 * y2) = 353.15/(273.15+20) * 1.007/(1 + 1.6 * 0.007) = 1.1997 kg/m3 = A 3 / (1 + y3) = 60000/1.0378 = 57814.0 kg/h = Ad3 * y3 = 57814 * 0.0378 = 2186.0 kg/h = 353.15 / (273.15 + t 3) * (1 + y3) / (1 + 1.6 * y3) = 353.15/(273.15 + 79.43) * 1.0378/(1 + 1.6 * 0.0378) = 0.9804 kg/m3

The volume of air V is calculated as follows:

V=

A  ρ

Thus: V1 = A1/1 = 50000/0.9484 = 52722 m3/h V2 = A2/2 = 10000/1.1997 = 8336 m3/h V3 = A3/3 = 60000/0.9804 = 61199 m3/h

102

[6,23]

6. Technical calculations

6.5. Air velocity in ducts Using a Pitot tube (Fig.6.1.) it is possible to measure the static pressure Ps, dynamic pressure Pd and total pressure P t.

Fig. 6.1. Pitot tube with U-tube

Pd = Pt − Ps Pd =

[6,24]

ρ 2 2*g *v

[6,25]

and,

v=

2 ∗ Pd ∗ g ρ



[6,26]

where: Pd = dynamic pressure in mm water gauge g = gravity constant 9.81 m/s²  = density of air kg/m³ v = air velocity in m/s.

6.6 Air flow measurements In practice, it is not easy to measure exactly the amount of air passing through a duct, filter or spray dryer. The methods available are listed below: a) Measuring the air velocity and duct area. If the air velocity is measured by Pitot tube or by wind or hot wire anemometer in a duct of SA area, then the volume of air flow V in m3/h is:

V = v ∗ SA ∗ 3600

[6,27]

Knowing the air temperature t and humidity y, the air rate A in kg/h can be calculated using equations [3,12], [3,13] and [6,23]. In a similar way, for round ducts of diameter D, the equation is: 103



[6,28]

b) Measuring pressure drop across the cyclone:

A = K ∗ n ∗ D 2 ∗ ρ ∗ ∆P 

[6,29]

where: A = air rate in kg/h D = cyclone diameter in m n = number of cyclones  = density of air in kg/m3 P= pressure drop across the cyclone in mm WG K = cyclone constant The cyclone constant depends upon the cyclone design and powder loading in air. For various types of the cyclones constants lay between 200 - 1000, the exact values being proprietary manufacturer know-how. However, if the air flow has been measured by one of the described methods and at the same time the pressure drop over the cyclone measured the cyclone constant can be calculated backwards and used in later routine measurements. c) Measuring the amount of heat necessary for air heating: If air is heated from a temperature t1 to a temperature t 2 the amount of air can be calculated from the amount of heat used and the temperature difference. In principle the same method of calculation is used for steam, oil, gas and electric air heaters. The basic equation is:

A=

X ∗ E t 2 ∗ ca 2 − t1 ∗ ca1 

[6,30]

where: E = efficiency of the heater t1 = air temperature at heater inlet t 2 = air temperature at heater outlet ca1 = heat capacity of air at heater inlet ca2 = heat capacity of air at heater outlet X = the heat consumed kcal/h. Calculation of X for various types of heaters is explained below. c1) Measuring of the condensate from the steam heater: X for the equation [6,30] is:

X = W ∗ ( h s − h c ) where: W = amount of the condensate in kg/h hs = enthalpy of steam at heater inlet in kcal/kg hc = enthalpy of the condensate (which is equal to the temperature)

104

[6,31]

6. Technical calculations

c2) Measuring of gas or oil consumption: X for equation [6,30] is:

X = G ∗ Q h

[6,32]

where: G = amount of oil in kg/h or gas in Nm3/h Qh = caloric value of the fuel in kcal/kg or kcal/Nm3. c3) Measuring of consumption of electricity: X for the equation [6,30] is:

X = kWh ∗ 860 

[6,33]

d) Measuring the water evaporation rate: The dryer is operated on water under constant t1 and t 2 for at least 1 hour to obtain stable conditions. The amount of water, supplied to the dryer over a period of time is measured. The most suitable way is to measure the level difference h in a cylindrical feed tank of diameter D. The volume and weight of the evaporated water and the evaporation rate are:



W=

V ρw

[6,34]

[6,35]

DER = 3600 ∗

W t



[6,36]

where: D = feed tank diameter in m h = level difference in m t = time of measuring in s w = density of water at feed temperature tf in °C DER = rate of evaporation in kg/h. The drying air rate A is then:

A=

DER ∗ (597.3 + cv 2 ∗ t 2 − t f )+ Ar ∗ K ∗ ( t 2 − t1 ) [6,37] t1 ∗ ca1 − t 2 ∗ ca 2

105

where: t1 and t 2 = the air inlet and outlet temperatures, ca1 and ca2 = heat capacities of air at t1 and t 2, cv2 = heat capacity of water vapour at t 2 t f = feed temperature, Ar = surface area of the dryer in m², t s = spray dryer surrounding temperature, K = radiation constant in kcal/m²/h.

6.7. Barometric distribution law The barometric pressure in an altitude of m meters above sea level is calculated:

p m = p0 ∗ e

-M ∗ g ∗ m  RT

[6,38]

where: p 0 = barometric pressure at sea level at 0°C, M = molecular weight of air (= 0.029 kg/mol), g = gravity constant (= 9.807 m/s2), R = gas constant (=8.3144 J/K/mol) T = absolute temperature in °K, and m = altitude in m.

6.8. The heat balance of a spray dryer The spray dryer in operation is a system where air and product move through under changing temperatures and humidities and as the product is concerned, also changing physical properties. Entering components are: drying gas, which is usually heated ambient air, some auxiliary air flows (as cooling air, fines transport air etc.) and the feed to be dried. The humidity of the entering air corresponds to the ambient air humidity, possibly increased somewhat by moisture generated during combustion (in case of direct gas heating) or by moisture picked up by the air on passage through the building. The feed is the milk concentrate. Exhaust air and powder leave the system. The exhaust air is made up of all entering air flows plus water formed from the evaporation and besides it contains also some traces of the dried solids (fine particles). The dry products in powder form contain practically all the feed solids, but have residual moisture. The amount of air necessary to evaporate a required amount of water from a given amount of feed can be found by calculating all the individual heat requirements necessary for evaporating the water, heating or cooling each individual component from its inlet to its outlet temperature, while compensating for heat losses. The sum of these contributions is then recalculated into the air rate on the basis that the drying air enters the system with a temperature t1 and leaves with a temperature t 2. The following example is a demonstration of the calculation of the heat requirement and drying air rate of a spray dryer as specified below by points a) through g). The source of drying air is ambient thus the inlet humidity is considered as ambient humidity ya:

106

6. Technical calculations

a) The drying air of the inlet temperature t1 and humidity ya, and the drying air rate Ad (which has to be calculated), b) The auxiliary cooling air for the atomizing device of temperature tc, humidity ya and rate Ac, c) The fines transport air of temperature tt, humidity ya and rate At, d) The feed concentrate of temperature tf, rate DFR and solids content DFS, e) Recycled fines, collected from all cyclones, into the dryer in the amount given by ratio R to the total powder production and temperature tt (i.e. the same as of fines transport air). The mass flows at the outlet of the system are: f) The exhaust air consisting of all the entering air flows plus the moisture generated during drying, having final temperature t2 and humidity y2, g) The powder consisting of the feed solids plus some residual moisture. For the calculation, the operating conditions at the inlet and the outlet have to be first set. Product experience, knowledge of the drying installation and applied process is used to estimate the permissible air inlet temperature and feed total solids content and the required air outlet temperature for the specified powder moisture. Besides there is also some heat loss due to the radiation from the equipment surface of area SA. The radiation coefficient is K and the surrounding temperature around the dryer is estimated to be 20°C above the ambient temperature. Having fixed the operating parameters the heat balance can be calculated as follows: Heat of evaporation: Qev = DER * (597.3 + cv2 * t 2 - t f )[6,39] Heat of product solids: Q pr = DPR * (tp - t f ) * (c s* DPS / 100 + (1 - DPS / 100))

[6,40]

Heat of cooling air: Qco = Ac / (1 + ya) * ((t 2 * ca2 - ta * caa) + ya * (t 2 * cv2 - ta * cva))[6,41] Heat of fines transport air: Qtr = At / (1 +ya) * ((t 2 * ca2 - t t * caa) + ya * (t 2 * cv2 - t t * cva))

[6,42]

Heat of fines: Q fi = DPR * R * (t 2 - tt) * (c s * DPS / 100 + (1 – DPS / 100)) 

[6,43]

Heat of radiation loss: Q rl = SA * K * (t 2 - t a - 20)

[6,44]

The sum of heat requirements: Q = Q ev + Q pr + Q co + Qtr + Q fi + Q rl

[6,45] 107

The drying air rate: [6,46]

Example: Calculate the drying air rate and size of dryer for following duty: Feed rate Feed concentration Feed temperature Product solids content Ambient temperature Ambient humidity Inlet temperature Outlet temperature Cooling air rate Fines transport air Transport air temperature

DFR = 4000 kg/h DFS = 48 % tf = 60 °C DPS = 95 % t a = 15 °C ya = 0.01 kg/kg t1 = 200 °C t 2 = 80 °C A c = 200 kg/h At = 500 kg/h tt = 60 °C

The values for operating parameters were estimated from product experience with respect to the required quality specification. The powder temperature is estimated to be 5°C below the outlet air temperature, fines recirculation ratio R = 0.5, radiation loss coefficient K = 3.0 kcal/ m2/h and dryer surface area 300 m2. The heat capacities of air and water vapour are taken from Equation 3.8 and 3.10 (ca1=0.245, ca2 = 0.241, caa = 0.24, cat = 0.241, cv1 = 0.463, cv2 = 0.45, cva = 0.444 and cvt = 0.448kcal/kg/°C). The product is whole milk with 28% fat, thus the heat capacity of solids, using values from Table 3.2. is: c s = (28 * 0.5 + (100 - 28) * 0.3)/100 = 0.356 kcal/kg/°C Calculation (according to equations [6,9], [6,7] and [6,39] through [6,46]: DPR = 4000 * 48/95 = 2021.1 kg/h DER = 4000 - 2021.1 = 1978.9 kg/h 1134530.5 kcal/h Qev = 1978.9 * (597.3 + 0.45 * 80 - 60) = 11768.6 kcal/h Qpr = 2021.1 * (80 - 5 - 60) * (0.356 * 95 / 100 + (1 - 95 / 100)) = 3163.0 kcal/h Qco = 200 / 1.01 * ((80 * 0.241 - 15 * 0.24) + * (80 * 0.45 - 15 * 0.444)) =  Qtr = 500 / 1.01 * ((80 * 0.241 - 60 * 0.241) + 0.01 * (80 * 0.45 - 60 * 0.448)) =  2431.3 kcal/h 5884.3 kcal/h Qfi = 2021.1 * 0.5 *(80 - 5 - 60) * (0.356 * 95 / 100 +(1 - 95 / 100)) =  40500.0 kcal/h Qrl = 300 * 3.0 * (80 - 15 - 20) = Q = 1134530.5 + 11768.6 + 3163 + 2431.3 + 5884.3 + 40500 = 1198277.7 kcal/h 39565 kg/h Adr = 1198277.7 / (200 * .245 - 80 * .241 + 0.01 * (200 * .463 - 80 * 0.45)) =  The above calculation can be simplified by neglecting the air moisture content and powder moisture and using for heat capacities of air and of water vapour constants 0.24 and 0.46 kcal/ kg/°C respectively:

108

6. Technical calculations

Q ev = 1978.9 * (597.3 + 0.46 * 80 - 60) =  Q pr = 2021.1 * 0.356 * (80 - 5 - 60) =  Q co = 200 * 0.24 * (80 - 15) =  Qtr = 500 * 0.24 * (80 - 60) =  Q fi = 2021.1 * 0.356 * (80 - 5 - 60) = Q rl = 300 * 3.0 / (80 - 15 - 20) =  Q =  Adr = 1198322.2 / (0.24 * (200 - 80)) = 

1136113.7 kcal/h 10792.4 kcal/h 3120.0 kcal/h 2400.0 kcal/h 5396.1 kcal/h 40500.0 kcal/h 1198322.2 kcal/h 41608 kg/h

This comparison demonstrates that the simplified calculation results in more than 5% higher amount of air and emphasizes the importance of calculation on the enthalpy basis. The difference is even greater if the available ambient air has high humidity values. The absolute humidity of the exhaust air is:

39565.4 + 200 + 500 ∗ 0.01 + 1978.9 1.01 = 0.0585 kg / kg y2 = ( 39565.4 + 200 + 500 ∗ 1.01 ) Total exhaust air = 39565.4 + 200 + 500 + 1978.9 = 42244.3 kg/h Exhaust air density exair and volume Vexair are then:

Vexair =

42244.3 0.968

= 43644.4 m 3 / h

The dryer should have two main cyclones of cyclone constant 380 operating at a pressure drop of 150 mm WG. The cyclone diameter according to the equation [6.29] will be:

D=

42244.3 380 ∗ 2 ∗ 0.968 ∗ 150

= 2.15 m

109

7. Principles of industrial production 7.1. Commissioning of a new plant After erection of a new spray drying installation it has to be brought into operation. This phase is called commissioning and proceeds in several stages: a) Mechanical test of all individual components, b) Test and calibration of control instruments, c) Performance test with air, d) Performance test on water, e) Performance test on the product(s) for which the spray dryer was designed, f) Fine tuning of the plant to achieve the design specification (both as to product quality, consumption of energy and capacity), g) Training of operators, and h) Final acceptance test and issue of a performance certificate. With the acceptance test, the plant fulfils the requirements specified by the supplier in the contract. The whole commissioning period normally takes 1 to 4 weeks, depending on the complexity of the installation and number of products. However, on multi-purpose plant drying a number of products, it may take longer.

7.2. Causes for trouble-shooting If during the intended commissioning period, the specified parameters have not been achieved, efforts are concentrated on how to identify, locate and remove the reasons for the problem. This involves a trouble-shooting exercise conducted as part of the commissioning program. However, commissioning is usually finalized successfully to time schedule and the plant quickly in commercial production. In factories faced with their first spray drying installation there has to be a training period during which the factory staff learns how to operate the dryer and the overall behaviour of the plant. However, even in factories having long experience with spray drying and possessing high level of technological knowledge, a training period is still needed, during which both the operators, engineers and maintenance staff acquaint themselves with the new plant since every spray drying installation has its own operational features. The experiences from one spray dryer cannot automatically be applied directly to another one. The development in spray drying has been very fast during the last 20 - 30 years. Therefore an existing spray dryer may not represent the best available technology. Experience has shown that problems can occur suddenly even in a well-established, well operated and well maintained plant with experienced staff after many years of successful operation. Such situation is in fact the most typical case calling for trouble-shooting and the problems the trouble-shooter is faced with normally fall into following categories:

110

7. Principles of industrial production

a) Lack of capacity, b) Excessive deposits somewhere in the system, c) Excessive energy consumption, d) Product quality defects, e) Excessive powder stack loss, f) Bacteriological contamination.

In broader sense, trouble-shooting involves also the development of technology for a new product or a product of upgraded quality. Generally speaking all advances in current technology aiming to upgrade the quality, to reduce the consumption of energy or increase the capacity can be considered as the outcome of successful trouble-shooting. However, such projects should be planned in advance and conducted in off-peak season. On the other hand, sudden occurrence of problems indicated above requires immediate action, especially if they appear in the peak season. The first approach to a trouble-shooting operation is to find out when the problem appeared and what changes of technology and/or which modifications of a product composition might have been implemented, just before the problem occurred. For such an evaluation availability of production documentation, such as production log books, installation maintenance books, laboratory analysis records both of raw milk and all raw materials, intermediate control and final product control data, is extremely helpful. It is therefore very important to keep the log sheets and other production records and reports up-to-date at all time.

7.3. Production documentation If a need for trouble-shooting suddenly occurs, the most appropriate approach after briefing about the problem and - if the first routine check of the plant and operation has not brought any explanation - is to find the answers on the above when-what-which questions basis, i.e. to trace when it has appeared and possibly in which consequence. The answers to such questions can be found by interviewing the staff and referring to the records and production documentation. Sometimes it is necessary to go through the documentation of the last few months or even to compare data with the same season of the previous year. The trouble-shooting is greatly facilitated, if such documentation is readily available and kept intact.

111

7.3.1. Production log sheets It is strongly recommended to keep all production run records containing data written down by the operators with ½ or at least 1 hour intervals. The importance of such practice cannot be emphasized strongly enough, even with modern, computerized plants having full data logging and trend facilities. In the modern electronic data processing it is dangerous to assume that the computer can fully replace the human factor. There are many good reasons to continue with manual operational data recording. First of all, the tabular type log sheet representing one page for one day operation is easier to survey than a computer print-out. Computer print-outs often involve plenty of paper. Secondly, it is well known that in modern life direct contact with figures can easily be lost. Therefore an operator, who writes down for 8 hours a day about 20 figures every half hour and does that throughout the whole year, can definitely keep in mind the important parameters better than his colleague just watching a computer screen. The general rules for keeping production log books are: a) Separate log-book (or a file of daily log-sheets) for the evaporator and the spray dryer, b) Separate log-books for each product, c) Headings describing each production run by type of product, required quality (possibly a number of quality specifications, if any), number of operational parameters specification (if any, see later), the plant configuration (as to optional possibilities which remain unchanged during the operation, i.e. size of nozzle inserts or where fines are introduced etc.), the names of supervisor, operators, laboratory worker responsible for the process control and any other information considered of local importance. d) A number of vertical columns sufficient to cover the time of reading all the important technological parameters which are shown on the control panel or measured directly by special instruments. It is also very useful to provide columns for the process control of the most important characteristics as final moisture, bulk density, solubility index, and possibly others, as measured by the laboratory or by operators themselves. It is also useful to have some columns expressing the production capacity (as raw milk intake to the evaporator, number of bags or load cell reading of a silo etc.) e) Some space has to be reserved as a last column for remarks which serve for comments as to production irregularities, i.e. reasons for production interruptions with stop and restart times, observations as to component malfunctions which did not stop the production (i.e. if a nozzle blocked and had to be replaced), comments as to operator reaction to such irregularities and replies from maintenance staff regarding reported malfunctions (see Fig. 7.2. columns Faults and Repairs: Reported to: Date Repaired:) etc. f) A number of rows for interval readings. Intervals should not be longer than 1 hour, preferably ½ hour. An example of a production log sheet for an evaporator and spray dryer with fluid bed and fines return for manufacture of instant whole milk powder is shown on Fig. 7.1 and 7.2.

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7. Principles of industrial production

Fig. 7.1. An example of a log sheet form for an evaporator. 113

Fig. 7.2. An example of a log sheet form for a spray dryer. 114

7. Principles of industrial production

7.3.2. General maintenance log book All components of the whole installation have to be serviced at regular intervals as specified by the manufacturer in the Plant Instruction Manual. Such service comprises oil change, lubrication, replacing of gaskets, filters etc. Beside a routine service there is sometimes a need for break-down service in a case of an accident, which requires immediate action.

Fig.7.3. - Check list for a spray dryer 115

A log book must be kept to record all service or intervention steps taken with description of the defect and the way the problem was solved. The remarks from the production log book as to malfunction or possible suspicion of malfunction of a component should be followed up by inspection of that component at the next production stop and the results of that inspection should also be recorded. It is also very useful to have a check list for control of the various components and to conduct such a control at regular intervals preferably each month. Such control involves, for instance, pressure drops across the air filters, cyclones and perforated plates, checking of thermometer and flow meter readings, safety pressure switches, functioning of the fire extinguishing equipment etc. A visual inspection of state of hygiene not only of the plant but of the whole plant building should belong to the everyday routine checks. It is also recommendable to organize a yearly inspection for potential hair cracks and welding failures in the dryer walls. An example of a check list for a small spray dryer with pneumatic transport system is shown on Fig.7.3.

Fig.7.4. An example of product quality specification. 116

7. Principles of industrial production

7.3.3. Product quality specification It is seldom that a factory produces only one product. Normally a number of products are produced differing in composition or identical as to composition, but different in quality requirements. For instance, a skim milk powder can be produced as a non-agglomerated, high bulk density powder or agglomerated with various requirements as regards bulk density, high-, medium- or low-heat. It is not unusual that a factory produces skim milk powder in more than ten various quality categories. For each quality a Product Quality Specification has to be elaborated. An example of such specification for Instant Whole Milk Powder is shown on Fig.7.4.

7.3.4. Operational parameter specification For every Product Quality Specification an Operational Parameter Specification is required. The name is self-explanatory. For internal factory communication, these specifications can be referred to by numbers and written in the headings of production log sheets together with numbers of Product Quality Specifications, as mentioned above. In modern computerized plants the parameter set points will be stored in ‘Recipes’, which are downloaded in the computer prior to start-up of the plant. Only the production management should have access for changes of these recipes. An example of Operational Parameter Specification is shown on Fig. 7.5.

7.4. Product quality control The target of any production is to obtain a final product, which meets all the requirements as specified by the appropriate Product Quality Specification and which has been manufactured using conditions as prescribed by the relevant Operational Parameter Specification as outlined above. However the latter has to be considered just as a general guide line. Such a specification usually expresses the operating parameters by a single figure which is valid for average ambient conditions or by a range covering extremes. Throughout the year, atmospheric conditions and milk composition are subject to variations. As to the former, quite considerable seasonal fluctuations occur, but also during a single day’s production. Thus successful manufacture of a product of standard and consistent quality is impossible without well-organized Process Quality Control and Final Quality Control.

7.4.1. Process quality control The aim of controlling the selected quality characteristics of both intermediate and final product during production is to provide information to the operating staff about possible deviations from the standard. Based on this information the operator has to evaluate the magnitude of possible deviation from the standard, if any, and decide whether production should either continue under unchanged conditions or whether adjustments have to be taken, i.e. which parameters and to what extent these should be altered to re-establish the standard quality parameter. The results of such process control are of value only when available regularly and shortly after sampling. Therefore in-process quality control has to be organised with rapid routine analysis methods. 117

Furthermore, the capacity of the installation must be taken into consideration, as it is obvious that with high capacity plant, substantial quantities of product can be lost as non-standard in a short time. Generally the sampling and intermediate quality control should be done in one or maximum two hours intervals. It should involve such properties, which are influenced directly by the spray drying operation and which are decisive for possible downgrading or rejection of the product. The following properties should be controlled:

Fig.7.5. An example of operational parameter specification.

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7. Principles of industrial production

a) Milk acidity at the beginning and end of each tank batch, b) Concentration from the evaporator, c) Scorched particles of the powder from the spray dryer (this is mainly for safety reasons indicating a fire risk), d) Moisture content of the final powder (in the case of a two-stage dryer, also several times during a day the moisture content after each drying step), e) Solubility index, f) Bulk density. The analytical methods for process control are often rapid routine methods with inferior reproducibility in comparison with the methods used for final control. It is therefore useful to compare frequently their results with the results of final control in order to modify their conditions, if necessary. Some of the above mentioned parameters (for instance, moisture) can be measured directly by in-line apparatus or can even control the installation. However, even such instruments need frequent check and calibration for each type of product by established laboratory methods. The results of process quality control are preferably recorded in Production Log Book together with processing parameters.

7.4.2. Final quality control The laboratory for final quality control must be equipped with all apparatus, instruments, chemicals and aids which are prescribed by the appropriate standard testing methods for all properties prescribed by Product Quality Specification. Besides, the laboratory should have facilities for such analytical methods, which are not directly related to the quality standards, but which are essential for special investigations, i.e. trouble-shooting. The laboratory staff should be qualified to conduct all types of analytical work. The sampling for final quality control has to be done in agreement with the appropriate standard as to sampling frequency, number of samples drawn from a batch, production run, silo, tote bin etc. The results are recorded in a Product Quality Book and are kept separately for each product. What is meant by the best quality of a product? It might be a subjective evaluation depending on the preferences of the person involved. One of the definitions is, of course, the quality required by the customer. A product is often specified according to more than ten various quality characteristics. However, none of them remains strictly constant during the whole production run. Each one is subject to some variations. A target of good manufacture is to obtain not only a top quality, but also a consistent quality. The judgement of quality is difficult, when comparing two products with ten characteristics, which deviate only slightly from the standard. Therefore it is useful to elaborate a Classification System, in which the whole production run (for instance 20 hours run with one hour sampling intervals) is evaluated as to quality and consistency, using a key based on penalty points deducted from the total sum of points for an ideal product. Such a system has to be elaborated individually and similar systems are in use by various companies in various countries. The laboratory manager is one of the key persons of a factory and should report directly to the factory manager. The responsibilities of the laboratory management include:

119

a) To follow daily the results of process quality control and its agreement with final quality control, b) To inform daily the production manager about the quality development, c) To keep the survey of quality development over longer periods expressing averages and deviations, d) To control the results of own laboratory by split-sample tests or in cooperation with other, possibly independent laboratories.

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8. Dried milk products

8. Dried milk products There are a great number of products manufactured in the milk powder industry including milk powders, modified milk powders, powdered milk by-products, and dried milk based products containing either milk solids only or also other foodstuff components. Therefore milk powder technology is a broad subject. The target of industrial production is to produce products which fulfil all the qualitative requirements. The influence of various technological parameters on powder properties is discussed in detail in section 10. Achieving product properties. This chapter presents just a survey of products with the required qualitative properties and technological guidelines for operation on various installations. Milk powders are defined as follows: a) Dehydrated products based on non-fat milk solids and milk fat, i.e. natural milk, the fat content of which has been adjusted by centrifugation or addition of cream or skimmed milk to achieve the fat content required in the final product. This is between 0.5 and 30% (expressed on total solids). Besides fat standardization, it is becoming more and more usual to standardize the protein content, i.e. to adjust it to a level of Frisian cows’ milk, i.e. 37-39% (total protein in non-fat solids) by means of the addition of lactose or permeate if the natural protein content of the milk supply is too high. Products involved in this group are skim milk and whole milk powders (designated often also as full cream milk powder) defined as milk powder with max. 1% fat and min. 26% fat respectively. Occasionally there are produced also powders with other fat content such as half cream milk powder with 14% fat or even others with a fat content inbetween these figures. Another product of this group is cream powder with a fat content 3580%. Permitted additives are the vitamins in various forms and some minerals.

b) Fat filled milk powders based on skim milk and vegetable or animal fat, possibly a mix of those, with a fat content 10-80%. Beside the components and additives mentioned above, other functional additives are used such as emulsifiers, stabilizers, flavouring and colouring agents etc. The inlet air temperatures as stated for the individual products in the subsequent text are valid at normal ambient conditions, i.e. with absolute air humidity max. 7 g/kg dry air. Higher air humidity will require reducing the inlet air temperature, (equal to reducing the evaporative capacity of a given dryer) or dehumidifying the air. Similarly, the feed concentrations are valid for milk of normal composition with max. 39% proteins in non-fat solids. Higher protein content requires reducing the concentration or standardizing with lactose or permeate to keep the viscosity below 100 cP at 40°C.

8.1. Regular milk powders Regular or ordinary is the usual designation for non-agglomerated products. Practically all types of drying installations can be applied. Generally, two stage and three stage drying processes bring many benefits to powder quality and economy. On the other hand they have adverse effects on bulk density which for regular milk powders is required to be as high as possible and is in fact one of the most important properties. This is due to some unavoidable agglomeration. A blow line transport to the silo or bagging-off point is normally sufficient to break down the agglomerates of two-stage process dried powders and to achieve bulk 121

density higher than with corresponding single stage process. Three stage drying systems however result in low bulk density and a mill may be needed to get the desired bulk density. As to bulk density the use of pressure nozzles gives higher values than the use of a wheel atomizer.

8.1.1. Regular skim milk powder Regular skim milk powder is a very important raw material in the food industry for further processing, and each application has some specific requirements to powder properties. The main users of skim milk powder are the bakery industry, recombining industry (for sweetened condensed and evaporated milks) and also the milk powder industry for fat filled powders for feeding calves. This involves dry mixing of regular skim milk powder with 40-60% fat concentrates to get a fat content corresponding to that of whole milk powder and to handle peak season surplus. Smaller amounts of skim milk powder, again produced during the peak season, go to the cheese industry for increasing the cheese production in winter. While the bakery industry requires powder of good water binding properties, achieved by denaturing of whey proteins, the cheese industry is asking for a powder with good rennet ability i.e. having whey proteins non-denatured. Therefore regular skim milk powder is produced in numerous varieties to satisfy the wishes of each user. The process step which influences the degree of whey proteins denaturation is pasteurization, and the conditions will be discussed in chapter 10: Whey Protein Nitrogen Index. Bulk density is another important property from the economical point of view (also discussed in chapter 10). The process involves separation, clarification, pasteurization in the evaporator to 70-120°C with 15-600s holding time, evaporation and spray drying. Pre-heating of the concentrate up to 80°C prior to atomization is strongly recommended.

Concentrate properties Solids content: 48-50% TS Viscosity: max. 100 cP at 40°C measured on the feed to be atomized. Measurements on concentrate not older than 15 min. and kept under vacuum between evaporator and sampling for viscosity measurements. Method Brookfield viscometer model LVT with spindle 2, rotation 60 rpm, measured at 40°C. Protein denaturation: For high bulk density powders the WPNI should be max. 1.0 mg. Solubility index: no measurable amount. Sieving test: no visible insoluble (cheesy flakes) on 250 micron mesh after passing 1 litre of concentrate through the mesh and washing with water. Scorched particles: no measurable amount. The obtainable bulk densities together with main operation conditions as given in table 8.1. (see also micro photo on Fig. 8.1.) anticipate that the powder is high-heat, the insolubility index 0.1-0.2 ml, and that the final powder is transported by air to a silo. The table indicates also the relative heat consumption based on the given conditions. The usual requirements for a good quality regular skim milk powder are: residual moisture of max. 4.0%, bulk density min. 0.72 g/ cm3 tapped 1250 times, insolubility index max. 0.2 ml and scorched particles disc A.

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Table 8.1. Bulk density of ordinary skim milk powder

8.1.2. Regular whole milk powder The process is identical to that used for skim milk powder production plus standardization to the required fat content and homogenization of the concentrate. Whole milk is more heat sensitive than skim milk; therefore heating of the feed prior to atomization is even more important. The homogenization is expected to be not higher than 80 bar total pressure with 20-30 bar in the second stage. The pasteurization in the evaporator is usually 95-110°C with 15150s holding time. Table 8.2. gives the obtainable bulk density for the various dryer systems. The quality of a good regular whole milk powder is characterized by moisture content max. 3.0%, insolubility index max. 0.2 ml, bulk density min. 0.60 g/cm3 tapped 1250 times, free fat content max. 1.5% and scorched particles disc A.

Concentrate properties Solids content: 48-50% TS Viscosity: Max. 60 cP at 40°C measured on the feed to be atomized. Measurements on concentrate not older than 15 min. and kept under vacuum between evaporator and sampling for viscosity measurements. Method Brookfield viscometer model LVT with spindle 2, rotation 60 rpm, measured at 40°C. Protein denaturation: For high bulk density powders the WPNI should be max. 1.0 mg, based on SNF. Solubility index: no measurable amount. Sieving test: no visible insoluble (cheesy flakes) on 250 micron mesh after passing 1 litre of concentrate through the mesh and washing with water. Scorched particles: no measurable amount.

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Table 8.2.- Bulk density of ordinary whole milk powder

8.1.3. Whole milk powder with high free fat content The traditional raw material for chocolate industry has always been a drum dried whole milk powder mainly because almost the total fat content in this product appears as free fat. In the milk powder industry, however, the spray drying process is, for many reasons, much more lenient than drum drying. Therefore efforts were made to produce a whole milk powder suitable for chocolate manufacture by spray drying. And successful results have been made. In a normal whole milk powder the continuous phase of a particle is the amorphous lactose which forms a very tight membrane protecting the globular fat against extraction. Thus an important condition for achieving high free fat content is to transform a substantial part of lactose into -lactose-monohydrate. Besides removing part of the lactose from the protective shell, the crystals formed create a net with craters and channels in between, through which the solvent for free fat determination can penetrate into the particles. The crystal formation as such exhibits also a positive effect on creation of free fat, affecting the fat globules with sharp edges. Thus crystallization of lactose is an essential part of the process. The industrial production is based on simultaneous atomization of two feeds, one of them a pre-crystallized skim milk concentrate of 45-48% total solids and temperature 30-35°C and the other, cream with fat content 50-90% (on total solids basis and possibly standardized with precrystallized skim milk concentrate to 45-48% total solids) in such proportion as to get a powder with 26-28% fat. This is done by a dual feed system supplying either two separate nozzle systems or a twin wheel (a double-deck wheel being able to atomize two feeds separately). The production can be done on any type of single stage dryer with cooling bed (see Fig. 5.3.) or Multi Stage Dryer (Fig. 5.9.). Obviously, because of high free fat content the biggest problem of this process is deposits in the dryer and especially in the cyclones. Therefore the cyclones are replaced by a bag filter. The deposit problems can also be reduced by introducing skim milk powder through a fines return system, i.e. to use the so-called powdering techniques, again in such rate as to get the required fat content of the final product. The use of the Multi Stage Dryer has the advantage that the crystallization of lactose can continue on the static fluid bed, if this is operated with an air temperature allowing high moisture content in the powder, contributing furthermore to the creation of free fat. 124

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The most important property is the free fat content usually required to be 85-95% of the total fat and final moisture max. 2.5%. There are no requirements as to insolubility index which is often quite high.

8.1.4. Butter milk powder Butter milk powder is a not too interesting or marketable product and is produced if other possibilities to get rid of butter milk are not available. Sweet butter milk in an amount up to about 25% of the total evaporator feed is sometimes used to standardize whole milk for making whole milk powder. However, the addition of butter milk increases the free fat content of whole milk powder and therefore degrades quality, especially of instant whole milk powder. Butter milk powder, both sweet and acid is used for cattle feeding blends.

8.1.4.1. Sweet butter milk powder The drying technology as to feed concentration and drying temperatures is identical to that of regular skim milk powder.

8.1.4.2. Acid butter milk powder Acid butter milk is difficult to evaporate due to the high acidity and therefore the highest concentration is about 30% total solids. Drying is also difficult because of the high content of lactic acid. The inlet air temperature should not be higher than 160°C in order to avoid excessive deposits.

8.1.5. Fat filled milk powder Fat filled milk powder is a common name for fat containing milk powder in which, instead of natural butter fat, animal fat or vegetable oil or even other types of emulsified fat-like material as lecithin, glycerin-monostearate etc. are used. The fat content can be up to 80% - provided that the carrier is maltodextrin - and products are used for further industrial processing in various food industries, mainly bakery, for dry mixing during preparation of mixtures for feeding calves and special kinds for human consumption i.e. so-called whole milk replacers. The production technology is based on mixing skim milk concentrate with a fat blend at the temperature well above the melting point of the fat, usually 50-60°C. The concentration of skim milk must be such as to get, after the addition of fat, a concentration of the feed for spray drying of 45-55%. Fat blends usually contain emulsifiers and stabilizers such as glycerinmonostearate, lecithin etc. Also other additives such as vitamins and minerals are often applied. For the calculation of total solids content of skim milk concentrate (TS-skim) and the amount of fat to be added to each 100 kg of that concentrate (kg fat/100 kg) to obtain the required total solids (TSR) and required fat content (FR), the following equations are used:

TS − SKIM =

100 TSR * TSR FR 100 − TSR [8,1] 1+ + 100 − TSR 100 − FR 125

kgFat / 10kg =

FR *TS − skim [8,2] 100 − FR

Example: Required fat content (FR) Required total solids of the mix

TS − skim =

52% 50%

100 50 * = 32.43kg 50 52 100 − 50 1+ + 100 − 50 100 − 52

For quick orientation, the results can be found using graph in Fig. 8.2. in which the above example is shown (the amount of fat in kg to be added to each 100 kg of skim milk concentrate is found using steps 1-2-3 and concentration of skim milk by steps 1-3-4).

Fig. 8.1. Required fat content in total solid The drying technology and type of spray dryer depends very much on the required total fat content of the product. For fat contents up to 35%, the same spray dryer design is used as for whole milk powder. For higher fat contents up to 60%, designs with cooling bed are required with falling tube transport of the fines to the bed from the cyclones which have to be designed for a low pressure drop. Powders with higher fat content than 60% require either tall form, multi stage or integrated belt dryer designs. Generally the higher the fat content, the lower the air inlet temperature and homogenization pressure and the higher the feed concentration.

8.2. Agglomerated milk powders The process technology is identical to that of regular powders, but the drying process requires separation and return of the fines to the atomizing device as described in chapter 4. Fines return system. The conditions of agglomeration are discussed in chapter 10. Agglomeration and instant properties. Contrary to regular powders, any kind of air transport of the final powder 126

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has an influence on the quality. However, new developed dense-phase powder transport systems with low conveying velocity and low air-to-powder ratio treats the agglomerated powder gently. However, the mean particle size after transport is always lower and the amount of fines and the bulk density are higher than before transport. Consequently, the functional properties are affected too. If the best possible quality of agglomerated powder is requested then use of any form of air transport is undesirable. The best way to handle agglomerated powders between the spray dryer and packing is a Tote-Bin system or possibly Big bag system. Agglomerated powders from the Multi Stage Dryer will however exhibit acceptable functional properties also after having been conveyed in modern conveying systems as mentioned in chapter 4.

8.2.1. Agglomerated skim milk powder Drying conditions: - Inlet air temperature for single stage processing in a conventional and TALL FORM DRYER™ with cooling bed 180°C, for two stage drying in conventional, TALL FORM DRYER™ and COMPACT DRYER™ 200°C, for Multi Stage Dryer, MSD™ up to 240°C, - Feed concentration for pressure nozzles and wheel atomizer 48-50%. All other requirements to the skim milk concentrate similar to what is mentioned in 8.1.1., however the WPNI should be between 2.5 and 3.5 mg.

Agglomerated skim milk powder has instant properties, if the mean particle size is higher than 180 μm and bulk density not higher than 0.48 g/cm3, the amount of particles smaller than 125 μm less than 20% and fat content not higher and preferably well below 1%. Even traces of free fat can be detrimental to wettability. The multi stage dryer equipped with fines return to the nozzles achieves bulk densities as low as 0.40 g/cm3. See also micro photo on Fig. 8.2. The usual quality requirements for instant skim milk powder are: moisture content max. 4.0%, bulk density max. 0.48 g/cm3, wettability max. 30 s, IDF-dispensability min 90%, insolubility index max 0.2 ml and scorched particles disc A.

8.2.2. Agglomerated whole milk powder Drying conditions: - Inlet air temperature for single stage process in conventional and TALL FORM DRYER™ with cooling bed 180°C, for two stage drying in conventional, TALL FORM DRYER™ and COMPACT DRYER™ 200°C, for Multi Stage Dryer, MSD™ up to 220°C, - F  eed concentration for pressure nozzles and wheel atomizer 48-50%, homogenization, preferably two stages at 80 bars for first stage and 30 bars for second stage. Agglomerated whole milk powder, in spite of better appearance, flowability and reconstitubility than regular powder is still not instant in cold water, only in water of temperature higher than 40°C.

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8.2.3. Instant whole milk powder The basic technology is identical to that incorporated in the production of agglomerated whole milk powder. However, the final product must fulfil many qualitative requirements which are, to a great extent, influenced by the individual processing steps. For a top quality product it is necessary to select carefully all decisive parameters, as follows: - standardization of fat, - standardization of protein content by addition of lactose solution or milk permeate to 37-39% (on non-fat-solids), - fortification with vitamins. Addition can take place as batch dosing in the standardization tank, or continuous dosing at the inlet to the evaporator or after pasteurization prior to the first evaporator stage. However, vitamin C in the form of ascorbic acid solution has to be added to the cold milk. In batch processing addition has to be carried out slowly under good agitation. In continuous processing, dosing is done preferably into the milk pipeline of good flow capacity. This is important, because it is an acid and poor mixing can cause local over-acidification and consequently precipitation. Another way to overcome these problems is buffering of ascorbic acid solution by sodium citrate or using ascorbic acid palmitate. - Pasteurization in the evaporator at 85-95°C with 0-180s holding, resulting in WPNI 2.5-3.5 (consult 10.7.4. Heat stability), - Homogenization two stage using 80 bar in first stage and 30 bar for the second stage, - Inlet air temperature for two stage drying in Conventional, TALL FORM DRYER™ and COMPACT DRYER™ 180°C, for Multi Stage Dryer, MSD™ up to 220°C, - Feed concentration for pressure nozzles and wheel atomization 48-50%, - Lecithin treatment using powdered lecithin dissolved in butter oil in 25-50% solution and dosing rate to get 0.15-0.25 lecithin on powder. The temperature of lecithin solution must be 60-65°C. The lecithin treatment in two stage drying systems is done usually between the two fluid beds. In the Multi Stage Dryer, MSD™ or the COMPACT DRYER™, the lecithination is done at the outlet from the static fluid bed just above the first section of an external fluid bed, - After lecithination, the powder has to be fluidized using warm air to keep the temperature well above 40°C, preferably 45°C, - The final powder is collected in tote bins or similar containers or conveyed to silos and kept at the above temperature until filling into tins or bulk transport containers, - The lecithinated whole milk powder has to be gas packed under inert gas, usually nitrogen in mixture with carbon dioxide to achieve residual oxygen content less than 2%. Otherwise the technological conditions for the production of instant whole milk powder can be found in Fig. 7.5. and the quality specification in Fig. 7.4. The instant whole milk powder is, as it has been emphasized before, the most important dried milk product and therefore the final quality is evaluated by many properties. 128

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The properties of a top quality instant whole milk powder are as follows: Moisture content max. Bulk density max. Scorched particles Wettability 25°Cmax. IDF-dispersibility NZ-dispersibility Sludge 25°C SDP 25°C Sludge 85°C SDP 85°C Free fat content Flowability

3.0% 0.48 g/cm3 disc A 15 s min. 95% max. Disc 2 max. 0.1 g max. B max. 0.1 g max. B max.1.5% max. 50 s

Fig. 8.2. Micro photo of an MSD™-produced instant whole milk powder (Copyright GEA Niro)

8.2.4. Agglomerated fat filled milk powder The technology for manufacturing of agglomerated fat filled milk powders is identical to that of agglomerated whole milk powder (see 8.2.2.).

8.2.5. Instant fat filled milk powder The technology for manufacturing of instant fat filled milk powders is identical to that of instant whole milk powder (see 8.2.3.).

8.3. Whey and whey related products Whey, both sweet and acid, can be dehydrated as such and also used as a raw material for a number of products. Modern processes such as demineralization, ultrafiltration and enzyme hydrolyzation have further expanded the product spectrum to modified whey powders, i.e. demineralized and hydrolysed products, whey protein powders and dried permeate. Furthermore, whey can be used as a carrier for fat during the production of fat filled whey powders. Most of these products are difficult to dry requiring special technologies and special equipment. 129

Whey is a valuable raw material requiring the same treatment and care as given to milk. The recommended procedure is cooling down below 10°C just after it is drained from the cheese vats to slow the bacterial activity. It is also strongly recommended to remove the so-called cheese dust by clarification and excess of fat by centrifugation, as residues of both will affect further processing. Lack of treatment resulting in developed acidity degrades the quality of final products and causes difficulties during drying. The composition of sweet and acid whey can vary very much, but average values for both liquid (after clarification and fat-centrifugation), solids and dry powder are given in Table 8.3. Table 8.3. The percentage composition of whey.

8.3.1. Ordinary sweet whey powder Sweet whey is a by-product from the manufacture of rennet fermented cheese and has usually a pH higher than 6.4. Lower pH is an indication of a developed acidity. Ordinary sweet whey powder can be obtained either by drying the whey concentrate directly from the evaporator or as a pre-crystallized whey concentrate. The operating conditions for ordinary whey powder without pre-crystallization are: concentrate total solids 42-45%, inlet drying air temperature 180°C and an outlet drying air temperature (around 90 oC) resulting in a moisture content of below 2% in the powder. For pre-crystallized ordinary whey powder, the concentration is about 55%, inlet drying air temperature 180-200°C and outlet drying air temperature (around 92°C) resulting in a “free” moisture content of below 2%. In both cases a single stage drying plant is used. The pre-crystallization process involves flash cooling of the concentrate from the evaporator to 30-35°C and transfer to crystallization tanks. These must be equipped with efficient agitation and cooling jackets. When the level in the tank is high enough so that the contents are rigorously agitated a seeding material is added. This is lactose in the form of finely ground -lactose-monohydrate, in the amount of 1 kg per each 1000 kg of the concentrate. The full crystallization tank is gradually cooled by the rate 2-3°C/hour down below 20°C. The whole crystallization process takes, inclusive tank filling, 24 hours, so that drying of the tank batch starts 24 hours after commencement of filling. The control of the pre-crystallization process is conducted by means of refractometrical readings before and during crystallization process. 130

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A good quality pre-crystallized whey concentrate should have the average crystal size of 3050 μm with single crystals not larger than 100 μm and the crystallization degree should not be lower than 70%. This powder will however still exhibit caking and hygroscopic tendencies if the powder is exposed to humid air. If a 100% non-caking and non-hygroscopic product is aimed at, then it is necessary to dry the pre-crystallized product in a plant with an after-crystallization belt see Fig. 5.13. or use the TIXOTHERM™ plant see Fig 5.14. Chapter 5

8.3.2. Ordinary acid whey powder There are several types of acid whey. All have pH 4.0-4.5, but depending on the type of acid they are very different as to behaviour during processing. The most difficult whey is lactic acid whey originating from the production of cottage cheese or quark. On the other hand, the hydrochloric acid whey from the production of casein is almost as easy to dry as sweet whey. Thus it is not the pH which causes problems, but the amount of acid. Lactic acid is a weak acid and to obtain the pH 4.6, which is the isoelectric point of casein, requires much higher amount of acid than in case of hydrochloric acid, which is a strong acid. Just like sweet whey, it is possible to dry acid whey either with or without pre-crystallization. The operating conditions for hydrochloric acid whey for both alternatives are identical to those for processing sweet whey. The hydrochloric acid whey can under certain circumstances cause pit corrosion of stainless steel and therefore it is advisable to use acid resistant steel in equipment fabrication. However, this corrosion does not take place during operation but usually when the plant stands. Therefore a spray dryer processing hydrochloric acid whey, irrespective of which material is used, should never be left in a shutdown mode for a longer period with powder on the walls. Cottage cheese and quark whey, especially without pre-crystallization, are very difficult products to dry. Neutralization by means of calcium hydroxide to transform the lactic acid to calcium lactate makes it easier, but this increases the amount of minerals significantly and thus is not very popular. However when used, it is very essential to add the lime (water suspension of calcium hydroxide) to the concentrate very slowly and under vigorous agitation to avoid local over neutralization. This will result in an almost unmanageable concentrate to dry. Using magnesium hydroxide to form Mg-lactate which is a dry salt is preferred by many end-users, and it does not result in dark colouring of the meat of the animals, typically calves eating the product. Sodium hydroxide for neutralization is not at all recommendable as Na-lactate is very hygroscopic and thermoplastic, and means just asking for more troubles. Single stage drying of both pre-crystallized and non-pre-crystallized concentrate requires 42 and 48% solids respectively, inlet air temperature 160°C and outlet air temperature (around 92°C) to secure a powder with a residual moisture content of less than 2%. Furthermore this process is very sensitive to ambient humidity and therefore it is almost impossible to operate during high humidity periods. These occur normally in the Northern hemisphere during late summer, i.e. in the period when most cottage- or quark whey is produced. Thus for factories specializing in drying lactic acid whey the FILTERMAT™ type spray drying plant is much more recommendable. Lactic acid whey concentrates are very thixotropic. Their viscosity, especially during precrystallization, can be so high that the concentrates almost solidify when cooled down below 20°C. Therefore the maximum concentration is 48% (maybe 50% but generally even 48% TS is too high) with the final temperature not lower than 20°C. 131

8.3.3. Non-caking sweet whey powder The production of non-caking sweet whey powder requires a pre-crystallization as described in 8.3.1. Concentration can be up to 62%. During crystallization, the viscosity increases whereby the higher the pasteurization temperature, the higher the viscosity. Thus, in order to control the viscosity within manageable limits, the pasteurization temperature should not be higher than 82°C with 15 s holding. Efficient pre-crystallization, which is 70% with 50% TS and 80% with 60% TS is very essential for this process. The drying is conducted in a straight-through drying system with 180°C inlet air and 80°C outlet air temperatures. The powder leaves the chamber with about 4.5% moisture content. This is reduced to 2% in the attached fluid bed. Even better non-caking quality and much higher drying economy is achieved on the so-called belt dryer. The transport belt, socalled crystallization or timing belt, provides the residence time of about 8 minutes between the drying chamber and the fluid bed after-dryer. The flow sheet of a belt dryer is shown on Fig. 5.13. An alternative to the crystallization belt is the successfully proven rotating disk. This is completely enclosed in a stainless steel housing and hence suitable for CIP, ideal from the hygienic point of view. The process using a belt dryer can operate with inlet air temperature up to 160°C and outlet air temperature 55°C. The powder leaves the chamber with 10-14% moisture. The residence time of 8 minutes on the crystallization belt creates conditions favourable for further crystallization, i.e. after-crystallization of lactose. The final product from this process has almost 100% of the lactose as -lactose-monohydrate. CDI and MSD™ plants can also be used. Here the static fluid bed functions like the crystallization belt, however the moisture content is not high enough and the powder residence time in the static fluid bed is not long enough to secure that 100% of the lactose is pre-crystallized. The best quality product is produced in a spray dryer with an external crystallization belt followed by a fluid bed after-dryer. (Fig. 5.13.). The same quality product can be obtained from the TIXOTHERM™ plant (Fig. 5.14.). The Integrated Belt Dryer, the FILTERMAT™, Fig 5.12. is also very recommendable.

8.3.4. Non-caking acid whey powder The drying plant described in the previous section can be used also for drying acid whey concentrates. As regards to lactic acid whey, all the problems and limitations discussed in section 8.3.2 can be expected here. Hydrochloric acid whey is processed under the same conditions as sweet whey. The drying can also be conducted in a straight-through plant (without the crystallization belt) with the following conditions: 48-50% total solids, inlet and outlet air temperatures 160 and 82°C respectively. The operating conditions for drying lactic acid whey with the belt process are: 48-50% total solids, inlet air temperature 160°C and outlet air temperature 56-60°C. The after crystallization process proceeds better, and the moist powder on the belt is not that sticky if the whey proteins are completely denatured by pasteurization at 90°C with 10 minutes holding.

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As a general rule, a straight-through plant with or without the crystallization belt has the advantage that other milk powders, both ordinary and agglomerated, can also be processed in the same installation (the dryer can be designed in such a way that the belt can be by-passed). If, however, producing lactic acid whey powder is the main duty, then the FILTERMAT™ dryer (see 5.3.5.) is preferable.

8.3.5. Fat filled whey powder Fat filled whey powder is used as an ingredient for stock food-mixtures. It contains 30-60% fat, usually lard or tallow. The higher the fat content the greater the difficulties with deposits both in the chamber and in the cyclones. Cyclones are therefore replaced by CIP-able bag filters, see chapter 4. With fat content 50% and higher, it is recommendable to use the socalled powdering techniques, in which a dry whey powder is introduced through a fines return system into the atomized spray cloud. In this case, the concentration of fat in the atomized feed and the amount of dry powder must be adjusted to get the required total content of fat in the final powder. Installations suitable for fat-filled whey include single stage dryers with cooling beds (5.1.3), and both CDI and MSD™ plants with the static fluid beds, operating with cold air. However, if the dryer is just for fat-filled whey powder the FILTERMAT™ concept is the most advantageous.

8.3.6. Hydrolysed whey powder The lactose in whey can be enzymatically hydrolysed whereby as much as 90% lactose can be transformed into glucose and galactose. Both these sugars have a high solubility in water and cannot be crystallised. The only drying plant which can successfully handle this duty is the FILTERMAT™ dryer (see 5.3.5.). The product is extremely hygroscopic, and therefore powder handling has to be done in a well air-conditioned room. Whey powder with 90% hydrolysed lactose, when exposed to the air of 50% humidity, can pick up more than 10% moisture within a few minutes. As both glucose and galactose have much higher sweetening power than lactose, the main use for this product is as an ice-cream sweetener etc.

8.3.7. Whey protein powder Production of whey protein powder is a good alternative to processing whey, since the product is much sought and has high value on the market. It is used mainly as a component in baby food formulations and also as a protein fortification in various food formulae. Standard products on the market contain 35, 60 and 80% protein. 80% is the most used and valuable product, which originates from an ultrafiltration plant as an approximately 25-30% total solids concentrate, which can be further concentrated in the evaporator up to almost 40%. Due to the high protein content, the powder tends to be very light and fluffy with a high content of occluded air. To minimize this, pressure nozzle atomization is preferred. The most used plant for this product is a two stage TALL FORM DRYER™ (TFD). Two stage drying is used to protect the proteins against denaturation. Other dryer types such as SDI and CDI operating with pressure nozzles can be used as well. If the final product should be agglomerated, then the MSD™ plant is used. Whichever dryer is selected, a bag filter is necessary.

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Fig. 8.3. shows the relationship between total solids content and density of 80% protein concentrate at 20°C.

8.3.8. Permeate powders A troublesome by-product in the manufacture of whey protein concentrate is the permeate consisting mainly of lactose and salts. It can be dried in similar plants and with similar technology as described for non-caking whey powder. High concentration, up to 66% and pre-crystallization are recommendable, but problems can be expected in the evaporator due to precipitation of calcium phosphate, especially with acid permeate. The drying process is either a single stage dryer with an inlet air temperature of 160 oC and outlet air temperature (about 90°C) to result in a powder with a maximum of 2% free moisture, or a two stage process with outlet 60-65°C and after crystallization belt followed by an after dryer. Also the TIXOTHERM™ process as described in chapter 5 can be used.

8.3.9. Mother liquor Mother liquor is a by-product from lactose production and is the liquor remaining after separation of the lactose crystals by centrifugation. The technology of lactose manufacture varies considerably and so does the composition of the mother liquor. The protein content can vary between 30-45% and generally the product having protein content higher than 40% is very difficult to dry. The pre-crystallization is, due to high protein content, difficult to conduct and therefore it is usually not applied. The recommended process is single stage drying with a cooling bed with inlet and outlet air temperatures 160-180 and 90°C respectively.

8.4. Other Dried Milk Products Almost every liquid milk or milk-containing drink or product existing on the market can be manufactured in form of a powder and therefore there are a huge number of different products which are used both as household consumer products and as raw materials in further industrial processing. The main products of this group are baby food, coffee-whitener, cocoamilk-sugar powder, cheese pow-der, butter powder and caseinate powders. 134

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8.4.1. Baby food The group of products designated generally as baby food or Infant Formulas contains a wide range of products of very different compositions and ingredients requiring diverse processing technology. The common feature of all infant formula products is compliance with high microbiological and hygienic standards together with an ease of reconstitution in lukewarm water without un-dissolved lumps. The basic baby food product is a whole milk powder and modified milk powder. The aim of this modification is to adapt the composition to closely resemble human milk. Such a product is designated as humanized milk powder. The composition of cow’s milk and human milk is shown in Table 8.4. Table 8.4.- The percentage composition of cow’s and human milk.

The ideal food for infants is breast milk. The first approach to developing alternative feeding methods was the application of sanitary standards for milk handling together with heat treatment ensuring the required microbiological standards. However, cow’s milk as such is inappropriate for feeding infants. The way to humanized milk was long, and was governed both by the level of paediatrician’s knowledge and the available technological possibilities of the dairy industry. The first step was just addition of sugar in the amount of about 20% in the dry matter. This type of baby food, i.e. whole milk powder with sugar is still produced in some Asiatic countries. The second step was the modification of fat by the addition of some vegetable oils with high content of polyunsaturated fatty acids, especially the essential acids as linoleic and arachidonic. The last step, i.e. the modification of the proteins became possible by the processes of demineralization and ultrafiltration. The full humanization step consists of replacing a part of butter fat with vegetable fat to get a similar composition as human milk fat, then increasing the content of lactose, replacing part of casein with lactalbumin, vitamin and mineral fortification. The main raw materials for manufacture are lactose, demineralized whey powder, whey protein concentrate, caseinate, malto-dextrin, lactulose (galactosido-fructose), fractionated coconut oil, sunflower, and corn and soy oil. The mineral additives are tri-calcium-phosphate, sodium and potassium citrates, magnesium and potassium chlorides, calcium carbonate and zinc, ferrous, cupric and manganese sulphates (possibly ferro-lactate or ferro-sacharate). The most common vitamins used include ascorbic acid, alpha-tocopheryl acetate, riboflavin, vitamin A 135

palmitate and vitamin D-3. Mono- and di-glycerol-stearate, lecithin and carrageenan are used as emulsifiers. However, apart from humanized milk powder, a number of specific nutritional and dietetic products have been formulated in the paediatric nutrition area for normal infants, premature infants and lactose-intolerant infants etc. including various acidified and fermented products. The composition of these products is a result of cooperation between research scientists, dairy chemists, clinical nutritionists, paediatricians and milk powder manufacturers. An extensive clinical testing and shelf-life evaluation of a new infant formula product are essential prior to marketing. The processing technology for each specific formula is proprietary to the manufacturer. However, the wet process for the dryer feed preparation employs traditional dairy processing equipment. Either batch or continuous processing is used. In general, the major ingredients are dissolved or dispersed in water or skim milk. The minerals, vitamins and emulsifiers are usually added at the end. After blending but before spray drying, the blend is heat treated and homogenized. The most important spray dryer requirement for baby food is the possibility of long periods of continuous operation without extensive deposits and maintenance of high hygienic standards. There are two philosophies as to operating a baby food spray dryer: - t o operate non-stop for as long as possible, usually one week and then wet wash the installation after each stop of production, - to operate non-stop for as long as possible but to use dry cleaning only, i.e. never water. To meet these criteria the most suitable types of spray dryer for infant food powders are TALL FORM DRYERS™ (TFD) and Multi Stage Dryer MSD™, both with a fluid bed for after drying and cooling. Use of a conventional spray dryer with either cooling bed or pneumatic transport system is also possible. From the viewpoint of drying, infant formulae are categorized as being the more difficultto-dry products. This is mainly due to high content of lactose (up to 60%) and other carbohydrates and, possibly also the acid content, either as added lactic acid or as a product of bacterial fermentation using cultures of streptococcus lactis and lactobacillus. Baby food products do not need to be truly instant, i.e. instant in cold water, because they are normally reconstituted in lukewarm water at the human body temperature. However, it is important that they are completely dissolved without even a trace of lumps which can clog the baby bottle teat. Therefore it is advantageous if the powder is slightly agglomerated. Due to the high hygroscopicity and stickiness of the powder, two stage drying is applicable only to a certain degree, and the products have to be spray dried to a moisture content of 3-4% followed by fluid bed drying and cooling to 2.5% moisture . The dry matter content of the feed can vary between 22-25% for fermented products to 55% for high-carbohydrate products. The drying air inlet temperature is in the range of 160-180°C. The feed temperature for high carbohydrate products is 80°C which is one of the conditions to ensure long operation times without bacteriological problems.

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8.4.2. Caseinate powder The most important caseinate product is sodium caseinate. The feed for the dryer is produced by dissolving, preferably freshly precipitated, casein curd in sodium hydroxide to obtain a neutral point around pH 7.0. The casein curd is prepared by precipitating skim milk by an acid (to bring the pH slightly below the isoelectric point, i.e. about 4.6), separating the whey and washing with water (usually twice depending on the desired purity). The acid used can be hydrochloric acid or lactic acid created by bacterial fermentation. The solids content of the feed for spray drying should be not much higher than 20% due to viscosity problems. Spray drying is conducted preferably using pressure nozzle atomization in order to get acceptable bulk density (wheel atomizer results in max 0.35 g/ml while pressure nozzles can give 0.5 g/ml). Bag filters are recommended to avoid unacceptable powder stack losses. The feed is spray dried using a feed temperature 90°C and inlet air temperature up to 250°C in conventional dryers but up to 320°C in a Multi Stage Dryer, MSD™. The outlet air temperature to get moisture content of 4-5% is about 90°C. The manufacture of calcium caseinate is in principle the same as for sodium caseinate. However, for re-dispersion and dissolving calcium hydroxide is used. Also drying conditions are similar. The composition of both sodium and calcium caseinates is given in Table 8.5. Table 8.5. The percentage composition of sodium and calcium caseinate.

Both sodium and calcium caseinate functions as a water binder, emulsifier, whipping agent and filler for food and meat products. It is used as a source of protein in dry cereal products, infant food, dietetic and diabetic products and it is also a useful component in coffee whiteners and toppings. In meat products such as sausages and other processed meat products, it improves the texture, binding the moisture and fat, while inhibiting shrinkage.

8.4.3. Coffee whitener The development of the so-called coffee whiteners or coffee creamers took place almost 40 years ago after experiencing that the milk powders, which were available at that time, when used in hot drinks like coffee and tea, flocculated creating an unpleasant appearance in the cup and sediment at the bottom. It was recognised that the main cause of these phenomena was flocculation of the whey proteins. The pH of coffee or tea is quite low, sometimes well below 5, which together with the temperature, in many circumstances almost 100°C, creates favourable conditions for denaturation of the whey proteins on the surface of the particles 137

before acceptable dispersion and dissolving could take place. The logical consequence was to develop a product without any whey proteins but that will exhibit the whitening power and taste of milk. The protein component of coffee whiteners is sodium caseinate; the carbohydrates are represented mostly by malto-dextrin and the fat by a mixture of vegetable oils. Emulsifiers, stabilizers and colouring agents are also used. The composition of various types of coffee whiteners, as referred to in the literature, is shown on Table 8.6. Today the industry master the technology for achieving excellent coffee stability of plain milk powders, and both instant skim milk powder and instant whole milk powder can be used for preparing coffee or tea with milk. However, the coffee whiteners were already well introduced on the market and as they are cheaper than milk powders they survive. The disadvantage of coffee whiteners compared to instant milk powders is their poor instant characteristics when the coffee is not that hot (i.e. below 50°C). Therefore further product development was directed to make a cold coffee instant powder, involving a lecithin treatment, conducted in the same way as for instant whole milk powder. Table 8.6. The percentage composition of 5 various coffee whiteners.

Coffee whiteners have to be well agglomerated, however without the presence of too large an agglomerate size, which otherwise will create so called floaters, appearing on the surface of the coffee as small lumps. The feed for spray drying is prepared by blending the components. Due to high content of malto-dextrin, the solids content of the feed can be rather high i.e. 6467%. Any type of two stage dryers with fines recycling, and spray dryers with integrated fluid beds can be used. The inlet air temperatures are 180°C (for MSD™ 220°C).

8.4.4. Cocoa-milk-sugar powder This type of product is a typical household and vending machine product for quick preparation of both hot and cold chocolate drinks. The composition of the final product can vary to a wide extent as shown in table 8.7. 138

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Table 8.7. The percentage composition of cocoa-milk-sugar powder.

With low amounts of sugar, up to 20%, it is possible to prepare the whole blend with 45-50% total solids and dry it using inlet air temperature of 180°C. With higher amounts of sugar, it is necessary to add a part of the sugar and possibly also cocoa powder in dry form through the fines return system to achieve agglomeration. For use in vending machines the product must possess good free-flowing properties. Most plants with fines return system are suitable. Most recommendable is the Multi Stage Dryer, MSD™.

8.4.5. Cheese powder Feed preparation for spray drying is similar to that of processed cheese. Good ripened cheese is first crushed into small pieces and agitated in a jacketed vat with water to obtain slurry of about 40% dry matter. A solution of stabilizing salts i.e. sodium citrate and disodium phosphate is added under vigorous agitation and the mix is gradually heated up to 80°C. Before spray drying the mix is submitted to a two stage homogenization. There may appear viscosity problems requiring adjustment of the total solids content. For spray drying, any type of spray dryer with cooling bed can be used with inlet air temperature 180°C. Cheese powder is a difficult product to dry as it has a tendency to deposit. Therefore chamber types with low possibility to form deposits, i.e. TALL FORM DRYERS™, are preferred. Another problem is the possible unpleasant smell of the exhaust air, requiring treatment by absorbing filters or bioscrubbers, especially if the factory is placed in a populated area. As there is a great variety of cheese types, the technology involved in feed preparation, drying and product composition can vary. However, cheese powder has roughly 50% fat, 40% protein, 3% carbohydrates, 4% minerals and 3% moisture. Cheese powder is used as an industrial ingredient in cheese biscuits, dressings and dipmixtures. As a consumer product, it serves the same purpose as grated parmesan cheese.

8.4.6. Butter powder The product which is called butter powder contains about 80% milk fat, i.e. almost the same content as in normal table butter. Obviously, this product is difficult to dry and handle. Sodium caseinate, non-fat milk solids, emulsifiers and stabilizing salts are used for preparing the mix for spray drying. The final moisture of the product is below 1%. To enable easier further processing, a free-flowing agent is added. The product is used exclusively in bakeries as a source of milk fat in dry form for making croissants. The Multi Stage Dryer, MSD™ and the FILTERMAT™ dryer are the types of dryers which have proved most successful for drying butter powder.

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9. The composition and properties of milk A great number of operational parameters and conditions influence the performance of a spray drying plant. One essential factor is the quality of the processed milk, evaluated not only according to general quality standards, but also composition and physical-chemical properties as influenced by the composition itself.

9.1. Raw milk quality The quality of the processed milk, both physical-chemical, bacteriological and organoleptic, must comply with the appropriate quality standard, which specifies the requirements to raw milk quality, taking into account the required quality of the produced product. Therefore this chapter deals only with the quality variations, which do not downgrade general quality.

9.2. Milk composition As with any other material of biological origin, milk composition is subject to geographical and seasonal variations as influenced by the breed of animal, lactation period, climate, region, feeding etc. All can have, under certain circumstances, great influence on the quality of the final product, especially if peculiarities of that milk supply have not been recognized and allowed for when choosing the processing technology. The gross composition of milk is given in Table 9.1. Table 9.1. - Gross composition of milk

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For a given region the main differences occur between spring and late autumn milk. From the technological point of view, the attention has to be paid to the quantitative variations of individual components especially those of non-fat-solids (NFS): a) fat/NFS ratio b) content of total protein in NFS c) content of lactose in NFS d) total protein/lactose ratio e) casein/albumin ratio f) mineral salts/protein ratio. Some of these variations are given in Table 9.2. Table 9.2. - Variations of some components

The above tables do not express any abnormalities, but just the variations of an average milk of various origins. These variations are an important factor in milk powder production, since extremes can influence the sensitivity of milk to heat treatment, heat stability, tendency to lactose crystallization in concentrates etc. The seasonal variations of the components in nonfat-solids in an area of North Island of New Zealand are shown on Fig. 9.1.

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Fig. 9.1. An example of seasonal variations of skim milk components.

9.3. Components of milk solids In this chapter only properties of individual components, which have direct connection to milk powder technology, will be highlighted.

9.3.1. Milk proteins Milk proteins consist of a complex of caseins (-, -, - and minor-caseins), which can be precipitated by acids or rennet, and serum proteins. Both groups exhibit different chemical and physical properties. Casein is the most important and the most characteristic protein of milk. In milk, it is in a form of a fine dispersion of particles (colloidal system) of deformed globule shape (casein micelles). It is sensitive to acids and rennet enzymes, which cause aggregation of individual particles forming flake-like or continuous gel precipitates. In the production of milk powders, it is important to avoid any casein flocculation and to retain its fine dispersion.

Among serum proteins belong -lactalbumin and -lactoglobulin. They are water soluble, and on casein precipitation they remain in the whey. Whey proteins in cow’s milk, in comparison with milk of other mammals and also with human breast milk, represent a relatively small part (about 22%) of total protein. Therefore in the production of humanized baby food powders, cow’s milk is enriched with whey proteins up to about 60% whey protein.

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9.3.2. Milk fat Milk fat is present as a dispersed emulsion of fat globules and their conglomerates. The size of fat globules varies considerably depending on various factors. Chemically, fat consists of triglycerides of fatty acids, both saturated and unsaturated. The proportion of individual fatty acids determines the chemical and physical properties of milk fat. Under the influence of lipolytic enzymes, milk fat decomposes into glycerol and fatty acids, resulting in a characteristic, unpleasant taste and smell (mainly of butyric acid). Oxidative decomposition induced or accelerated by day light or the presence of certain metal ions manifests itself by rancid taste. Milk fat globules are covered by a phospholipids/protein membrane. Under the influence of heat above 80°C, free sulfhydryl groups are created. They act as antioxidants protecting fat against oxidation. This is utilized during the production of whole milk powders. During homogenization, especially of concentrated milk, the size of fat globules is considerably reduced. This contributes to digestibility and reduces the amount of so-called free fat in milk powders.

9.3.3. Milk sugar Milk sugar or lactose is a carbohydrate, existing only in milk, in true water solution. It is a disaccharide C12H22O11 consisting of glucose and galactose and occurs in two isomeric forms - and -lactose (see Fig. 9.2). They have different physical properties, especially solubility in water and polarized light rotation. Both forms can crystallize, but for the production of milk and whey powders the most important is -lactose, which crystallizes as -lactose-monohydrate from the supersaturated solution of lactose below the temperature of 93.5°C. During the production of normal milk powders, the water evaporation during spray drying is so fast that despite supersaturation, the lactose cannot crystallize but remains in the powder as amorphous Fig. 9.2. Configuration of - and lactose, also called lactose glass. Amorphous lactose -lactose. is very hygroscopic. This can cause caking problems with powders having high content of lactose, as for instance whey powders. To avoid caking the lactose has to be crystallized as -lactosemonohydrate, which is non-hygroscopic. This is done by pre-crystallization of the concentrates. The rate of crystallization in solutions of lactose is controlled by the rate of mutarotation, i.e. transformation of -lactose into -lactose. The rate of mutarotation decreases with falling temperature, being fairly high in the range of 40-20°C, but practically zero at temperatures below 10°C. The solubility of lactose is shown on Fig.9.3.

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Fig. 9.3. Solubility of lactose. The specific optical rotation of -lactose in water is [] D/20°C = +89.4° and melting point 201.6°C under disintegration. The corresponding values for -lactose are [] D/20°C = +33.5° and 252.2°C. The equilibrium specific rotation is [] D/20°C = +55.3°. -lactose-monohydrate crystallizes in prism shapes often called tomahawk. These crystals in milk products are detectable on one’s teeth. With crystals larger than 10 μm there is a “flourish” and over 15 μm a “sandy” feel on the teeth. The density of pure -lactose-monohydrate is 1.54 g/ml. The sweetness of lactose is only 30% of that of sucrose. For whey powder manufacturing technology a most important physical property is the heat of crystallization, which is 10.63 kcal/kg. This must be taken into consideration when calculating the consumption of cooling water for the crystallization tanks. The relationships between various forms of lactose acc. to. King [3] are shown in Fig. 9.4.

Fig. 9.4. The relationships between different forms of lactose. 144

9. The composition and properties of milk

9.3.4. Minerals of milk From the point of view of the technology involved, the most important cations are the calcium and magnesium ions. These are bound to casein and to the phosphoric and citric acid anions. Casein has the maximum thermostability when bound to the optimum quantity of calcium. This is referred to as the salt balance. Low thermostability is mostly caused by a surplus of calcium. Thus addition of citric acid or phosphoric acid anions will improve the thermostability. The stabilizing salts used, also called sequestering agents, are secondary di-sodium phosphate or tri-sodium citrate.

9.4. Physical properties of milk Milk is a very complex polydisperse system. The individual components have various influences on its physical properties. The lactose and salts appear in true water solution. Fat exists in emulsion and partially in suspension, especially in deep chilled milk. The colloidal system is stabilized mainly by phosphoric and citric acid salts. Water is the main component of the colloidal medium. The fat appears as microscopic globules 0.1 - 20 μm in diameter. Casein occurs as submicroscopic particles 10 - 300 nm, albumin and globulin 5 - 15 nm. Lactose and minerals (both as molecules and dissociated salts) have the size 0.4 - 0.5 nm. Milk exhibits properties of both colloidal and true solution’s nature, which explains the various characters of its physical behaviour. Casein, which occurs as a calcium phosphoric complex, is hydrophobic and tends to cluster. It can be precipitated by acids at the isoelectric point, pH 4.6, or enzymatically by rennet. The degree of casein dispersion can be increased by dilution or reduced by intensive agitation, heating or boiling. The phosphoric and citric acid salts act as stabilizers of the colloidal system. The most important physical properties of milk in the production of dried milk powders are viscosity, density and heat stability. Variations in milk composition, mainly the protein/lactose ratio and the salt balance have greatest influence on these properties. Lactose is the main component of milk solids. Therefore the physical properties of lactose are reflected to a great extent in the behaviour of milk.

9.4.1. Viscosity Shear stress in a flowing fluid occurs due to friction between adjacent layers, moving at different velocities. The unit of dynamic viscosity is Newton second per square meter, Ns/m², but in industrial practice, the unit of centipoises, cP is used. The viscosity of liquid, untreated milk is influenced mainly by composition, temperature and age. The viscosity of milk concentrates during the evaporation process reflects the viscosity of the incoming milk. However, the viscosity is also influenced by heat treatment (i.e. by the degree of denaturation of whey proteins as caused by the sum of all heat treatment effects), temperature and concentration. Agitation and especially homogenization also increase viscosity. Milk and milk concentrates are non-newtonian fluids and exhibit thixotropic behaviour. There are two viscosity components: basic viscosity and structural viscosity. The structural component develops during storage (i.e. without movement) progressively with temperature. This viscosity increase in milk concentrates is known as age thickening and can be eliminated almost completely by high shear stress treatment. The viscosity measurement of milk concentrates has to be done at high shear rate or extrapolated to infinite shear rate to get the basic viscosity. 145

Viscosity plays an important role during atomization, since it is one of the decisive factors for the droplets size distribution and thus for the whole drying process. The structural component, unless it is excessively high, does not have too much influence during atomization, which proceeds under high shear stress. On the other hand it can have a strong influence where the milk flow is relatively slow as in the last stages of the evaporator, concentrate heat exchangers etc. High viscosity prior to these treatments may result in further increase of viscosity due to overheating of the boundary layer on the heating surface. The most important factor for increase of basic viscosity is the denaturation of the whey proteins by heat treatment, as expressed by Whey Protein Nitrogen Index (usually called WPNI). This increase in viscosity is caused by the ability of denatured whey proteins to bind an amount of water up to seven times their own weight. Therefore high heat milk concentrates require drying at higher outlet air temperature than low heat concentrates, if all other conditions are the same, to get powder of the same moisture content. The concentrate viscosity can be reduced by heating. This produces smaller droplets during atomization and thereby lower outlet drying temperature. This effect, however, is more noticeable at concentrations above 46% total solids. It is almost negligible below 44% TS. Extensive studies on viscosities of milk concentrates have been done by Snoeren et al. [48] who concluded that the viscosity depends on the volume fraction of the macromolecular material of milk and the viscosity of milk serum. They used Eiler’s [9] relationship between viscosity and the volume fraction occupied by the proteins. The latter depends on the content of casein, denatured whey protein and native whey protein (and thus on the heat treatment) of the liquid milk, plus concentration and temperature. Fig. 9.5-9.8 show Snoeren’s relationships of the viscosity vs. solids content as influenced by heat treatment (% denaturation of whey proteins), total protein content in non-fat-solids, temperature and homogenization. All these calculations are based on whey protein content of total protein 22.5%, liquid skim milk total solids content 9% with 0.5% fat in total solids, and liquid whole milk total solids content 12.5% with fat content 28% in total solids. Apart from the variable properties, the calculations relate to a total protein in non-fat-solids content 38%, denaturation of whey proteins 50% (medium heat) and concentrate temperature 50°C. Fig. 9.5. The influence of heat treatment on viscosity

146

9. The composition and properties of milk

Fig. 9.6. The influence of temperature on viscosity.

Fig. 9.7. The influence of protein content on viscosity.

Fig. 9.8. The influence of homogenization on viscosity.

147

Regarding the computation of Snoeren’s relationships, it must be emphasized that it expresses the viscosity as measured with infinite shear rate, i.e. it is the true viscosity without the structural component. The calculations are very sensitive and their accuracy or agreement with measured viscosities is to a great extent dependent on many factors such as calculation of concentrate and milk densities, water density and viscosity, viscosity of lactose solution, volume fractions of various components etc. Nevertheless these calculations are very useful in practice to predict or explain the influence of the various factors involved. These calculations are based on following equations:

ηConc

  φapp ∗ φ  1.25 φ  = ηserum 1 + φapp / φ   ∗φ  1− φ max  

ηserum = ηwater +

2



[9,1]

(0.002 ∗ηwater + η5%lactose )∗ DMconc 

[9, 2]

DM milk

φapp

for Phomog 0, 50, 75,100,125 bar  = 1,1.01,1.06,1.07,1.09 φ  

[9,3]



ηwater = 1.66 − 0.036244 t + 0.00033276 t 2 0.0000010631 t 3

[9,4]



[9,5]

η5%lactose = 0.102977 − 0.000603 ∗ t

[9,6]

φmilk = cc ∗ vc + cdw ∗ vdw + cdw ∗ vnw + cf ∗ vf 

[9,7]

c tp = c tp/ NFS ∗

100 − cf  100

 c wp  cc = c tp ∗ 1 −   100  cdw = c tp ∗ 148

c wp ∗ d wp 10000

[9,8]

[9,9]



[9,10]

9. The composition and properties of milk

c nw = c tp ∗

c wp 100

− cdw

[9,11]



in which: conc serum water 5% lactose app/  max c x

concentrate viscosity in mPa*s, viscosity of milk serum in mPa*s, viscosity of water in mPa*s, viscosity of 5%-lactose solution in mPa*s, the apparent volume fraction occupied by proteins, volume fraction occupied by proteins,

DM 

dry matter content in percent, density in g/ml as calculated using equations [9,14], [9,16],[9,17] and [9,18].

[9,12]

maximum volume fraction = 0.79, concentrations of total protein in %

of non-fat solids (c tp/NFS) and total protein (c tp),casein (cc), denatured whey proteins (cdw), native whey proteins (cnw) and fat (c f ) in % total solids, v  volume fractions of casein vc=3.57, denatured whey proteins vdw=3.09, native whey proteins vnw=1.07 and fat v f=1.075,

The viscosity of cream with various fat contents according to Free [11] is shown in Fig. 9.10.

Fig. 9.9. Viscosity of cream (acc. to free).

149

9.4.2. Density Density is the mass of a certain quantity of a material divided by its volume. It is expressed in kg/m³ or g/cm³ or g/ml. For milk concentrates of known composition, the approximate calculation of the density follows the Hunziker’s [12] equation:



[9,13]

where: %F



= percent of fat %NFS = percent of non-fat-solids %W = percent water fat = specific gravity of fat (acc. Hunziker 0.93) NFS = specific gravity of non-fat-solids (acc. Hunziker 1.608) water = specific gravity of water (Hunziker used 1 for t=15°C)

Concentrate density at a temperature, t, can be calculated with a reasonable degree of approximation by: 

[9,14]

Gosselin [13] modified Hunziker’s equation, so that it can be used directly over the entire temperature range using for specific gravities of fat, non-fat-solids and water equations: [9,15]

[9,16]

[9,17] The equation [9,13] can be used also for calculating the theoretical density of milk powder solids at laboratory temperature. The specific gravities of individual constituents use constant values: 0.94 for fat, 1.52 for non-fat-solids and 1 for water. For non-fat solids of whey powder 1.58 is used. The densities of skim and whole milk concentrates calculated using the modified Gosselin’s relationships are shown on Fig. 9.10 and 9.11 respectively.

150

9. The composition and properties of milk

Fig. 9.10. Density of skim milk concentrate.

Fig. 9.11. Density of whole milk concentrate with 28% fat in TS.

151

Fig. 9.12. Density of whey concentrate. The density of whey concentrates can be calculated using the empirical equation: 

[9,18]

in which %TS = concentration and t = concentrate temperature. The densities of whey concentrates are given in Fig. 9.12.

9.4.3. Boiling point The boiling point of milk under normal atmospheric pressure is about 100.6°C. It increases with higher concentrations. The increase is proportional to the molar concentration of the dissolved components. In practice it can be calculated by the equation: Boiling point increase in °C

=

% TS 92 - % TS

* 1.28 - 0.11[9,19]

9.4.5. Acidity Fresh milk is a complex buffering system of proteins, phosphates, citrates, carbon-dioxide and other minor components. During bacterial activity, lactic acid and other organic acids are created. The acidity of milk is normally determined by titration using NaOH solution and phenolphthalein as indicator (corresponding to pH 8.3). In various countries various expressions of titratable acidity are used. It is usually expressed in ml of NaOH solution, the strength of which and the comparison between various methods are given in Table 9.3. and Table 11.4. In spite of titratable acidity often being expressed “as lactic acid” the acidity of good fresh milk has only negligible amounts of true lactic acid. True lactic acid can be determined by an 152

9. The composition and properties of milk

enzymatic method and gives the real expression of acidity increase due to bacterial activity. Active acidity is expressed by pH. The pH of normal milk is 6.5 - 6.65 and decreases with increasing temperature. During concentration, the pH value decreases. Table 9.3. - Conversion factors of titratable acidities.

(multiply A by factor to get B)

9.4.5. Redox potential Redox potential expresses the energy with which a system can oxidize or reduce present or added components. It depends on the presence of oxygen, ascorbic acid, free sulfhydryl groups, trace elements and products of bacterial activity. It is also influenced by acidity.

9.4.6. Crystallization of lactose Lactose is quantitatively the major component of milk, as shown in Fig. 9.3. The solubility of lactose in water is relatively low and concentration and drying thus yield a supersaturated solution. This happens already when milk is concentrated to 45 - 50% total solids and has temperature below 45°C. Under normal circumstances during the production of whole or skim milk powders, however, the lactose will not crystallize and will appear in the final product as amorphous lactose or lactose glass, which can be considered a very high viscous or solidified solution. Also the ratio of - to -lactose remains unchanged i.e. same as in the concentrate. If, however, some crystal nuclei are present and the concentrate is kept some time in the supersaturated state, crystallization occurs. The same process will take place in moist, warm powder having moisture content higher than about 6 - 7%. From a supersaturated solution at temperatures below 93°C -lactose will crystallize as monohydrate since it is less soluble than -lactose. The rate of crystallization is governed by mutarotation, i.e. transformation of -form into -lactose. The rate of mutarotation is temperature dependent and is shown on Fig. 9.13.

153

Fig. 9.13. Mutarotation rate of lactose.

For the production of normal milk powders the crystallization of lactose is undesirable and has to be avoided. It is very undesirable in the production of whole milk powder. The consequence of lactose crystallization in whole milk powder is an increase of the free fat content because the amorphous lactose, being a continuous phase of milk powder particles, creates a very tight membrane (encapsulation) through which the solvent cannot penetrate thus protecting the fat against extraction (and also against oxidation). Crystallization transfers the lactose from the continuous phase to a discontinuous phase, thereby creating craters and channels, enabling penetration of solvent. This can happen if some nuclei of lactose are present already in the concentrate due to high concentration and low temperature, and possibly also with long holding times before drying and/or if a semi-dried product is held for longer periods with a moisture content above 6 - 7%. On the other hand, high free fat content is an advantage for milk powder for chocolate industry. Establishing intentionally the above conditions makes possible the production of whole milk powder with over 90% free fat content (of total fat content). The presence of crystallized lactose in milk powders can be traced by scanning electron microscopy (SEM) and microphoto techniques in polarized light. An example of lactose crystals in whole milk powder is shown on the polarized light-microphoto Fig. 9.14. The reason for the presence of lactose in whole milk powder was too high moisture content from the first drying stage in an SDI dryer. In baby food powder the reason was not fully dissolved lactose in the mixture prior to drying. In both cases the presence of lactose crystals has resulted in high free fat content. Amorphous lactose is very hygroscopic and is the main reason for hygroscopicity and caking of powders with high content of lactose such as whey or permeate powders. In order to reduce hygroscopicity and caking tendencies, the lactose has to be transformed into -lactose-monohydrate, which is non-hygroscopic. This is done by pre-crystallization of the concentrate prior to spray drying. It is possible to achieve above 80% pre-crystallization. Even higher crystallization in the final product is possible using a special drying process in which the crystallization, so-called after-crystallization, continues in the moist powder leaving the spray dryer chamber before final drying. Final powder can have over 90% of lactose as -lactose-monohydrate and such product is non-caking. 154

9. The composition and properties of milk

Fig. 9.14. Polarized light photos of whole milk powder particles with lactose crystals.

155

9.4.7. Water activity The water activity (aw) of dried milk products is largely a function of moisture content and temperature. The composition and state of the individual components, as influenced by various processing techniques, also play an important role. The composition of the solids is given more or less by the contents of proteins. At low moisture content characterized by a w <0.2 (corresponding for instance to skim milk at <4% moisture) it is the casein, which is the main water absorber. Within the intermediate range of a w >0.2 and <0.6 (about 4 - 16% moisture on skim milk) the sorption behaviour is dominated by the transformation of the physical state of lactose. Above this level the salts have a marked influence. As to the influence of temperature at low moisture levels up to about 12-18%, water activity increases with rising temperature. Above 12% moisture temperature has only minor influence, as reported by Warburton and Pixton [54] and above 18% moisture, i.e. in the area where the salts start to contribute significantly to the water activity, the influence of temperature is opposite. The water activity of milk powders consisting of milk non-fat-solids and milk fat is predominantly controlled by the moisture content expressed on non-fat solids since the fat has no influence. Thus differences in water activities are due mostly to the state of proteins and physical state of lactose. The main interest in water activity relationships concerns the connection between predicting and controlling the shelf life of foods. It has been recognized that the growth of most bacteria is inhibited at water activities lower than 0.9 and for mould and yeast strains between 0.880.80. Furthermore, many physicochemical changes as enzymatic reactions, Maillard reaction, lipid oxidation, textural changes, crystallization of carbohydrates, aroma retention etc. are, to a great extent, controlled by water activity. As water activity plays important roles during the dehydration process, knowledge of desorption isotherms can provide useful guidelines for the design, engineering and the control of the drying process. Numerous publications are available in the literature presenting sorption isotherms of various dried dairy products. Attention has been paid to milk powders and other dehydrated products, such as whey powders, whey protein concentrates, caseinates, baby food powders etc. It has been recognized that water activity, beside the moisture content and overall composition, also depends on the pre-treatment to which the material has been subjected prior to and during dehydration. Practically all the isotherms of milk powders, which can be found in the literature, were obtained using the final product as the starting material. This starting material is exposed to air of well-defined and known relative humidity and brought to equilibrium, after which the moisture content is determined. Thus the published isotherms are designated as adsorption, desorption and re-adsorption (second adsorption) isotherms. Establishing equilibrium can very often take weeks and especially during adsorption many changes can take place when the moisture is rising. As to milk powders it is mainly crystallization of lactose which takes place when the moisture level rises to around 7%. The amorphous lactose forms about half of the content of non-fat-solids of milk and about three quarters of whey solids. Transformation to -lactose-monohydrate, which is almost inert, has a quite dramatic influence on the shape of isotherms. They exhibit a sudden “break” at which the moisture drops at constant water activity. This drop corresponds often to the theoretical amount of water consumed by the crystallization of lactose. However, the test material has been irreversibly changed and any 156

9. The composition and properties of milk

further data during the continuation of absorption, desorption and reabsorption are of little value because it has been conducted on a product different from the starting material. Many mathematical equations, both theoretical and empirical, have been reported in the literature for expressing water sorption isotherms of milk powders. Iglesias and Chirife [14] have compiled from the literature experimental data on water sorption isotherms of hundreds of food products, among them many powdered milk based products. Referring to several mathematical two parameters equations for expressing water sorption isotherms, they applied regression programmes to the published experimental data. For instance as regards skim milk powder, they applied Halsey’s [15] equation,

 b a w = exp  − a [9,20]  x  expressing the moisture content x on a per cent dry basis, i.e. kg H2O/kg dry matter*100 on the data of Berlin et al. [16] measured at 34°C and found the constants: a = 2.0544 and b = 54.3870 for 1.cycle desorption and, a = 1.7764 and b = 24.8439 for 2.cycle adsorption (reabsorption). Obviously no attempts were done to express first cycle adsorption due to the mentioned irregularities, i.e. curve break. Two other sorption models often used are the BET (Braunauer, Emmett & Teller) and the GAB (Guggenheim, Anderson & de Boer) models: BET model:



[9,21]

GAB model:

[9,22]

where:  aw = water activity

m = moisture content in g/g total solids mo = monolayer moisture content in g/g total solids c & k = constants

157

To avoid these problems connected with crystallization of lactose during prolonged exposure to humid atmosphere and to obtain water activity values under the primary desorption. Písecký [17] conducted series of measurements placing the sensor for water activity and temperature directly in the drying chamber. The measurements were conducted on skim milk powder, whole milk powder with 27% fat and two fat filled milk powders with 42 and 46% fat respectively. Based upon regression analysis of the measured values, the following empirical equations have been developed for calculation of water activity during primary desorption, i.e. during the dehydration:

[9,23]



[9,24]

[9,25]

[9,26] or, 1

x SNF

1n a w  n =    m 

[9,27]

where the constants are: a = -1.2988 and b = -1.208*103 for [9,24] c = 1.4626 and d = -3.7668*105 for [9,25] and:

%M = percent moisture on wet basis

%F = percent fat on powder xNFS = moisture content kg/kg non-fat-solids Agreement between measured and calculated values for 144 measurements of samples in the fat content range 0 - 46%, moisture range x NFS 0.0345 - 0.1337 and temperature range 20 - 60°C was expressed by mean relative deviation E = 2.59% and variance v = 0.0758 [17]. The empirical equations are for general use in predicting moisture content of milk powders on discharge from the spray dryer from measured values of water activity, temperature and fat content, and vice versa. The equations have been shown applicable over a wide range of all variables. Fig. 9.15 illustrates the moisture content xNFS plotted against water activities at 20, 34 and 80°C. For comparison are shown also published data by Berlin for skim milk powder at 34°C on first cycle adsorption, exhibiting the typical break at a w 0.45, due to crystallization of lactose.

158

9. The composition and properties of milk

Fig. 9.15. Water activity of milk powder acc. to Písecký with data by Berlin for first cycle absorption. The water activity is one of the main factors governing many of the phenomena occurring during thermal dehydration, mainly: a) ease of which water is evaporated from a liquid droplet, b) particle temperature history during the whole water removal process (see also section 3.2.3. Droplet temperature and rate of drying), c) the equilibrium moisture content which can be achieved under given conditions at infinite residence time, d)  the stickiness of the product (sticking temperature) and the outlet conditions (air temperature and moisture content) that can be used to dry without sticking problems occurring.

9.4.8. Stickiness and glass transition Powder build-up on drying chamber walls is a well-known phenomenon, caused by certain product properties described with terms as Stickiness or Thermoplasticity. Thermoplasticity is a very descriptive term that implies that the product plasticizes at elevated temperatures. It is well known that increased powder moisture increases the stickiness of the products. The relation between powder moisture and T-out, influenced by other drying parameters as well, is shown later in Fig. 10.1. It shows that increased T-in or % TS changes that increase the plant capacity - will result in increased powder moisture content and potentially in powder build-up if not compensated for by T-out. Increased ambient air humidity will have the same effect. However, T-out can only be increased within limits, partly for product quality reasons, but also because the powder particles become sticky at a certain T-out due to thermoplastic behaviour, even though the moisture content may be quite low. This phenomenon is shown in Fig. 9.16. It should be emphasized that the sticking curve is a product property depending on product composition only, whereas the T-out – moisture relation (and hence T-particle – moisture relation) as well is dependent on product properties (composition) as drying conditions (T-in, % TS, etc). 159

The sticking curve approach described above is a rather empirical one, but the use of the glass transition concept in the last decade or more has formed a more theoretical basis of understanding the phenomena.

Fig. 9.16. Empirical sticking curve. Relationship between the moisture content, outlet air temperature, particle temperature and the sticking point temperature.

A glass is an amorphous, high-viscous liquid in a non-equilibrium state, exhibiting mechanical properties of a solid, but with structural characteristics of a liquid, i.e. contrary to the crystalline state a glass is without any ordered molecular arrangement. Due to the non-equilibrium state, a glass is thermodynamically unstable and can undergo phase transitions. The glass transition in amorphous systems is a reversible change in the physical state from a mechanically solid glass to a visco-elastic, rubbery state, which takes place at the characteristic glass transition temperature, Tg. Tg of a product depends on the product composition and in particular on the presence of plasticizers, of which water is very potent, that is the Tg declines drastically at increased moisture content. In spray dried milk powders the lactose is usually in an amorphous state because the drying takes place so fast that there is no time for the molecular ordering required for crystallization of the lactose. Hence normal milk powders exhibit glass transition. Although the glass transition temperature of a milk powder is not identical to the ‘sticking temperature’, defined as the temperature at which powder build-up in dryers may occur, there is still a relation between Tg and the sticking temperature. Indications are that the ‘sticking’ temperature is about 20 - 25°C above the Tg. Glass transition is accompanied by measurable physical changes in viscosity, heat capacity and others. The change in heat capacity can be measured Differential Scanning Calorimetry (DSC).

160

9. The composition and properties of milk

Models describing Tg in binary system (one component and a plasticizer) are available. The Gordon – Taylor model [9,28] has been used extensively, but also the more elaborate Couchman – Karasz equation [9,29] is used.

Tg =

Tg =

w1 ∗ Tg1 + k ∗ w 2 ∗ Tg2 w1 + k ∗ w 2



w1 ∗ c p1 ∗ Tg1 + w 2 ∗ c p2 ∗ Tg2 w1 + w 2 ∗ c p2

[9,28]



[9,29]

where: wi = weight fraction of component i, Tgi = glass transition temperature of component i, C pi = specific heat change of component i. Table 9.4 shows the glass transition temperature of some food components. It can be seen that lower molecular weight carbohydrates exhibit lower Tg (mono-saccharides < di-saccharides and the higher the DE of maltodextrines, the lower the Tg). The high Tg of starch and low DE maltodextrines explains why these components are good ‘carriers’ for more difficult components in spray drying. The extremely low Tg of lactic acid also explains the difficulties of drying acid whey. Table 9.4. Glass transition temperature of different food components

Component

Tg (oC)

Fructose

5

Glucose

31

Galactose

32

Sucrose

62

Maltose

87

Lactose

101

Maltodextrin DE 36 (MW ~ 550)

100

Maltodextrin DE 25 (MW ~ 720)

121

Maltodextrin DE 20(MW ~ 900)

141

Maltodextrin DE 10 (MW ~ 1800)

160

Maltodextrin DE 5 (MW ~ 3600)

188

Starch

243

Lactic acid

-60

Water (amorphous)

-135

161

Although the relation between a w of milk powders and Tg is basically sigmoid it has been found to be almost linear in the aw range from 0.2 – 0.65,

Tg = -143.6 a w + 77.8 

[9,30]

Knowledge of the parameters in [9,28] or [9,29] for a given product and drying process together with knowledge of the corresponding desorption isotherm and the glass transition temperature as a function of aw can be very helpful in defining optimized, but still safe drying conditions.

162

10. Achieving product properties

10. Achieving product properties About 40 years ago milk powder quality was evaluated using the same criteria as for liquid milk products. The aim of such evaluation was: a) to ensure that the final product met the specified composition i.e. fat content, total solids content and possibly content of other ingredients, if any (sugar etc.). b) To ensure that the product during processing was not affected by some undesirable microbiological or chemical processes. The only properties related specifically to milk powder were solubility index and content of scorched particles. Later on it was found that it was possible to influence the properties of the final product by certain pre-treatment processes, by choosing certain conditions for evaporation and spray drying, by dividing the water removal process into spray drying and fluid bed drying and by applying various after-treatment processes. This resulted in the development of many new products with properties tailor-made for a special end-use, i.e. having special functional properties. The properties of the final products are influenced by a number of factors involving quality and composition of the raw milk and operating conditions applied. As some of the factors are subjected to both seasonal and daily variations, it is necessary to frequently control those properties, which might be affected by those variations and to make the appropriate correction to the operation parameters.

10.1. Moisture content The moisture content of the final product is a property, which is required by the product specification, defining the permissible maximum level (for instance max 3 %). From the point of view of functionality, too high a final moisture may result in inferior shelf life due to Maillard reaction, creation of lumps, and possibly bacteriological problems or growth of yeast and mould. Thus the moisture contents for individual products and that required by legal specification have been chosen with respect to the above. The final moisture content is important from the point of view of final powder quality and achieving a standard product. From the economical point of view, it is important to operate as close as possible to the limit. In large spray drying installations each 0.1% of moisture can represent a great sum of money on a yearly basis. However, not less important is the intermediate moisture content of a product leaving the individual processing steps during two stage or three stage drying. The intermediate moisture content has great influence on such properties as solubility index, bulk density, particle density, agglomeration (i.e. particle size distribution) and also on overall drying economy. In a single-stage dryer the final moisture is influenced by combination of factors involving properties of the feed (concentration, temperature and viscosity), conditions of atomization (rotating wheel atomizer speed or atomization pressure with pressure nozzles) and conditions of the drying air (inlet and outlet temperature and absolute humidity). The magnitude of influence of some factors on the moisture levels ex-drying chamber is known. For instance, as shown in Fig. 10.1, an increase of inlet temperature by 10°C, ambient air absolute humidity by 2.8 g/kg or total solids of the feed by 1% and reduction of the outlet temperature by 1°C 163

will result in an increase of powder moisture by 0.2% with skim milk and by 0.16% with whole milk. The direction of change for other factors is indicated by the ±-symbols but not the exact magnitude.

Fig. 10.1. The influence of various factors on powder moisture Fig. 10.1 shows how the moisture content is influenced by variations of a number of other factors. The absolute value of the outlet temperature is determined also by a number of other factors, mainly the efficiency of the mixing of the cloud of droplets with the drying air. Fig. 10.2 shows the powder moisture (ex-chamber) as a function of outlet temperature for both skim and whole milk at the inlet temperature 180°C and feed concentration 48%.

Fig. 10.2. The relation between powder moisture and outlet temperature

164

In a two stage drying system the intermediate moisture should be kept reasonably constant because, as mentioned above, it influences several other properties. The importance of the second drying stage is the fine adjustment of the final moisture below the rejection level from the standard quality point of view, but at the same time it should be as close as possible to that level from the economical processing point of view. Furthermore, it is also important during the second drying stage and cooling stage to ensure continuous reduction of moisture, as the powder passes through the drying/cooling fluid bed system. As mentioned in section 3.6, the milk powders, when cooled down to low final temperatures, can pick up moisture from the cooling air.

10. Achieving product properties

Depending on the type of product and humidity of the cooling air, this process may start already at a powder temperature of 34-40°C. Therefore it is advisable to check the powder moisture content both before and after the cooling section. The traditional way of in-process moisture control consists of sampling powder from the various stages of processing at regular intervals and checking the moisture by fast routine methods. The in-line infrared measurement is now more and more common. This can be combined with direct in-line moisture control and in computer controlled spray dryers even with feed-forward system adjusting the set-point of the outlet temperature according to variations of the factors shown on Fig. 10.1. Such a system is able to keep the standard deviation of the final moisture below 0.1%.

10.2. Insolubility index One of the first defects observed in milk powders was the presence of some insoluble material when centrifuging reconstituted powders. It was especially the case with whole milk powders. Waite and White [18] concluded that protein was carried into the fat layer and fat into the sediment layer in quantities directly related to the degree of insolubility. Furthermore they concluded that a well washed sediment consists mainly of calcium caseinate together with calcium and phosphorus in the same proportion as in tri-calcium-phosphate Ca3(PO4)2, with most of the calcium-phosphate probably being in the form of a casein-phosphate complex. Howat [19] and Wright [20] concluded that the creation of an insoluble form of milk protein during the drying process is most probably identical in nature to the ordinary heat coagulation of a protein. As mentioned above, the insolubility problem is much more severe with whole milk than with skim milk and is even more emphasized by homogenization. In more recent work Mol [21] came to the conclusion that whole milk concentrate is much more sensitive to drying temperatures, if the concentrate is standardized by “casein cream” (i.e. normal cream) than when using “casein-free cream” (based on whey proteins). Therefore he concluded that the impairment of the heat stability is due to casein micelles absorbed on the fat globule membrane. There have been several methods elaborated for determining the insolubility of milk powders, some of them gravimetric. For routine purposes the most widely used procedure was developed in USA by American Dry Milk Institute and illogically called Solubility Index. This method was modified and defined with more precision by IDF (International Dairy Federation) in 1988. In order to distinguish this method from the original ADMI method and also to follow a more logical approach it was named Insolubility Index. The main factor controlling the insolubility index is the particle temperature during the first drying stage from initial feed moisture content down to below about 10%. It is said that the most critical phase of first stage drying is when the powder moisture lies between 20 and 10%, whereby the critical factor is the powder temperature. The factors influencing the powder temperature are in fact all those shown in Fig. 10.1, i.e. all factors increasing the outlet temperature and thereby the whole profile of the particle temperature. Thus, when faced with insolubility index problems, attention must be given to all factors that increase the viscosity, droplet size and generally the outlet temperature. Means to reduce insolubility index include:

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a) reduce pasteurization effect, i.e. lower the temperature and/or reduce the holding time, b) increase the feed temperature by concentrate heating, c) avoid long holding of the concentrate before drying and especially after reheating, d) reduce the protein content by adding lactose, e)  reduce the homogenization to the lowest possible level and use two stage homogenization in case of fat containing powders, f) reduce total solids content of the feed, g) apply higher atomization pressure with nozzles or higher speeds with atomizer wheels, h) reduce the air inlet temperature, i) reduce the outlet temperature.

The above are just general rules and the decision which of them has to be applied depends on many other circumstances. However, in most cases just one of them is often sufficient to solve the problem. The strongest factor controlling the insolubility index is the powder temperature and this is mostly influenced by the outlet temperature.

Fig. 10.3. The influence of outlet temperature on insolubility index at constant inlet temperature of 200°C and 48% concentrate total solids

The relationship of insolubility index and outlet temperature for both skim and whole milk is shown on Fig. 10.3.

The rules above for troubleshooting insolubility index problems are valid generally. However, various types of dryers, even when operating with identical conditions, can produce powders of very different insolubility index. A very important factor is namely the efficiency of mixing the atomized droplets with the drying air, i.e. the design and the adjustment of the air disperser. Agglomeration also has a slight adverse effect on insolubility index as this affects the rate of evaporation by reduction of the evaporative surface and thereby increasing the outlet temperature.

10.3. Bulk density, particle density, occluded air Bulk density expresses the weight of a volume unit of a powder and in practice is expressed in g/cm3, kg/m3 and more seldom g/100ml. The reciprocal value of bulk density, often incorrectly called “bulk density” as well, is the bulk volume and is expressed as volume in ml of 100 g of powder. Bulk volume is also often used in the milk powder industry, as it is a value received directly from the analytical method.

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Bulk density of milk powders is a very important property, both from the point of view of economy, functionality and market requirements. When shipping milk powder in bulk over long distances the producer is interested in high bulk density to reduce shipping costs, since in most cases transportation costs relate to volume. Also, high bulk density saves packaging material for a given weight shipment. Low bulk density may be interesting from the marketing point of view, so that larger amounts of powder per given weight are seen on the shelves of a supermarket than in a package of a competitive brand. Furthermore low bulk density as achieved by agglomeration is an important factor influencing other powder properties, mainly flowability and instant properties. Currently the bulk density is determined by measuring the volume of 100 g of powder in a graduated glass cylinder of 250 ml after loose filling, followed by tapping manually or using apparatus designed solely for this purpose (10, 100, 600 or even 1250 times). See also 11.3. The bulk density of milk powders is a very complex property being the result of many other properties and being influenced by a number of factors shown in Fig. 10.4. The primary factors determining bulk density are: a) the density of the solids (see 3.1.2), b) the amount of the air entrapped in the particles (occluded air) or the particle density, c) the sphericity of the particles, determining the amount of interstitial air, i.e. the air between the particles or agglomerates, inclusive of the air inside porous agglomerates. Ideal spherical-shaped particles or agglomerates create low content of interstitial air resulting in higher bulk density powders, while irregularly shaped agglomerates with attached smaller particles lead to lower bulk density or bulky products. The content of occluded air together with the density of solids (given by the composition of the solids) determines the particle density. This together with the content of interstitial air results in bulk density of the final powder. For non-agglomerated powders the content of interstitial air is given exclusively by the particle size distribution. The broader the size distribution, the higher is the bulk density.

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Fig. 10.4. Factors influencing bulk density The mechanism for creation of occluded air is as follows: high protein content and especially non-denatured whey proteins support the feed foaming ability. Therefore low-heat products always have higher contents of occluded air than high-heat products. Aeration of the feed before spray drying, i.e. vigorous agitation or often just by filling the balance tank from the top with liquid falling onto the surface incorporates air into the feed. During atomization with rotating wheels, air is drawn into the feed, the extent depending upon wheel type (see 4.4.1). This effect is, however, negligible with pressure nozzles. It is difficult to predict the bulk density of agglomerated powders from particle size distribution. The content of interstitial air is a dominating factor here, overriding the influence of other factors. The final factor controlling the bulk density, especially of agglomerated powder and to a much lesser extent also of nonagglomerated powders, is attrition as a result of mechanical treatment of the powder during pneumatic transport, vigorous fluidization or even packaging in bag- or can-filling machines. Therefore mechanical treatment during production of high quality instant products must be minimised. 168

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The series of graphs in Fig. 10.5 demonstrate interesting work conducted by Mol [22]. The full lines here show the viscosity of the skim milk concentrates and various powder properties when working with constant concentration and using heat treatment resulting in values of WPNI 2, 4 and 6 respectively. The dotted lines show the same. However, here the viscosity was kept constant while the concentration was varied. Otherwise the drying conditions were identical. It can be seen that with constant total solids content from low heat to high heat: a) viscosity increases, b) final moisture increases as the denatured whey proteins formed by high heat treatment bind better the moisture, c)  occluded air content decreases as high heat treatment reduces the foaming properties, d) therefore bulk density increases and, e) solubility index decreases. The latter is probably due to particle temperatures being lower due to higher moisture content in spite of an identical outlet temperature. With constant viscosity the following trends can be observed: Fig. 10.5. Influence of WPNI and % TS on various skim milk powder properties

a)  the total solids content of the concentrate is lower with high heat treatment, b) the final moisture approaches a constant value and the same goes for occluded air content and bulk density, c) the solubility index decreases due to lower feed concentration.

The occluded air content has also a considerable influence on bulk density, as it increases the particle volume, i.e. decreasing particle density. High bulk density is desirable, for instance, for skim milk powder used in the Far East for recombining purposes. The amount of occluded air, as illustrated in Fig. 10.4, depends on a number of factors. One is mode of atomization, and in this respect pressure nozzles are superior to wheels. Other factors include conditions of the feed, especially the degree of denaturation of whey proteins, concentration and temperature. Decades ago with old-fashioned recirculation evaporators yielding a concentrate of low solids content, it was known that cooling of the concentrate resulted in high bulk density. Nowadays when working with total solids content of skim milk concentrate of around 50%, it is advantageous to heat the concentrate up to 80°C in order to get a low occluded air content and thereby a high bulk density. Fig. 10.6. explains this phenomenon.

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Fig. 10.6. Whipping ability of skim milk concentrate at various % TS and temperatures

Medium heat skim milk concentrates of 40, 44 and 50% total solids were whipped at various temperatures in a similar way as when testing whipping ability of egg albumen. Results indicate clearly that a concentrate of low solids content is less whippable (i.e. is able to incorporate less air) when cooled down, while a concentrate of high solids content exhibits an opposite effect. Concentrating to high solids and heating to high temperature is definitely favourable. The incorporation of air into the concentrate may take place either during transfer between the evaporator and spray dryer or during the atomization. Fig. 10.7. shows the contribution of the volume of occluded air to the total volume of the powder as a percentage. Under certain circumstances, the volume of occluded air can occupy up to about 10% of the total volume. Thus in order to obtain high bulk density powder, it is important to pay attention to the problem of occluded air as outlined above.

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Fig. 10.7. The contribution of occluded air (expressed as particle density) to the total powder volume of skim milk powder

A similar relationship has been found by Westergaard [23] who expressed the bulk density of skim milk powder directly as a function of heat treatment (expressed as WPNI - see Fig. 10.8.). The relationship seems to be close to linear, and as in a previous example, the lower bulk density at high WPNI (low heat treatment) is due to high foaming ability and therefore higher content of occluded air. In order to get high bulk density of low heat skim milk powder, it is therefore even more important to avoid incorporation of air into the concentrate prior to and during atomization, hence the preferred use of pressure nozzles. Fig. 10.8. Influence of heat treatment (as WPNI) on bulk density of skim milk powder The occluded air appearing as bubbles inside milk powder particle, often called vacuoles or entrapped air, is therefore one of the most important factors for controlling bulk density. Its volume is usually expressed in ml/100g powder or in percent of powder total volume. 171

High or low content of occluded air may be either an undesirable or wanted property depending on the powder bulk density specification. For high bulk density specification, high contents of occluded air are unwanted. The factors of creation of occluded air in connection with manufacture of high bulk density powders have been discussed previously. If low bulk density is required, the powder volume can be increased by agglomeration (see next section) or by increase of the content of occluded air. With rotating wheel atomization, a small increase of the volume can be achieved by changing to a straight vane wheel from a curved vane wheel. Injection of carbon dioxide (CO2) into the feed before atomization is more effective and controllable. CO2 injection can be done with a wheel atomizer. However, much better control of the injected amount of gas is achieved with a pressure nozzle atomizer. For wheel atomizer operation, CO2 may be injected in slight excess prior to the feed tank. When dosing directly into the feed line, overdosing must be avoided, as it will cause pulsation to the feed flow to the atomizer. With a pressure nozzle atomizer gas is injected into the pipeline prior to or after the high pressure pump. Commercial units, such as the GEA Niro DENSISET™ for CO2 injection on the high pressure side, is available. CO2 is soluble in water even under atmospheric conditions, but its solubility can be increased by increasing the pressure or lowering the temperature. After atomization the dissolved CO2 will be released from solution as a gas, expand and blow up the droplets. For obtaining the best results, the feed must be able to form voluminous and stable foam containing for instance non-denatured whey proteins and maltodextrins. The solubility of CO2 in water at various temperatures and various pressures is shown in Fig. 10.9 and Fig. 10.10, respectively.

Fig. 10.9. Solubility of CO2 in water at different temperatures

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Fig. 10.10. Solubility of CO2 in water at different pressures

Instead of CO2 also N2O can be used, but this is much more expensive and has therefore no practical application. Nitrogen has also been used for so-called foam spray drying. Nitrogen is, however, hardly soluble in water, and therefore it can be used only with high pressure nozzle atomization requiring dosing at high pressure into the high pressure line (between high pressure pump and nozzles). It is often believed that the higher the mean particle size, the higher the powder volume, i.e. the lower bulk density, but this is not always true. In a powder consisting of ideal spheres of the same size, packed in an ideal way in tetrahedron pattern, the solids would occupy 74% of the volume regardless particle size. The bulk density of such powder when considering air-free particles will be theoretically about 1.2 and 1.4 g/ml for whole and skim milk respectively, i.e. about 70% higher than experienced in practice. A distribution of different particle size when still maintaining the ideal spherical shape would contribute to even heavier powder. Thus the main factor controlling bulk density is not so much mean particle size, but particle shape. From the above it is obvious that to obtain high bulk density, the powder must consist of airfree particles having as near as possible spherical shape with smooth surfaces and appropriate particle size distribution without agglomeration. This can be best achieved using preheated feed of high total solids content and a two stage drying method. The effect of the latter is more a compensation of the influence of high concentration on other powder properties (mainly insolubility index). Otherwise the influence of two stage drying on bulk density is to reduce the volume of occluded air, however on the account of deforming spherical shape due to shrinking of the particle. Therefore, if all other conditions are the same, the nozzle atomization is superior to wheel atomization where the particles are subjected to higher shrinkage due to higher contents of air in the atomized droplets, i.e. their shape deviates more from an ideal sphere. Under all circumstances, two stage dried powders are always slightly agglomerated and this has an adverse effect on bulk density. The agglomerate bindings are, however, in this case weak requiring only gentle mechanical treatment, like pneumatic or blow line transport to break them down. The main tools for obtaining a low bulk density are agglomeration and increasing the occluded air content. An extremely low bulk density is achieved by the above discussed CO2 atomization. The agglomeration is discussed below.

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10.4. Agglomeration Agglomeration is the formation of porous clusters of single particles with the aim to increase the volume of interstitial air of a powder, which is one of the main factors for obtaining easily dispersible and easily dissolving - so-called instant - powders. Generally the agglomeration is achieved by intentional collisions between wet particles and dry particles. It can be done either by the so-called straight-through or rewet method. Straight-through agglomeration means agglomeration during the drying process, while for rewet agglomeration the ingoing material is a dried non-agglomerated powder. Primary and secondary agglomeration can be distinguished as follows: 1. Primary agglomeration takes place through coincidental collisions within the atomizer cloud between droplets of various sizes and in various stages of drying (see Fig. 10.11.). a) spontaneous primary agglomeration appears in any atomizer cloud. The higher the amount of feed flowing through a single atomizing device, the higher is the probability of collisions. b) forced primary agglomeration can be achieved by directing atomizer clouds of two or more atomizing nozzles (this type of agglomeration is only possible with nozzles) towards each other. 2. Secondary agglomeration takes place through coincidental collisions inside the atomizer cloud between droplets of various sizes and in various stages of drying and dry particles.

Fig. 10.11. Types of Agglomeration

c) spontaneous secondary agglomeration means agglomeration by coincidental recycling of dry particles into the atomizer cloud. It takes place due to the turbulent movement of the drying air in the drying chamber. It is most evident in chambers where air dispersers cause air rotational flow, but it is weak in chambers with streamline air flow (plug flow).

d) forced secondary agglomeration is obtained by classifying the agglomerated powder, i.e. separating the non-agglomerated particles and re-introducing them back into the atomizing cloud. Agglomeration is a complex process and its effect on mean particle size and powder bulk density depends on the equilibrium between several partial processes as shown schematically in Fig. 10.12. Agglomeration in this model means exclusively the formation of agglomerates in the atomizer cloud, and agglomeration efficiency means here the percentage of agglomerates of total powder after this stage. As mentioned in the above scheme, the efficiency of agglomeration depends on a number of factors, an important one being the amount of recycled fines. 174

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Fig. 10.12. Model of fines generation and recirculation The agglomeration stage is followed by separation, the aim of which is to separate the fines (or more correctly the non-agglomerated particles) from agglomerates. Separation efficiency means here the percentage of fines (of total fines) separated from the powder, leaving the drying chamber together with the exhaust air and passing to the cyclone for separation. The fraction of agglomerated powder leaving the chamber has usually an excess of moisture and has to be dried to final required value. This process takes place in a fluid bed. This can be either stationary integrated within the drying chamber base or an external vibrating fluid bed. The fluid bed has several functions. Besides removing the excess of moisture it also classifies the powder, i.e. to remove the excess of fines. An unwanted side effect, which takes place during fluidization, is attrition. Some attrition is unavoidable yet acceptable, but excessive attrition has an adverse effect on particle size, and especially on the shape of agglomerates where grinding off the appended particles (which form the distance bridges important for high volume) and “polishing” their surface to round shape takes place. The result is an excessive loss of volume, i.e. too high bulk density. The extent of attrition depends on the conditions of fluidization and on the type of perforated plate. The main characteristics of perforated plates are the percentage of free area and the number of holes per given area determining the size of individual holes. Anticipating that the fluidizing velocity and the gas rate are given by the 175

required duty (see section 4.7.2.) the total fluid bed area is also fixed. Too small a free area of the perforated plate will require too high a pressure in the plenum below the plate. This results in a high gas inlet velocity through the individual holes - so-called jet velocity. Another expression for the destructive effect of fluidization on agglomerates is momentum, which is the jet velocity multiplied by the mass of gas through one hole. These are the two factors most important for attrition, along with the mechanical stability of the treated powder, which also plays an important role. The mechanical stability depends mainly on the composition of the powder and on the structure of the agglomerates. As to composition, stable agglomerates are obtained with powders containing carbohydrates creating a continuous phase of particle solids, which are sticky in wet state and relatively hard when dry. Lactose is an excellent binding agent for agglomeration and its natural content in milk powders is sufficient for stable agglomeration. The conditions of agglomeration, i.e. mainly the moisture content of droplets at the moment of collision have an influence on the type of agglomerates and on their mechanical stability and dispersibility in water. Too stable a powder can be difficult to disperse or even dissolve, and a too well dispersible powder may have poor mechanical stability. The types of agglomerates can be described as follows (see Fig. 10.13.):

Fig. 10.13. Types of agglomerates a) Onion: These are created when droplets of very high moisture, i.e. before any, or only very little, drying has taken place, contact the recycled fines. These droplets, still very fluid, just cover the surface of the fines, gradually building up layers and increasing the size of the original particle, thus forming a structure that resembles an onion. This type of “agglomerate” is obtained intentionally by the process called spray-fluidization, in which liquid feed is sprayed onto fluidized powder in a fluid bed. Obviously, such particles are of relatively large size, have high mechanical stability, but are difficult to dissolve and are degraded by heat. Therefore this type of agglomeration is not desirable for dairy powders. b) Raspberry: These are created if large droplets of high moisture collide with a high amount of fines. As with the onion type this type of agglomeration does not offer well dispersible, soluble or voluminous milk powders. c) Grape: These represent the proper structure of agglomerates for milk powders and are created by mutual collisions of wet particles and fines. The grape structure can be either loose or compact depending on the moisture content at the moment of collision. The compact 176

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grape structure is characterized by agglomerates having small amounts of interstitial air, and therefore the powders have low volume but high mechanical stability, whereas the loose grape type agglomerates support high volume, but they are more sensitive to mechanical treatment. The latter exhibits the very best reconstitution properties as to dispersibility, absence of slowly dispersible particles, fast rate of hydration etc. Generally, the higher the moisture content of the droplets at the point of collision with the returned fines, the larger and more stable are the agglomerates. Therefore two stage systems agglomerate better than single stage systems and three stage systems are superior to both of them. The distance from the atomizing device at which the fines are introduced into the cloud is also important. On the other hand, too large and too stable agglomerate can cause problems with reconstitution especially as to instant properties. Therefore the optimum lies between the loose and compact grape structure ensuring reasonably good mechanical stability without too much loss of reconstitution properties.

10.5. Flowability Good flowability is a property, which definitely increases the marketing value of a product, since it can be seen and compared visually with a competitive product. In some instances, good flowability is the necessary property of a product for its intended utilization, e.g. powders to be used in vending machines for various hot or cold drinks, for feeding calves by means of automatically working reconstituting and feeding apparatus (artificial cow machine) or for further industrial processing, equipped with mechanical handling and dosing devices. On the other hand, a better flowability is associated with higher bulk density, especially the loose value. The tapped-to-constant bulk density value remains almost unchanged. Different types of milk powder exhibit very different degrees of free-flowing ability, and the factors influencing this property can be summarized as follows: a) important factors in non-agglomerated powders are the particle size, shape and structure of the surface. Large mean particle size, narrow particle size distribution, spherical shape and smooth surfaces are the factors contributing to better flowability. Pressure nozzle powders are superior in this respect to rotary wheel powders due to their lower occluded air content. b) agglomeration improves the free-flowing properties and again the same factors as mentioned above apply. The shape of agglomerates can exhibit even greater deviation from a spherical shape of single particles, and therefore long chain-type agglomerates should be avoided. Low amount of fines is also an important condition. c) increasing fat content of the milk powder reduces flowability and it is well known that skim milk powder is more free-flowing than whole milk powder. On the other hand, it has been shown that with milk powders of very high fat content 65-80%, powders with fat content in the upper part of this range had better flowability than those in the lower. d) a very detrimental effect on flowability is a high free fat content, especially if low melting fat is involved. Therefore lecithin treatment of fat-containing powders used to achieve improved wettablility adversely affects flowability. e) addition of free-flowing agents improves flowability. However, there are limitations to the type of product to which they can be applied. Typical free flowing agents include sodiumaluminium silicates, calcium silicate, calcium phosphate, pre-crystallized whey powder or lactose as -lactose-monohydrate. 177

10.6. Free fat content The free fat of fat containing milk powders is (apart from exceptions discussed later) an undesirable property, jeopardizing the keeping quality due to fast development of oxidized flavour and tallowiness. This causes unpleasant appearance to reconstituted solutions with fat layers forming on the surface. It also deteriorates flowability. The free fat is sometimes defined as the fraction of fat, which is not protected by a protein film and is present in form of fat pools or patches rather than globules predominantly on the surface of fat containing milk powders. An excellent study of the appearance of free fat content has been presented by Buma [24, 25]. Based on his findings the free fat content can be defined as that fraction of fat, which is extractable by organic solvents under defined conditions (solvent type, time and temperature of extraction). He proved as well, that at least a part of free fat is located inside the powder particles and that the occurrence of free fat in whole milk powder can be related to the presence of micropores and cracks. There is a relationship between moisture and free fat content. With increasing moisture from 2 to 4-5% the free fat decreases, but with moisture content above 6-7%, the fat is 100% extractable. The former phenomenon can be ascribed to the swelling of particles and thus closing of the microspores thus retarding the penetration of the solvent into the particle interior. The latter is attributed to the crystallization of lactose, proven by microscopic observation in polarized light, which provokes the creation of a network of fine interstices and cracks along the side and edges of the lactose crystals, which makes the particle permeable to gases and solvents. The changes of free fat content taking place during moisture absorption, resulting in swelling of particle mass and closing the microspores are fully reversible as long as the critical moisture level of 6-7% is not exceeded. The continuous phase of milk powder particles is formed by amorphous lactose together with other milk serum constituents, in which fat globules and casein micelles are dispersed and which is impenetrable by organic solvents. During homogenization some casein particles adhere to fat globules covering them partly. The physical structure of a particle, mainly the particle size, distribution of fat and porosity plays a dominating role. High occluded air content results in high free fat and therefore powders made from concentrates with low solids content or foam spray dried have higher free fat contents. In spite of free fat content not being quite proportional to surface area, small particles have much more free fat than large. Buma’s conclusion was that small particles have more and wider micropores. However, the small and large particles in his study were cyclone fractions and chamber fractions of the same powders. Thus a more probable explanation is that the cyclone fraction had been exposed to considerable friction during pneumatic transport and especially in the cyclone, attacking the surface and liberating the fat from fat globules located close to the particle surface. Buma also explained why the amount of extractable fat is time dependent, whereby the major part of free fat is extracted during the first 10 minutes and the residual portion needs many hours. The difference between short and long extraction remains almost constant with increasing moisture up to a critical level. While long time extraction exhibits a critical moisture level between 6-7%, the short time extraction is reaching that point around 9-10%. Based on these findings, Buma presented a model of a whole milk particle with four forms of extractable fat. A free interpretation of Buma’s work dealing with the extraction of whole milk powder with 29% fat and various moisture content is presented in Fig. 10.15 and his model of a milk powder particle in Fig. 10.14. 178

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These four forms of extractable fat are: a) surface fat, present as pools or patches on the particle surface, particularly in the irregular folds or at contact points between particles. b) outer layer fat, formed by fat globules almost touching the particle surface and thus easily accessible to solvents.

Fig. 10.14. Buma’s model of four forms of extractable fat in whole milk powder

c) capillary fat, consisting of fat globules almost touching the surface of microspores and cracks and thus, similarly to b), can be reached by solvent.

d) dissolution fat, consisting of fat globules almost touching any hole remaining after dissolving the outer layer and capillary fat globules inclusive dissolution fat globules, forming a chain of fat globules. In good quality whole milk powder with 28% fat, the free fat content is low, in the range less than 1% up to 1.5%. It is obvious that free fat is accessible not only to solvents, but also to gases and atmospheric oxygen developing oxidized flavour and tallowiness. Wewala [26] recommended an increase of moisture content of commercial whole milk powder from the usual 2.5-3.0% to 3.4%. More about the background of this recommendation is given in section 10.10., but a strongly contributing effect of such a Fig. 10.15. Influence of moisture content on step for extended shelf life is definitely the decrease of free fat content as a function free fat (acc. To Buma) of moisture described above. Moreover, according to Buma’s findings, the free fat decrease between 2 and 7% moisture is close to linear. Nevertheless, it exhibits a very significant drop, followed again by an increase, around 3.5% moisture. This phenomenon may be caused by onset of lactose crystallisation, which eventually may lead to severe product quality deterioration during storage. The work of Snoeren [4 - 8] on viscosity of milk concentrates has already been mentioned. It can be seen in Fig. 9.5 that the viscosity of whole milk concentrate increases with both increasing concentration and increasing homogenization pressure. No doubt the homogenization effect on the size of fat globules has a dominating effect on decreasing free fat content. However, Snoeren proved that it is also a function of viscosity (which controls the particle size distribution and thereby many other properties). In other words low free fat content can also be achieved by a combination of high concentration with low homogenization pressure instead of low concentration with excessive homogenization, without any adverse effects on 179

other properties. This finding can probably be utilized in practice to achieve more economical conditions. However, it will require a deep knowledge of the influence of various factors on the viscosity and standardization of protein content. The factors controlling the level of free fat are: a) total fat content of the powder. Below approximately 26% fat, the free fat fraction is low but above this level it increases rapidly. b) type of fat, i.e. vegetable fats of low melting point tend to increase the free fat level. c)  product composition, i.e. if the composition of non-fat-solids is dominated by carbohydrates, especially lactose, the free fat level is low; with dominating proteins it is higher. d) physical state of lactose, i.e. amorphous lactose protects the fat against extraction while crystallized lactose provokes free fat, e) gentle drying conditions result in particles with smooth surface, giving lower free fat than with high temperature drying, which creates cracks and microspores, f) gentle powder treatment, i.e. avoid pneumatic transport, use a dryer type giving a low cyclone fraction, operate with a low pressure drop over the cyclones and cool the powder in a fluid bed, avoid too high powder moisture from the first drying stage. g) avoid standardization by buttermilk of fat of whole milk. h) use high feed concentrations up to the level which will still ensure good solubility index. i) avoid using lecithin as emulsifier for fat filled powders. j) avoid over drying of the product to too low final moisture content. k) use two stage homogenization with medium pressures. l) using crystallized lactose as ingredient dissolves it completely.

10.7. Instant properties Reconstitution of a milk powder is a complicated process and many analytical methods have been developed to determine how successfully it has been completed and what the defects of the reconstituted solution are. The most important instant properties are wettability and Dispersibility. Originally sinkability was considered as a part of the reconstitution process also expressing how fast particles sink to the bottom of the glass, but later on it was found that this was of secondary importance and difficult to measure. Sometimes the expression ‘sinkability’ is used as a synonym of wettability. Ideally, the reconstitution of milk powder in water should result in a homogeneous solution and suspension having the appearance of pasteurized milk, but in practice there is almost always some un-dissolved or un-dispersed residue. This may consist either of inside un-wetted lumps or slurry at the bottom of a glass, agglomerates, single particles or fine flakes floating in the reconstituted solution or tiny flakes and possibly some particles or small lumps floating on the surface, so-called floaters. Apart from wettability all the other tests try to detect this insoluble residue. The biggest insoluble elements are determined by the Sludge and the Dispersibility tests. The flakes floating in the solution after removing the Sludge by filtration through a 600 μm mesh are detected visually in the milk film, remaining on the walls when emptying the test tube as slowly dispersible particles abbreviated to SDP. Even smaller flakes, designated as white flecks, are expressed by a White Fleck Number. Instant milk powders were originally developed to be used as a fresh milk equivalent, i.e. consumed as cold drinks and therefore they should preferably be cold water instant. However, consumer demands have shown that they must also be reconstitutable in hot water or even 180

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hot coffee or tea. In countries with a lack of good quality drinking water, this has to be boiled before use. This water is often used for reconstitution of the powder just after boiling. Therefore good instant powder must exhibit an absence of Sludge and slowly dispersible particles also under these conditions. Thus, we are talking about cold and hot Sludge and cold and hot SDP. Moreover, there must be good thermo-stability in hot beverages which is expressed by the Coffee Test and Hot Water test, also called hot sediment. For cold tests the temperature is 25°C while for hot tests 80 - 85°C. For agglomerated but non-lecithinated powder 45°C has been accepted for cold tests. When the hot milk is allowed to stand for 15 minutes a skin is formed on the surface. Its thickness and colour is evaluated visually and expressed as skin index. Generally, the essential conditions for good instant behaviour are good agglomeration and wettable surfaces. Milk powders with fat content less than 1% have a wettable surface. Fat containing powders, as instant whole milk powder, have due to the free fat a hydrophobic surface. Lecithin, being both hydrophilic and lipophilic and, being also a natural component of milk and therefore acceptable as an additive, is most suitable to be used for preparing the wetting agent, which is a solution of lecithin in oil. This is sprayed on the final powder when still warm and exposed to violent fluidization to ensure as complete distribution on the total surface of the agglomerates and particles as possible. Any fat with low melting point can be used as the solvent for the lecithin. Originally pure butter oil was used for ethical reasons to avoid any ‘foreign’ fat. For functional reasons, however, low melting vegetable oils or fractionated butter oil with melting point well below 18°C is superior. Practice has shown that there are no objections to using a vegetable oil for this purpose, and it is now legal in most countries.

10.7.1. Wettability The Wettability or wetting time determines the time necessary for a given amount of powder, dropped onto still water, to pass through the surface. Factors of importance for good Wettability of whole milk and other fat-containing powders are the following: a) good agglomeration with an absolute minimum mean particle size of 180 μm (preferably 200 - 300 μm), having a size fraction below 125 μm less than 20% (but preferably lower than 15 or even 10%) and size fraction greater than 500 μm not higher than 10%. b) efficient lecithin treatment - the details are explained separately below. c) particle density at least 1.15 g/cm3 but preferably above 1.2 g/cm3. d) conditioning of the powder after lecithin treatment to get a final temperature about 45°C. e) gentle transport to silos or hoppers. f) packaging in cans or transport boxes before the temperature drops below 40°C or alternatively after it has dropped below 25°C. The philosophy and theoretical backgrounds behind lecithin treatment are: The occurrence of free fat in fat-containing powders was discussed in previous section. As shown in Fig. 10.14, some part of this free fat appears on the surface. The fat is hydrophobic, i.e. water-repellent, which is the reason why such powders when dropped on the surface of cold water remains on the surface almost un-wetted. Lecithin is a component which is both 181

hydrophilic and lipophilic and is fat-soluble. Thus incorporation of lecithin into the free fat is able to convert the hydrophobic surface to hydrophilic one. In whole milk powder the milk fat is a mixture of a number of triglycerides with melting points between 0 and 45°C. The powder should be instant at room temperature, i.e. around 20°C. At this temperature, part of the fat is in a solid state while a part is still liquid. These are the high melting and low melting fractions. The latter constitutes about 30-50% in a normal milk fat. For achieving good Wettability by means of lecithin treatment, the active part is the low melting fraction, which remains liquid after the powder has been cooled down to the surrounding temperature. The high melting fraction will crystallize. The solution of lecithin in oil is sprayed on the treated powder and it is mixed with the original free fat. It is an advantage if the fat used as a solvent is a low melting fat, i.e. oil, which is liquid at normal temperature. However, using milk fat (in form of butter oil) is possible as well. The disadvantage of using butter oil is that the solution is more viscous and besides, it requires higher addition (because together with the active low melting component the high melting fraction is added). This so-called wetting agent has to be sprayed onto the powder in such a way as to ensure both good distribution and, during subsequent treatment (fluidization with hot air), good mixing with the original free fat to finally create a homogeneous solution of lecithin in melted free fat together with added oil. During cooling, the high melting fractions gradually crystallize. They are transformed to solid crystals, whereby the lecithin concentration in the low melting fractions increases. At the very end of this process (which in fact should terminate in cans or transport bins) the agglomerates and particles are covered with crystals of the high melting fat fraction and the whole surface is coated by the remaining low melting fraction with dissolved lecithin. If the powder is cooled rapidly and exposed simultaneously to mechanical treatment, as for instance fluidization in a fluid bed cooler, the whole solution will become very viscous. Later on the high melting fraction will also very slowly crystallize. However, the crystals will be very small and spread throughout the whole coating layer, appearing partly also on the surface. These crystals are hydrophobic and such powder will not be wettable. Keeping in mind the above theoretical considerations, the condition for achieving good wettable powders are: a) lecithin concentration in the liquid fraction of the free fat must be 15-25% and the absolute amount of lecithin on powder 0.12-0.22%. b) wetting agent consisting of 30-50% lecithin in oil solvent. c) the total amount of free fat inclusive lecithin should be between 0.8-1.8% whereby the amount of low melting fraction should be 0.5-1.2% to ensure that the whole surface of the agglomerates and particles will be coated. d) the oil used as solvent for lecithin should be without any foreign taste and odour and have a melting point preferably below 12°C. If butter oil is used, it must match the requirements for pure butter oil. From the above it is obvious that the amount of low melting fraction and therefore also of the lecithin depends on the specific surface area of the powder, and that good agglomerated powders of large mean particle size require less lecithin than poorly agglomerated powders. Furthermore, using low melting point oil results in lower final free fat content than with butter oil. The low limits of the above ranges refer to powders having low specific surface area. A good Wettability can also be achieved when the amount of lecithin and free fat are higher than the above levels. However, high levels will adversely affect the other instant properties. Proper composition of the wetting agent, the kind of oil used, and dosing are in practice found empirically. 182

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For the lecithination process as such, the requirements are as follows: a) the wetting agent temperature should be 60-65°C to secure a reasonably low viscosity and good atomization, b) the powder temperature must be min. 50°C, c) after lecithination the powder should be fluidized at min. 45°C for at least 5 minutes.

10.7.2. Dispersibility Dispersibility determines how completely the product has been dissolved without leaving any un-dispersed residue, which can be detected visually as Sludge or lumps and separated from the solution by means of sifting. Sifting of the reconstituted solution and estimation of the amount of non-reconstituted residue retained on the sifter mesh are the operations used for the determination of the Dispersibility and Sludge. These two methods are using different sifter mesh sizes: 150 μm for IDF-Dispersibility, 300 μm for NZ-Dispersibility and 600 μm for Sludge. The part of retained residue is also determined by the Sludge method. Thus the Sludge test is in a way also a dispersibility test. Both the IDF Dispersibility and Sludge express quantitatively the un-dispersed residue, the NZ-Dispersibility just by comparing with the standard photo-scale. Poor dispersibility is due to all kinds of un-dispersed material, which cannot pass the mesh. These are: 1. Large and compact agglomerates (raspberry or compact grape type), for which the conditions of the analytical methods are insufficient - as to the time and turbulence available - to complete the dispersion. Such agglomerates are created when droplet/droplet or droplet/particle collisions take place at high moisture content, i.e. close to the atomizing device. Agglomeration of poorly dispersed fines with wet droplets can also create compact agglomerates. 2. Large lumps and slurry settling at the bottom of the glass as a result of poor agglomeration. The reason for this is too small mean particle size and an excessive amount of fines smaller than 125 μm. A too high level or poor distribution of lecithin can result in creation of a slurry. The experiences with various types of spray dryer design as to dispersibility have shown that the Multi-Stage (MSD™) concept is in this respect superior to any other design, obtaining almost constantly the highest classifications. This is most probably due to the large mean particle size and low amount of fines together with spontaneous primary and secondary agglomeration taking place in this type of dryer.

10.7.3. Sludge The cold Sludge is determined at 25°C for instant powder and at 45°C for agglomerated non-lecithinated powder. 85°C is used for hot Sludge. These are currently called Sludge 25, Sludge 45 and Sludge 85. As commented above, the Sludge test is also a kind of Dispersibility test, determining a part of un-dispersed residue. The standard for Sludge is max. 0.1 g. As the weighed wet Sludge has about 50% moisture, expressing this amount in terms of IDFDispersibility would correspond to 99.6%. In practice, however, the Sludge of a good quality powder is much lower than 0.1 g, and therefore the contribution of Sludge to Dispersibility is minimal. 183

Nevertheless the cold Sludge is a useful and fast routine test especially for drying systems where achievement of reasonably high mean particle size and low amount of fines can be a problem. On the other hand, there can be expressed doubts about the usefulness of hot Sludge. It is seldom too different from cold Sludge.

10.7.4. Heat stability Coagulation of proteins leading to precipitation is the reason for many powder faults. The constituents of milk solids, which are the direct cause of precipitation, are mainly casein and -lactalbumin. The size of precipitated matter can vary widely, ranging from microscopic to macroscopic. The precipitation can take place in any processing step, during which the milk, the concentrate during the evaporation process, the droplet or wet powder are exposed to heat. The heat exposure can be indirect, in which the heat from the heating medium (steam or hot water) is transferred to the milk solids via a heating surface. In direct heating, the heating medium (steam) can be either injected into the heated liquid (Direct Steam Injection or DSI) or the heated liquid injected into the heating medium (Steam Infusion). In any case, there is a surface layer of heated material, which is primarily exposed to the first heat chock. Besides, precipitation can take place also during the analytical tests, in which the powder is exposed to the heat of the water or coffee used for dissolving. Whether precipitation will occur under the various conditions of processing depends on following factors: 1. The temperature difference (T) between the heating medium and heated milk solids. 2. How fast, if at all, the material of the surface layer is replaced by new material. In case of liquid heating this depends on the product viscosity and on the flow velocity or turbulence or on the effectiveness of agitators. On the other hand during spray drying, where the surface layer of the droplet remains constant, the important factor is the air velocity. Direct heating by means of steam injection is usually considered gentler. However, even in this case a surface layer of the heated liquid on steam bubbles can occur, if the T is too high and mixing is unsatisfactory due to low flow velocities. 3. Heat stability of the milk solids can be defined as a relative resistance to precipitation during processing or testing. Generally speaking the heat stability of milk decreases during the evaporation process. Moreover, the danger of precipitation increases due to rising viscosity, influencing the flow characteristics. An important contributing factor in the evaporator is insufficient coverage of the tubes when liquid distribution is unequal. All properties based on dissolving powder in hot water or hot beverages are in a way expressing heat stability of the product. Good heat stability is thus a common precondition for achieving good SDP 85, Hot Water test, Coffee Test and also Sludge 85, although the latter is much less sensitive. The factors affecting the heat stability of milk powders can be divided into three groups: 1. Factors defining the basic properties of the processed milk such as chemical composition, including properties that express freshness. 2. Factors induced by the whole manufacturing process. 3. Factors given by the physical structure of the powder. 4. Factors given by the conditions of the test method. The basic properties of the milk involve mainly the acidity, total protein concentration, 184

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-lactalbumin/casein proportion and salt balance. The influence of acidity is shown in the next section on SDP 85. High protein concentration increases the concentrate viscosity and thereby the size of primary particles, i.e. affecting the physical structure of the powder. The manufacturer is obviously mostly interested in the performance of the final powder in which the effect of precipitation, which took place during processing or during the analysis, will be as little as possible. This means that the amount of the precipitate will be below the level specified for the individual tests or that the size of the precipitated elements will be undetectable by the method in question. The proteins are the constituent subjected to precipitation under certain circumstances and therefore a reduction of total protein content is a useful step to reduce the susceptibility of the milk to precipitation. However, in many countries, the protein content is much higher than the average of that from Frisian cows. A reduction to 38-39% total protein of non-fat solids by the addition of lactose is definitely one of the most useful steps for the control of hot properties. Such content is also still well above the Frisian milk average. This standardization of the protein content is sometimes called frisianisation. Since 1999 the FAO Codex 207-1999 has allowed for protein standardisation down to 34% protein of non-fat solids using either milk permeate or lactose. The main protein components are casein and -lactalbumin. The amount of -lactalbumin can vary over a wide range and can be very high especially at the beginning of the season. Normally the milk powder manufacturer has no information about the actual content of -lactalbumin. During heat treatment, it always coagulates creating, depending on the conditions of the heat treatment, coarse or fine precipitates. Pasteurization prior to evaporation is a necessary step not only for bacteriological reasons but also to ensure the required shelf life. Acceptable pasteurization conditions for both heat stability and shelf life control should result in WPNI 2.5-3.5. However, lower temperatures with long holding time have preference for the control of the heat stability. A useful step, which has been experienced in the production of UHT-milk for years, is holding the processed milk at 80°C for 6 minutes before the main pasteurization. This step is called stabilization and leads to an extremely fine precipitation not detectable by the naked eye. The homogenization of the concentrate, used for the control of free fat, progressively decreases the heat stability with rising pressure. Therefore high homogenizing pressures, single-stage homogenization and recirculation of the concentrate over the homogenizer (when using constant speed homogenizer as a feed pump for the dryer) must be avoided. Experience has shown, however, that it is seldom necessary to homogenize with total pressure higher than 80 bar with 20 bars in the second stage. One of the most important factors influencing the susceptibility to precipitation is the salt balance. In connection with the sterilization of evaporated milk, it has been found that casein has maximum heat stability when combined with an optimum amount of calcium. When the calcium content is higher or lower than this optimum, the casein/calcium complex is less stable. The calcium in normal milk is distributed between casein, phosphates and citrates. The optimum salt balance is achieved when the calcium (and also magnesium) cations, which are not bound in the casein complex, are in balance with citric and phosphoric acid anions. Obviously, the optimum balance can be achieved by addition of the component, which is in deficiency or removing the surplus component. Normal milk usually has an excess of calcium, 185

thus the increase of heat stability can be achieved by the addition of the acid anions in form of sodium phosphates or citrates. Citrates are more expensive than phosphates but are more effective. The tri-sodium citrate is much better soluble in water than the corresponding phosphate. Moreover the presence of the natural calcium phosphate content in milk is one of the reasons for precipitation and thus any increase should be avoided. The addition of citrates for improving the heat stability is permitted, because citrates are a natural component of milk. Furthermore they are listed among the additives approved by WHO and FAO for baby food. Nevertheless, in some countries they can still be considered undesirable, probably because they have to be declared as additives, and this can spoil the product image. The heating of the concentrate prior to atomization improves the so-called hot properties (tests where powder is reconstituted in hot liquid). One effect of this heating is reduction of the viscosity. Another effect may be that due to the decrease of tri-calcium phosphate solubility with rising temperature, this treatment removes the excess of calcium ions from the solution. The factors for increasing the heat stability and thereby improving all properties at higher reconstitution temperatures can be summarized as follows: 1. Standardization of total protein content below 39% (in non-fat-solids) or lower preferably by lactose. However, if problems should occur, it is recommendable to run a comparable production with lactose only. 2. The addition of ascorbic acid as an antioxidant, if required, should be done in cold milk giving good dilution and by adding where milk flow is turbulent. The higher the level of Vitamin C added the more careful the dosing. Addition in the form of a neutral salt of ascorbic acid or ascorbyl-palmitate eliminates dosing problems. 3. Addition of citrates or polyphosphates for milk in the off-peak milk season (where milk is unstable) is recommendable to achieve the desired heat stability. 4. Pasteurization resulting in WPNI 2.5-3.5. Low temperatures with long holding time are to be preferred. Pre-treatment with holding for 4-6 minutes at 80°C may be advantageous. 5. Concentrate leaving the evaporator to be free of any insoluble residue. Rapid fouling of the evaporator is usually a good indication of problems in this respect and appears especially in the early season where the milk stability is poor. It has been experienced that rapid fouling of the evaporator is accompanied by poor functionality results. Poor stability of milk is the main reason for both these problems, but undoubtedly fouling increases the functionality problems. Thus improvement of quality can be achieved by limiting any fouling through modification to evaporator components that foul up, conditions of pasteurization used, better mixing between milk and steam in case of direct steam injection (DSI), milk flow distribution and coverage of the tubes (possibly by addition of water). 6. Heating of the concentrate to min. 75° (preferably to 80°C). 7. Avoid excessive homogenization. Usually two stage homogenization is sufficient with 80 bar total pressure drop, 20 bar in the second stage. 186

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10.7.5. Slowly dispersible particles Slowly dispersible particles, called SDP 25 and SDP 85 for cold and hot SDP, respectively, are the properties creating most problems. They are based both on powder structure as well as on the chemical nature of the product. There is often a significant difference between hot and cold SDP, the latter being most problematic. As the name indicates, this property expresses also a kind of dispersibility evaluation. While the standard NZ-Dispersibility test retains all elements greater than 300 μm on the screen, SDP test is conducted with reconstituted milk after the Sludge test, i.e. from which all elements greater than 600 μm have been separated by sifting. Experience has also shown that there is no connection between Dispersibility and cold SDP. A powder can exhibit excellent or poor Dispersibility combined with either excellent or poor cold SDP. The appearance of the white spots observed in the film of milk on the wall of the test tube in this test does not indicate the presence of particles or agglomerates. Moreover, the fact that it remains at all in this film indicates that these are rather flakes than particles. A comprehensive investigation conducted over 8 months at a large spray dryer of the MSD™ design has given the following findings:

Fig. 10.16. Influence of titratable acidity on Hot Water Test and SDP 85

1. No correlation between SDP 25 and any other property or operating parameter was found. 2. Strong relation between SDP 85 and many other properties and also to some operating parameters was found. The correlation of SDP 85 to Hot Water sediment and titratable acidity is shown in Fig. 10.16. Table 10.1 shows the relation between SDP 85 and WPNI, titratable acidity, Hot Water test and Coffee Test. The operating parameters, which were of influence, were the fluidizing velocity in static fluid bed, the outlet drying air temperature, and atomization pressure.

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Table 10.1. - The Influence of various parameters on SDP 85.

The other average properties of approximately 400 samples were: - Sludge 25 and Sludge 85 was 0.02 and 0.03 with no single results higher than 0.05, - Dispersibility was within the range 1-2 with average 1.22, - SDP 25 was A-D with average 1.54, i.e. between B and C. It can be concluded that there is no correlation between both SDPs and neither Dispersibility nor other properties, indicating a kind of dispersibility, like Sludge 25 and 85. However, there is a strong correlation between SDP 85 and all other hot properties, i.e. Hot Water sediment and Coffee Test. All depend mostly on WPNI and acidity. As to the latter, it is necessary to comment that none of the samples indicated really sour milk as all measured acidities were in the sweet range. Another example is from a large spray dryer of CDI design with fines return system of FRADdesign where, apart from Dispersibility, very good results were achieved as shown in table 10.2. However, in contrast to the previous example, this powder had a very poor Dispersibility between 4 and 6. In the previous example the constantly excellent Dispersibility and Sludge values were achieved together with SDP 85 varying between A - E and SDP 25 A - D. In this case excellent Sludge, very good SDP 25 and reasonable SDP 25 appeared together with poor Dispersibility. As can be seen, the oversize fraction was quite high, i.e. 17.70% which was probably the reason for that Dispersibility, however without any significant influence on SDPs. One of the reasons for much better SDP 85 in this CDI-powder in comparison with previous MSDTM-powder is the WPNI, which is on the high side of the recommended range, while in previous example it was even outside the low side.

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Table 10.2. Powder quality results from CDI-plant with FRAD.

From these findings it can be concluded that SDP 85 is controlled by the same factors as the other hot properties, i.e. Hot Water sediment and Coffee Test. The reasons for poor hot properties are most probably oversize primary particles due to high viscosity or low atomizing pressure and also particles originating from the jelly-like lumps originating in the evaporator and concentrate heaters. The cold SDP is, as mentioned above, a more problematic property. It seems that it has more to do with white flecks than with dispersibility, but reliable evidence is not available. Experience indicates that feed temperatures around 72-74°C result in better cold SDP than 76-78°C. Conditions for improving heat stability as recommended in the previous section are important for control both SDP 25 and 85. Guidelines are: 1. Effective atomization whereby the exact conditions have to be found individually for each plant. The general rules for non-protein standardised milks are however: a) Nozzle atomization:

- concentrate total solids 46-48% - spray angle as close to 75° as possible - pressure min. 220-300 bar - frequent check of nozzle insert wear

b) Wheel atomization:

- c oncentrate total solids 48-49% - curved vane wheel - peripheral wheel speed 135-165 m/s - good dispersion of returned fines

2. Lecithin treatment:

- avoid excessive amount of lecithin - see precautions described in 10.7.1. - avoid recycling of lecithinated fines

3. Ensure gentle treatment of final powder and pack when powder temperature still exceeds 40°C.

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10.7.6. Hot water test and coffee test These two properties are closely related following the same trends, but the Hot Water test appears to be the most sensitive especially as to acidity. As described in the previous section, the main factor for both properties, apart from acidity, is WPNI which should be 2.5-3.5. In fact the higher the better, however, the upper limit is controlled by the requirements for shelf life. The relationship of both these tests to WPNI tested on a milk of generally good quality with 37-38% total proteins (on non-fat-solids) and without any additives is presented in Fig. 10.17. The higher sensitivity, i.e. higher amount of sediment of NZ-Hot Water test in comparison with NZ-Coffee Test is caused mainly by the higher specified amount of powder used (12.5 g against 2 g). Besides, experience has shown that when poor, quite high fluctuations (sometimes almost ±100%) are exhibited. Both tests are controlled by the same factors as all other hot properties. The NZ-Coffee Test is the only one of a number of coffee/ tea tests, whose results are expressed by a number. On the other hand, it does not give the complete picture of the performance of a powder in hot coffee. The precipitation appears usually both as a sediment on the bottom and floaters on the top. The floaters escape evaluation in the NZ-Coffee Test. The CCF-Coffee Test evaluates both floaters and sediment, unfortunately only by a subjective estimate. The so-called MiddleEast-Coffee Test is not at all a scientific test. The coffee and milk powder is stirred into almost boiling water in the same way as done by a busy consumer. This rough treatment creates good conditions for precipitation. The amount of sediment is evaluated visually after pouring out the liquid. Powders produced on spray dryers with rotating wheel atomizers are inferior to pressure nozzle powders with this test. Fig. 10.17. Influence of whey protein nitrogen index on coffee and hot water test

10.7.7. White Flecks Number (WFN) This is a fairly new test which has been elaborated by IDF and accepted as an IDF-standard in 1990. The white flecks are the minutest elements of a precipitate appearing in great numbers, however occupying a relatively small volume. When a reconstituted solution is allowed to stand in a glass for several minutes, the white flecks create a layer on the surface. When dipping a teaspoon and taking it out again, the white flecks stick to the spoon and can be seen by the naked eye. They remain also on the walls of the glass when it is emptied. The IDF-test is based on observations that these flecks can clog the holes of a fine mesh (the test is using 63μm mesh) and thus obstruct the flow. It is probable that white flecks relate to SDP 25, but as this test is very new, no industrial experience is available. A study of factors promoting the appearance of white flecks has been 190

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reported by Ruyck [27]. Unfortunately this work is of limited value, as it was conducted on agglomerated powder formed by rewetting powder from a single stage pilot plant spray dryer. None of the tested samples were truly instant. Following conclusions can be made from this work: 1. The White Flecks Number remained almost constant under all heat treatment conditions resulting in WPNI 2.98 - 6.68 (WFN 0.65 - 0.75) and <0.1 - 7.22 (WFN 0.86 - 0.89) in spite of the viscosity varying between 30.5 - 93.5 and 39.2 - 440.0 mPa*s respectively. The WFN remained also constant while the insolubility index increased from 0.05 ml (at WPNI 3.35 - 7.22) to 3.50 7.2 (at WPNI 0.81 - <0.1). 2. Homogenization pressure had a negative effect on WFN. However the work was conducted under pressures of (I.stage/II.stage) 50/50 to 150/50 bar (i.e. pressures which are not suitable for instant whole milk powder). 3. Ruyck concluded also that increasing the inlet drying air temperature and/or outlet temperature had negative effect on white flecks, the latter being most important. However during these tests the concentrate solids varied over a broad range, thus it appears that this was a dominating effect.

Fig. 10.18. Influence of outlet temperature and solids content on White Flecks Number (acc. To Ruyck)

The Ruyck’s results of the influence of the outlet drying air temperature on the White Flecks Number as a function of total solids content are presented in Fig. 10.18. It can be seen that there is a good relationship of both parameters to WFN. However, it can also be seen that during these trials, low outlet temperatures were combined with low solids and vice versa. Thus, it is difficult to decide, which of them is the dominating factor. Nevertheless results from other tests indicate that it is most likely the concentration.

There is no doubt that White Flecks Number is a useful test. However, more work is necessary to fully understand its significance.

10.8. Hygroscopicity, sticking and caking properties Hygroscopicity means the ability of a powder to absorb moisture from the surrounding air. All powdered milk products and their components, with the exception of fat, are hygroscopic. The relative tendency of how much moisture a given product will, under given conditions, pick up (or possibly loose) is given by the sorption isotherm of the product in question. Amorphous lactose, proteins and salts are strongly hygroscopic. However, while absorption of moisture by products containing high content of lactose is accompanied by increasing 191

stickiness and finally caking, protein-rich products absorb moisture almost without these phenomena. All three properties define the requirements on the packaging material and storage conditions of final products. However, they also set limits for the drying conditions as to inlet and outlet air temperatures and feed solids content. Probably all milk powder producers have experienced blocking of the ducts, cyclones etc. The reason for most of such incidents relate to these particular properties. Fig. 10.19 illustrates the meaning of stickiness and sticking point. The sticking temperature line in Fig.10.19 defines the combination of moisture content and temperature (Tp) of a product above which it is inside the sticking zone. If, for instance, under normal air humidity, the outlet air temperature is 89°C (line To-1) then the product temperature (line Tp-1) is just below the sticking temperature line, i.e. no sticking is taking place. The corresponding 2.5% moisture is, in case 1, the upper limit for drying of that product. It is possible, of course, to use higher outlet temperatures Fig. 10.19. Explanation of sticking phenomena resulting in lower moisture levels to increase the difference between sticking temperature and powder temperature. If suddenly the ambient air humidity increases, then the relationship of moisture content to outlet air temperature and powder temperature is given by lines To-2 and Tp-2. Continuation of the process with 90°C outlet temperature (case 2) will result in about 3.1% moisture and the corresponding powder temperature will be well inside the sticking zone. The solution is to increase the outlet air temperature to about 96°C (case 3) whereby the corresponding moisture content will drop below 2% and the product temperature will come just out of the sticking zone. This graph is illustrative and the figures must not be considered as absolute. However they are close to conditions for drying of non-pre-crystallized whey. One of the components contributing mostly to hygroscopicity and stickiness is amorphous lactose, and therefore the mentioned problems occur when drying whey and whey products. The acidity due to presence of lactic acid is strongly contributing to stickiness as well. These are the reasons why acid whey is one of the most difficult products to dry. One of the possibilities to reduce the hygroscopicity and thereby stickiness of whey powders is precrystallization of lactose. Pallansch [28] in his work on drying acid whey developed a method for determination of the sticking temperature and using this method presented the influence of lactose crystallization and presence of lactic acid on the temperature of sticking. These relationships are given on Fig. 10.20 and 10.21.

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As already mentioned, the hygroscopicity is controlled by water activity and this again depends on product composition. Hygroscopicity cannot be reduced without changing the composition or without changing the physical state of some components. The possibilities for reducing the hygroscopicity and disposition to sticking or caking are as follows:

Fig. 10.20. Influence of pre-crystallization on sticking temperature

1. Transformation of amorphous lactose to -lactose-mono-hydrate by precrystallization. The effect is shown in Fig. 10.20.

2. Denaturing of whey proteins by pasteurization. For acid whey, this pre-treatment was recommended as a must. However such treatment also increases the viscosity significantly, thus setting limits for maximum manageable concentration. 3. Neutralization of lactic acid by means of calcium hydroxide (lime). Use of sodium or potassium hydroxide is not advisable as the created lactic acid salts contribute to stickiness practically to the same extent as lactic acid. 4. If maltodextrin is a component, a maltodextrin in the lower dextrose equivalent range is recommended. 5. In order to delay the absorption of moisture and avoid caking, lecithin treatment can be applied in the same way as for instant whole milk. 6. Addition of a free flowing agent such as aluminium-sodium-silicate reduces the stickiness and tendency to caking.

Fig. 10.21. Influence of added lactic acid on sticking temperature

As far as whey powders and whey based products are concerned, the most usual and effective way is pre-crystallization of lactose. The degree of crystallization expressed as percent of crystalline lactose (of total) for a whey concentrate with 72% lactose in total solids, based on initial and final refracto metric reading and final temperature, can be found using the graphs in Fig. 10.22. In the example in this graph, point 1 indicates that a concentrate exhibiting an initial refractometric reading 53°Bx and the second one at 20°C 36°Bx has achieved about 76% crystallization (point 2) and the total solids content before crystallization has been 52%.

193

Fig. 10.22. Relation between initial and final refractometrical readings, total solids and degree of crystallization The glass transition theory approach to better understanding the sticking mechanisms was discussed in 9.4.7. 194

10. Achieving product properties

10.9. Whey Protein Nitrogen Index (WPNI) The whey protein nitrogen index, WPNI, expresses the amount of un-denatured whey protein. It is a measure of the sum of heat treatments to which the milk has been subjected. The heat treatment of a concentrate has only a negligible effect on WPNI and thus the main operation to adjust the required value is the pasteurization processes, possibly at the milk reception and in the evaporator, i.e. time/temperature combinations. However, there are many other factors influencing the WPNI including the total amount of whey protein and the overall composition of processed milk as influenced by animal breed, seasonal variations, and possibly protein standardization. The individual design of the processing equipment, i.e. the pasteurizer and holding cells, has also great significance. Therefore it is difficult to predict the conditions of achieving the required WPNI on a general basis, or to transfer the experiences from one plant to another or even from one season to the next. Obviously the primary purpose of heat treatment is to ensure that the product is free of quality jeopardizing bacteria, yeasts, moulds and milk enzymes. In milk powder production, the influence of heat treatment on denaturation of whey proteins for achieving the desired properties of the final product is just as important. Skim milk powder for cheese manufacture should have as much un-denatured proteins as possible, i.e. it should be low-heat (WPNI > 6), while for bakeries, high-heat powder with high denaturation is required (WPNI < 1.5). Denatured -lactalbumin can bind moisture about seven times its own weight and this is important for the structure of the dough and bread volume. The importance of heat treatment resulting in WPNI 2.5 - 3.5 for instant whole milk powder, i.e. in the middle of medium-heat (WPNI >1.5 - <6.0) range, is explained above. Generally, direct contact heat treatment requires about 5 - 6°C higher temperature than indirect heating in order to get the same WPNI. The time/temperature relationship for achieving the desired WPNI is presented in Fig. 10.23. However, from reasons given above, this relationship shown has to be understood as a guideline only. In order to bring the WPNI under control every powder plant should collect the results from each plant at least twice a week.

Fig. 10.23. Guideline for time/temperature relation of WPNI 195

10.10. Shelf life The shelf life is an important property especially of products for the retail market. Deterioration of product quality can be caused by moisture uptake with consequential chemical, physical and bacteriological changes and by oxidation of fat. As milk powders are hygroscopic, the only way to reduce the hygroscopicity is crystallization of lactose. However, such a step is applicable only for products where crystallization will not affect the functional or other desirable properties, and where there is a reasonably high content of lactose. In practice it is used only for whey powders. Therefore vapour-tight packaging is necessary to give the product the required shelf life. For fat containing products and mainly whole milk powder for the retail market, there is the danger of deterioration by fat oxidation. Free sulphhydryl-groups (SH-groups) which are created in milk by heat treatment have anti-oxidative properties and thus a positive influence on prolonging shelf life. The higher the pasteurization temperature the faster is the creation of SH-groups. Kirchmeier et al. [29] studied the rate of creation of SH-groups in milk at various temperatures. According to his results, the formation of free SH-groups commences at 72°C, reaching the maximum at 95°C and then declines at higher temperatures. This phenomenon is attributed to masking due to creation of a complex with casein. The results of Kirchmeier work are presented in Fig. 10.24 and 10.25.

Fig. 10.24. Creation of SH-groups in milk at various temperatures

196

10. Achieving product properties

Fig.10.25. Creation of SH-groups in milk at various temperatures and times According to Labuza’s [30] theory on stability of foods as a function of water activity, the reaction rate of lipid oxidation reaches the minimum at aw=0.24 (see Fig.10.26). This has inspired Wewala [26] to recommend to increase the moisture content of commercial whole milk powder from usual 2.5-3.0% to 3.4%, corresponding to water activity 0.24. This was confirmed by test work, which showed achievement of an extended shelf life of the powder.

Fig. 10. 26. Relative reaction rates of major deteriorative reactions in food (acc. to Labuza) Previously there have been tendencies in whole milk powder manufacture to keep the moisture content at a value sometimes as low as 2.3%. Thus the background of Wewala’s recommendation is theoretically correct and as mentioned in section 10.6. The strongly contributing effect of this step for extended shelf life is definitely the decrease of free fat content. Nevertheless, a critical analysis of the consequences of this recommendation cannot approve 197

of such practice, based upon practical experiences regarding actual shelf life of whole milk powder with such high moisture content. For example much instant whole milk powder is marketed in areas having very high ambient temperatures, even at above 50°C. The storage rooms are not always conditioned to temperatures recommended for milk powder storage. On the other hand, supermarkets in these areas are mostly well air-conditioned. The set of graphs in Fig. 10.27 shows the conditions of interstitial air in a tin of whole milk powder with moisture content 2.0 - 4.0% at temperatures 20 - 50°C. The figures are calculated for normal atmospheric air. The humidities for nitrogen would be slightly higher, but for carbon dioxide slightly lower. The usual mixture of N2 with CO2 will give similar values as for air. The dew point will be the same for all types of gas. It can be seen that the dew point of the gas in a can of whole milk powder with 3.4% moisture, heated to 50°C, is about 25°C. Therefore condensation can take place when this can is transferred from the storage room to an air-conditioned room. Thomsen et al. [57] studied the phase transition of pure amorphous lactose and showed instability, i.e. lactose crystallisation, at aw ~ 0.4 at 25°C and aw ~ 0.25 at 38°C. Thomsen et al. [58] also studied the temperature effect on lactose crystallisation, Maillard reactions, and lipid oxidation in whole milk powder with an initial aw of 0.23. Powder samples were stored on closed Fig. 10.27. The conditions of vials at 37, 45, and 55°C. At 37°C only minor changes interstitial air in powder cans under occurred during the 147 days of observation. At 45 various temperature conditions and 55°C quite dramatic deteriorative processes took place, at 55°C after just about 6 days and at 45°C after about 2 months. It is suggested that lactose crystallisation, being initiated at storage temperatures above the glass transition temperature, Tg, is the main cause of these quite dramatic and deteriorative changes in important quality parameters. The aw and hence the final moisture content should therefore be selected, depending on the expected thermal stress the milk powder product may be exposed to during transportation and storage.

198

11. Analytical methods

11. Analytical methods Analytical methods for dry milk products are an almost forgotten subject in the literature. The only existing publication was issued by GEA Niro in 1978 [31], but updated versions of analytical methods can now be found on GEA Niro’s homepage: http://www.niro.com/niro/cmsdoc.nsf/WebDoc/ndkw6dknxs Since the time of the first publication, many new methods, especially for testing instant properties, have been developed by various milk powder and baby food manufacturers and are considered the intellectual property by these companies. Hence descriptions of these new methods are generally not accessible and are found only sporadically in articles in dairy magazines. In this chapter, the methods commonly used in practice are described just briefly without unnecessary details of common analytical procedures, anticipating that such are known to the experienced analyst.

11.1. Moisture content The determination of moisture is probably the most used analytical procedure in a milk powder laboratory. The conventional oven drying method is quite laborious and time consuming. Equipment, which is based on determination of loss of weight of a sample exposed to some source of heat (infrared lamp, microwave etc.) over a period of time, is now available. Nowadays the dominating method in large milk powder factories is based on infrared reflection. All these new instruments reduce labour and time, the latter to just less than 1 minute. Nevertheless, the conventional oven drying method still remains the reference method, against which all other methods have to be calibrated. Moisture content is of great importance from a commercial point of view. If no other method is agreed on between two parties then the standard oven drying method is decisive. However, one has to be aware that the ‘moisture’ determined by this method is just the ‘loss of weight’ and does not necessarily expresses the true water content. From a scientific point of view, it may be interesting to know in which form water appears in the product. In dry milk products water can exist as free moisture or water of crystallization. The content of water of crystallization is negligible in normal milk powders, but it is high in whey powders.

11.1.1. Standard oven drying method (IDF Standard No.26-1964 [32]) Determination of loss of weight of about 3 g sample, exposed to drying in an oven heated to 102 ± 2°C to constant weight. First check weight after 2 hours and then after each hour. The constant weight is achieved when the difference between two successive weightings is ≤ 0.5 mg. The weight loss is expressed in percentage. This method determines not only the free moisture but also some part of the water of crystallization. A non-standard modification of this method is the routine procedure under the same conditions, but drying in oven for 3 hours.

199

11.1.2. Free moisture Determination of loss of weight of an about 3 g sample, exposed to drying in an oven heated to 87 ± 2°C for 6 hours. Fig. 11.1 shows the effect of various temperatures used for determination of ‘moisture’ for whey powder (W) and -lactose-monohydrate (L). It can be seen clearly that the temperature 87°C is the most preferable for the determination of free moisture.

Fig. 11.1. An example of the determination of ‘moisture’ under various conditions

11.1.3. Total moisture The total moisture, involving both free moisture and water of crystallization, is determined by Karl-Fisher titration. It is based on the reaction between iodine and sulphur dioxide in the presence of water. Apparatus and reagents for this method are commercially available with instruction manual for the procedure.

11.1.4. Water of crystallization The water of crystallization is the difference between the total moisture (11.1.3) and free moisture (11.1.2).

11.2. Insolubility index Insolubility index according to the IDF Standard 129A:1988 [33] expresses the volume of insoluble material in the product under the conditions of the method. This method is primarily defined for skim and whole milk powder, but can be used also for other powders which require reconstitution without any sediment. 10 g of skim milk powder or 13 g of whole milk powder are added to 100 ml water of 24°C (50°C for drum dried product) 200

11. Analytical methods

with 3 drops of silicone antifoaming agent in the mixing jar (Cenco-mixer) and agitated at 3600 RPM for 90 seconds (Fig. 11.2.), then left for 15 minutes. The content is then gently stirred and filled into a centrifuge glass up to the 50 ml mark. The glass is centrifuged for 5 minutes at 160 G. Using the vacuum pump, the supernatant is removed to leave about 5 ml liquid above the sediment. The glass is refilled with water up to the 50 ml mark, the sediment is dispersed by means of a wire and the centrifugation is repeated. The volume of the sediment is the insolubility index. In order to get good reproducibility, it is important to stick exactly to the prescribed conditions. One of the most important factors is temperature. Therefore, before the analysis, not only the water, but also the mixing jar must be tempered to 24°C. This high sensitivity to temperature can be utilized for increasing the classification ability of the method to distinguish powders with insolubility index less than 0.1 ml (which is according to the standard procedure the lowest obtainable reading). For instance when using 15°C instead of 24°C the sediment will increase about 10-times, i.e. the 0.1 ml/24°C result will be about 1 ml/15°C.

11.3. Bulk density Bulk density of a powdered product is determined as the volume of 100 g after exposure to compaction by standardized tapping. The most utilized apparatus is the Engelsmann tapping machine (the German Stampfvolumeter – see Fig. 11.3.A). The IDFStandard 134-1986 [34] uses a 250 ml glass cylinder and 625 taps which is supposed to be Fig. 11.2. The impeller of the insolubility almost tapped-to-extreme, but in practice also index mixer 0- (loose bulk density), 100- and 1250-times tapping are used. The GEA Niro-method uses the Engelsmann machine with a brass cylinder of 100 ml volume (Fig.11.3.B) with removable extension. After tapping, the extension is removed and the top of the powder levelled at the top edge. The advantage is that the bulk density is obtained by weighing directly in g/100ml. British Standard Method [35], used only seldom nowadays, applies manual tapping 10 times of a 250 ml glass cylinder on a folded towel. In Holland the Ledoux-method is still used in some factories. In this method a weight of 28.34g (1 ounce) is placed on top of 28.34g powder in the 100 ml measuring cylinder which is tapped 100 times. The results are expressed as ml/g.

201

Fig. 11.3. The principle of the Engelsmann Stampfvolumeter (A) and the GEA Niro method (B)

11.4. Particle density The particle density is the mass in g/ml of the particles. The particle density is always lower than the density of the solids due to the presence of vacuoles (occluded air). The most accurate method for determination of particle density is based on the air pycnometer method, the principle of which is shown on Fig. 11.4. The container with a weighed sample is inserted into the measuring cylinder and the apparatus is hermetically closed. By means of a screw arrangement both pistons are moved simultaneously forward increasing thus the air pressure in both cylinders, but keeping zero differential pressure between the cylinders. When the reference cylinder comes to a stop, the readout on the measuring cylinder indicates the powder volume

202

11. Analytical methods

Fig. 11.4. The principle of the air pycnometer

The air pycnometer method is excellent for normal milk powders, which have an impermeable continuous phase formed by lactose glass. On the other hand it may give too high results for protein powders as the compressed air can penetrate into the vacuoles. Anyhow, usable results can be obtained by fast operation. An alternative method is the petroleum ether method. 25 g of powder are transferred into a 100 ml calibrated measuring cylinder (with glass stopper). By means of a 50 ml pipette petroleum ether is added and the contents are shaken until all powder is suspended. After flushing the walls with further 10 ml petroleum ether in order to bring all the particles down into the liquid, the reading of the total volume is taken. This volume minus 60 is equal to the total volume of the powder particles. The particle density, occluded air and interstitial air content for both methods are calculated as follows:

Particle density g / cm3 =

sample weight (g) sample volume (ml)

Occluded air cm3 / 100g =

Interstitial air cm3 / 100g =

100 particle density 100 bulk density

−

−

[11,1]

100 density of solids 100

particle density





[11,2]

[11,3]

The density of the solids is calculated using the equation (3, 17) and densities of the components from the table 3.1. Both the content of occluded air and interstitial air can be expressed also in percentage of the total powder volume:

203





[11,4]

[11,5]

11.5. Scorched particles The determination of scorched particles is important not only for evaluation of product quality, but also from the point of view of safety, as this is a check of possible occurrence of some undesired combustion processes in the hot zone of the installation or heat generation in powder deposits due to Maillard reaction. The official ADMI method [36] uses 25 g of skim milk powder or 32.5 g whole milk powder mixed for 50 seconds with 250 ml water of 18 - 27°C and addition of antifoaming agent. Using compressed air or vacuum, the solution is filtered through a standard filter pad, 32 mm in diameter. The result is evaluated using the standard ADMI photographic scale. When used as a safety precaution, the frequency and quickness is most important and therefore volume measurement of amount of powder instead of weighing is recommendable.

11.6. Wettability Wettability is the essential requirement for instant products. Nowadays the IDF-Standard 87:1979 [37] is widely used where 10 g of skim milk powder or 13 g of whole milk powder is dropped into water of 25°C in a beaker of 400 ml. The weighed amount of powder is transferred into a glass cylinder placed on a glass plate over the beaker (see Fig. 11.4). The glass plate is then withdrawn and simultaneously the stop watch started. The wettability or wetting time is taken when the last particles of the powder penetrate the water surface. The Niro method uses a funnel made of antistatic plastic foil which is placed on the beaker edge with a glass pestle as a stopper above the water of 20°C. 10 g of powder is spread around the pestle. The stop watch is started simultaneously with lifting the pestle.

204

11. Analytical methods

Fig. 11.5. Wettability method. A: IDF. B: GEA Niro

11.7. Dispersibility Dispersibility is often considered the most important characteristic to decide whether a product is instant or not on the basis of a single property. There are many methods for the determination of dispersibility. All of them are based on reconstitution of a powder under standard conditions and sifting the solution through a defined mesh. Most of the available methods evaluate the residue on the screen in comparison with a standard photographic scale. The IDF Standard 87:1979 is based on the determination of the total solids of the solution and expressing the dissolved amount in percentage. 250 g of water at 25°C is weighed into a dry glass beaker. Using arrangement shown in Fig. 11.6 with glass tubing fixed by a clamp on a stand (the glass plate should remain free enough to be withdrawn) 26 g of skim milk or 34 g of whole milk powder are weighed and transferred to the glass plate inside the tubing.

205

Fig. 11.6. Equipment for determination of IDF-dispersibility The glass plate is withdrawn by gentle continuous movement and a stop watch started simultaneously. Remove the beaker and after 5 seconds insert the spatula along the wall until it touches the bottom. During the next 20 seconds stir the contents with the spatula touching continuously the bottom moving forth and back across the diameter making one complete movement per second. During the first 5 complete stirrings the spatula is slightly tilted to avoid that the upper part will touch the edge of the beaker, while during the next 15 movements it is held vertically. Simultaneously with the stirring, the beaker is slowly rotated along its axis to achieve a 360° turn at the end. Allow the contents to stand for 30 seconds (until the stop watch shows 55 seconds). Without disturbing any sediment, pour about 100 ml of the contents onto a test sieve 150 μm (diameter 200 mm) while collecting liquid in the Erlenmayer flask. Put on the stopper and mix the contents thoroughly. Carry out the determination of dry matter content of the filtrate in duplicate and use the mean value for the calculation:

Dispersibility % =

T ∗ F 100 − W + T



[11,6]

where: T is the total solids of the liquid W is the powder moisture (method 11.1.1) F is 962 for skim milk and 735 for whole milk. The IDF-method is very laborious and time consuming. The dispersibility, being one of the most important instant properties, requires frequent checking in at least 2 hours intervals. In practice methods are preferred using similar procedures, but for the end-result a visual evaluation of the residue on the sieve is applied and compared with standard photos. The NZDB-method is such a method and uses 300 μm stainless steel screen cut into strips to fit into a sediment testing funnel. 26 g of powder is reconstituted in 200 ml of distilled water, the contents are stirred using a fork and while still swirling, the contents are poured into the sediment funnel/sieve assembly and sieved under vacuum within 5 seconds. The residue remaining on the funnel is rinsed with 100 ml of 45°C water; the sieve is removed, dried at 40°C and compared with the standard chart. 206

11. Analytical methods

11.8. Other methods for determination of instant properties Originally the only official IDF methods for the determination of instant properties were the Wettability and Dispersibility, but later the methods for determination of White Flecks Number and Coffee Test have been accepted (see section 11.8.4 and 11.8.5.). These properties are important, but they are not sufficient to evaluate the complex instant performance. Therefore a number of methods have been developed by various producers with the aim of detecting all other possible reconstitution defects of the powder, which can be observed by the consumer. It has been emphasized several times that instant milk products and especially whole milk powder are used in many different ways, and ideally they must not exhibit any unpleasant performance that can be visually detected. It is impossible to discuss all these methods, because many instant milk powder producers have their own methods, which they consider as confidential. It is probably not surprising that an excellent collection of methods has been prepared in New Zealand. The properties determined by these methods have been discussed in section 10. Achieving product properties.

11.8.1. Sludge The sludge test is a kind of dispersibility determination detecting those elements of undispersed material, which cannot pass a mesh of 600 μm. This material is not so much oversized agglomerates, but rather lumps created by conglomeration of fines to form thick slurry at the bottom of the beaker. The basic equipment for the determination of sludge is a 600 μm screen soldered to a brass ring of 75 mm diameter and 20 mm high. 12.5 g of powder is tipped on the surface of 100 ml water in a 250 ml beaker (water temperature see Table 11.1) and immediately stirred using a teaspoon. Stirring consists of 24 revolutions (6 clockwise, 6 anticlockwise, 6 clockwise, 6 anticlockwise) completed within 10 seconds. After standing (standing time see Table 11.1) and possibly removal of the skin by the teaspoon (see comment at Table 11.1), the content is resuspended gently with one circular and transverse movement and the content then poured onto the pre-weighed screen. The screen is then drained for 60 s on 4 layers of 2-ply tissue. After removing the liquid from the residues and the bottom and possibly walls of the screen, it is reweighed. The difference is expressed with two decimals as sludge. Table 11.1. Conditions for the determination of sludge

As indicated in Table 11.1, the cold sludge measurement is conducted at different temperatures 207

and standing times, depending on whether the powder is instant (i.e. agglomerated and lecithinated) or only agglomerated without lecithin treatment.

11.8.2. Slowly dispersible particles The test for slowly dispersible particles (SDP) is conducted simultaneously with sludge determination using the liquid from the screen filtration. This filtrate, which is collected in a 400 ml beaker is filled into a test tube (150 x 25 mm) and immediately poured back into the beaker. After 2 minutes the appearance of the film on the wall of the test tube is compared with the SDP index standard photo. It is essential that the tubes are absolutely clean and dry. Similarly to sludge, the measurement distinguishes cold SDP which is SDP 25 or SDP 45 and hot SDP or SDP 85 depending on the same criteria as outlined for sludge.

11.8.3. Hot water sediment Hot water sediment test follows in the same operation after conducting the sludge and SDP tests. The filtrate, poured back into the beaker from the SDP test tube is filled into two 50 ml centrifuge tubes (the same as for insolubility index) and centrifuged for 5 minutes at 164 G. The top liquid is then sucked off using a water jet vacuum pump down to 5 ml level. The tube is refilled with water to the 50 ml mark taking care not to disturb the sediment and then centrifuged again as before. The volume of the sediment is read on the nearest scale mark in each tube. The result is the sum of these two readings.

11.8.4. Coffee test A very important factor for good reproducibility of the coffee test is the origin and quality of the instant coffee powder. Suitable powders will have a pH of about 4.9 in 1% solution. The precipitate after conducting the test consists of so-called floaters (usually a few quite large particles remaining on the surface), flakes (dispersed tiny particles in the whole body of the solution) and sediment at the bottom of the beaker. Some methods count the floaters and sediment and express the results subjectively. It is obvious that floaters are the most undesirable because they cause an unpleasant appearance of the beverage that can be seen immediately by the consumer. The NZDB method expresses the result objectively with a number, however without distinguishing between the characters of the appearance of coffee and milk beverage. 100 ml of boiling water is added to 0.8 g instant coffee in a 250 ml beaker. After the black coffee temperature drops to 80 ± 0.5°C, 2 g of the milk powder is added and the stop watch started simultaneously. After 5 seconds, the contents are stirred with a teaspoon using a circular motion (6 complete revolutions clockwise followed by 6 complete revolutions anticlockwise). The total stirring time should be 5 seconds. After 10 minutes the sediment is re-suspended with a single gentle stir, and filled into two 50 ml centrifuge tubes (the same as for insolubility index). The tubes are centrifuged for 5 minutes at 164 G and the volume of sediment is read to the nearest scale mark in each tube. The result is the sum of both readings. In 2005 an IFD standard method ISO 15322/IDF 203:2005 based on the above described NZDB method was published as ‘Dried milk and dried products – Determination of their behaviour in hot coffee (Coffee test). 208

11. Analytical methods

11.8.5. White flecks number White flecks are tiny flakes floating in the reconstituted solution. If it is allowed to stand for several minutes they rise to the surface forming a thin layer. The original method for the detection of white flecks was just a visual observation of the reconstituted milk in a thin layer with a teaspoon placed close to the wall of a beaker as background. Alternatively the thickness of the layer could be expressed in millimetres. However, this is inaccurate and not very sensitive. When gently moving the beaker in a rotational manner the white flecks are seen as a rim on the wall above the solution. The same effect is obtained by quickly dipping a glass plate through this layer. In 1991 the IDF developed a method which expresses white flecks quantitatively. The apparatus is shown on Fig. 11.7.

24 g of tested powder is dissolved in 100 ± 1 ml distilled water at 20 ± 1°C in a 400 ml glass beaker. Stirring follows exactly the same procedure as described for IDF-dispersibility in section 11.6. Then another 100 ± 1 ml of water is added followed by 20 complete stirring Fig. 11.7. Apparatus for determination of White movements in 20 s while continuously rotating the beaker. After completion Flecks Number of stirring, the liquid is poured onto the 63 µm sieve and the stop watch started simultaneously. After 15 s, the volume of the liquid in the measuring cylinder is read to the nearest mark (value an in equation [11,7]). White flecks number is a figure between 0 and 1 expressed with two decimals:

White Flecks Number =

215 − a



215

[11,7]

The method utilizes the fact that white flecks clog the mesh, and depending on their quantity, allow only a limited amount of liquid to pass through.

11.9. Total fat content The standard method for the determination of total fat content in milk powders is the RöseGottlieb method as described in IDF-Standard 123A:1988 [38]. The amount of 1 g whole milk powder or 1.5 g skim milk powder is weighed into a graduated shaking cylinder with wellfitting stopper. Add 10 ml of water for dissolving and heat if necessary. Add 1.5 ml 25%-NH3209

solution and heat in a water bath for 15 minutes at 60 - 70°C with occasional shaking. Cool down; add 10 ml 96%-ethanol and mix. Add 25 ml ethyl ether (b.p.34 - 35°C), close the cylinder tightly and mix by turning upside down for 1 minute. Add 25 ml petroleum ether (b.p. 40 60°C) and repeat mixing as above. Allow to stand for at least 1 hour to achieve an ether phase clearly separated from the water phase. By means of a siphon, transfer the ether phase to a pre-weighed 150 ml Erlenmeyer flask rinsing at the end the siphon with a little ether. Take care not to introduce any water phase into the flask. Repeat the addition of 25 ml ethyl ether and 25 ml petroleum ether keeping the same procedure as above collecting the ether phase into the same flask. Evaporate the ether and finally dry the flask for 1 hour in an oven at 102 ± 2°C. Cool in a desiccator and weigh. The result is expressed in percentage on powder. The quick method for routine determination is the Gerber-Teichert method. Into a special butyrometer with scale 0 - 35 or 0 - 70% is added successively 10 ml sulphuric acid (90 - 91%, density 1.816 ± 0.003 g/ml), 8 ml distilled water (not to be mixed with the acid), exactly 2.5 g powder and 1 ml amyl alcohol (density 0.811 ± 0.002 g/ml). The butyrometer is closed with a rubber stopper, shaken vigorously for 5 minutes and turned several times upside-down to mix all the acid with the contents. The tube is then centrifuged for 15 minutes in a centrifuge heated to 65°C at 1200 RPM. The 5-minutes shaking and centrifuging is repeated once more. By means of the rubber stopper adjust the fat column to appear in the graduated part of the tube, spin again for 5 minutes and read the fat percentage directly.

11.10. Free fat content The determination of free fat content of a milk powder is based on extraction by fat solvents. There are many alternatives especially as to extraction time and used solvent. Suitable solvents include petroleum ether and carbon tetrachloride, although for environmental reasons the latter should not be used anymore. Extraction time can vary between 15 minutes and 24 hours. Longer extraction times give higher values. In case of free fat determination, however, the most interesting is the surface free fat which is extracted very quickly and therefore there is no reason for long extraction time. The routine method is as follows: Weigh 10 g powder into 250 ml Erlenmeyer flask with ground glass stopper. Add 50 ml solvent, close the flask and agitate in a shaking device for 15 minutes. Filter the solution into a 100 ml Erlenmeyer flask. Pipette 25 ml of the filtrate into a pre-weighed 50 ml Erlenmeyer flask. Evaporate the solvent on a hot plate (or similar) and dry in an oven at 105°C for 1 hour. The content of the free fat is expressed as percentage of the powder:

% free fat on powder =

A ∗ 25 ∗ 2 ∗ 100  ⎛ 25 - A ⎞ ∗ B ⎜ ⎟ 0.94 ⎠ ⎝

[11,8]

where: A = evaporation residue from 25 ml of solvent B = amount of used powder in g Alternatively the free fat can be expressed also as a percentage of total fat:

% free fat of total fat =

210

% free fat on powder % total fat on powder

*100 

[11,9]

11. Analytical methods

11.11. Particle size distribution The original methods for the determination of particle size distribution involved microscopic counting or sifting. Nowadays a number of sophisticated instruments are available, e.g. Malvern instruments, based on laser beam diffraction, Coulter counter etc. The microscopic counting method, used for non-agglomerated powders was very laborious, time consuming and the results were influenced by the subjective judgement of the analyst. It was necessary to count with simultaneous evaluation of the size of at least 1000 particles dispersed in toluene on the microscopic glass under the microscope. Nowadays this method has been virtually replaced by automatic modern methods. The sieving test is suitable only for agglomerated powders and is based on sifting 100 g of powder through a number of sieves in a shaking apparatus, usually for 5 minutes. The used screens are 200 mm in diameter and recommended mesh sizes are for instance: 500, 315, 250, 212, 180, 125 and 90 μm. Fat containing powders require an addition of 2% of free flowing agent, mixed gently with the powder, before testing. The expression of results can be found in Table 3.3. In the Malvern method, the powder is presented to the laser beam while airborne in a stream of air or kept in a suspension in isopropanol in a cuvette under magnetic agitation. The results are calculated by a computer and shown on a screen and print-out. Every method determining particle size distribution is influenced by the breaking down the agglomerates due to mechanical handling involved (friction on the sieve during shaking, agitation of the suspension etc.). A demonstration of this breaking-down effect can be seen in Fig. 11.8. An agglomerated sodium caseinate powder was kept under constant gentle agitation in the measuring cuvette for 600 s. A size print-out was taken every 100 s. During this time, the mean particle size dropped from 400 to 200 μm and the fines increased from roughly 2, 5 and 7 to 4, 22 and 30 μm respectively.

Fig. 11.8. The effect of agitation on mean particle size and generation of fines during Malvern measurement 211

However, the Malvern method is gentler than the sieving procedure. When comparing both methods, the Malvern method indicates smaller mean particle than the sieving test. As mean particle size increases the difference between measurements decreases and at about 200 μm the results are close to each other. Above 200 μm the Malvern method gives higher values. The former effect is because Malvern detects the fines better. The latter effect is due to the gentler conditions of treatment. The large agglomerates are not broken down as much with the Malvern method.

11.12. Mechanical stability The principle for determining mechanical stability of agglomerated powders is based on applying a defined mechanical treatment followed by determination of fines created. These are normally defined as the sifting fraction smaller than 150 μm. Before applying the mechanical treatment, this fraction must be removed from the original powder by gentle sifting. Mechanical treatment usually involves 10 minutes shaking. For a comparison of mechanical stability of various samples Malvern analysis can be used, as shown in Fig. 11.8.

11.13. Hygroscopicity The hygroscopicity is a property, which together with the method for determining degree of caking, is particularly suitable to classify whey powders as to their ability to pick up moisture from the surrounding air during storage. It is defined as the final moisture content of powder after exposure to humid air of 79.5% relative humidity at 20°C under the conditions of the method.

Fig. 11.9. Apparatus for determination of hygroscopicity

212

11. Analytical methods

The apparatus for the determination of hygroscopicity is shown on Fig. 11.9. The washing bottle is filled with a saturated solution of NH4Cl with surplus of crystals at the bottom and the apparatus is connected by means of a three-way cock to a vacuum pump. The other passage of the cock is open to the atmosphere. The cock must always be in this position when starting or stopping the vacuum pump. The surrounding temperature must be 20 ± 2°C. After assembling the apparatus with empty Gooch filter and starting the vacuum pump, the cock is turned to suck the air through the apparatus and the flow rate is adjusted to 300-400 ml/ min. After 5 minutes the flow is stopped, the Gooch filter weighed first empty (a) and then with about 0.5 g of the test powder (b). The apparatus is assembled again and the air circulation started. Check the weight increase after 4 hours and then after each hour. The measurement is completed when the difference in weight between two successive weightings is negative and the result is then the second but last weighing. 

[11,10]



[11,11]

where: a= weight of powder b= weight increase %FM= free moisture (see section 11.1.2.) The powder component, which is mainly responsible for moisture uptake, is the amorphous lactose, but also proteins and minerals can pick up moisture. During the determination the powder starts to pick up moisture, but when the level of moisture is sufficiently high the lactose starts to crystallize. When this happens, the water activity of the powder decreases and the powder loses moisture. If the process is allowed to continue to a condition of final equilibrium, then all whey powders of normal composition would exhibit final moisture content of about 12%. This occurs when the lactose is completely crystallised and the moisture increase is exclusively caused by other-than-lactose components. Well pre-crystallized whey powders reach this point with continuous moisture increase and quickly, while non-pre-crystallized powders can reach up to 30% weight increase before the moisture starts to decrease. It might be difficult to determine the maximum weight increase before the moisture begins to decrease. Therefore a much better expression for hygroscopicity is the next method, i.e. degree of caking. Thus the hygroscopicity of whey powders, expressed in terms of maximum weight increase, is classified as follows: Non-hygroscopic max. 10% slightly hygroscopic 10.1-15% hygroscopic 15.1-20% very hygroscopic 20.1-25% extremely hygroscopic > 25%

11.14. Degree of caking The degree of caking, called also cakeness is the portion of powder remaining on a given mesh when sifting under prescribed conditions after the powder has been exposed to hygroscopicity test (section 11.13.) and then re-dried. 213

After determination of hygroscopicity, the Gooch filter with the wet sample is oven-dried for 1 hour at 102 ± 2°C. After cooling in a desiccator, the caked sample is as quickly as possible transferred by means of a spatula onto a weighing paper, and the weighed amount transferred on a sieve 500 μm using a brush. The sieve is placed in a shaking apparatus and shaken for 5 minutes. The powder remaining on the sieve is again transferred on the weighing paper and weighed. The degree of caking is then:

Degree of caking % =

b a

∗ 100 

[11,12]



where: a = the amount of dried sample b = the amount remaining on the sieve. The results are evaluated by following scale: non-caking max 10% slightly caking 10.1-20% caking 20.1-50% very caking > 50% extremely caking 100%.

11.15. Total lactose and -lactose content The determination of -lactose content (of total lactose) is based on two refractometrical readings. For the first reading, all operations must be done at low temperature (below 5°C) to avoid mutarotation of the lactose. The second reading is then done after completing the mutarotation and achieving equilibrium. All the chemicals, measurement equipment and samples must be placed in the refrigerator overnight and the operation has to be done quickly to avoid a temperature increase above 5°C. Preferably, all operations should be done in a cold room. The procedure is as follows: The weighed amount of sample (a) corresponding to 1.0-1.5 g lactose is dissolved in about 10 ml cold distilled water using a mortar and pestle. The obtained paste is then diluted and transferred quantitatively into volumetric flask of 100 ml. Add 5 ml of 2.5% tannin solution, 10 ml of 10% lead acetate solution and fill up to the mark. Mix the contents and filter into a 200 ml Erlenmeyer flask. The first portions of the filtrate should be returned on the filter because it is sometimes not quite clear (if no cold room is available conduct the filtration inside the refrigerator). The solution is then filled into a pre-cooled polar metric tube. The polar metric reading (P1) must be done within the first minute. The rest of the filtrate is heated to 80°C and kept for 30 minutes. After cooling down to 5°C, the second polar metric reading (P2) is taken. Following equations apply:

Total lactose % = 90.25 *

P2 a



[11,13]

[11,14]  214

11. Analytical methods





% H 2 O cryst =

% TL − % AL 19

[11,15]

[11,16]



[11,17]



[11,18]

where: %TL % total lactose content (as anhydride) % LTL % -lactose of total lactose % A % amorphous lactose % L an % -lactose monohydrate (as anhydride) % H2Ocryst % water of crystallization % LM % -lactose monohydrate (as monohydrate) % cryst. % crystallized lactose of total lactose (degree of crystallization) The value y in the equation [11,15] is calculated from the value x which is the proportion of -lactose to -lactose at the temperature of the concentrate prior to drying. Both values are shown in Table 11.3. Table 11.3. Proportion of -lactose to -lactose at various temperatures

Otherwise a good indication of the content of -lactose monohydrate can be obtained from the difference between total and free powder moisture (methods 11.1.3. and 11.1.2.) and multiplying the result by 19 (compare with equations 10.16 and 10.17).

11.16. Titratable acidity The determination of titratable acidity is a very common laboratory procedure in the dairy industry. The various expressions for titratable acidity have been explained in section 9.4.4. Nowadays the mostly used procedure expresses acidity as ‘lactic acid’ according to ADMI standard described in Bulletin 916 [36]. The IDF-standard is IDF 86:1978 [39]. 10 g of skim milk or butter milk powder, 13 g of whole milk powder or 6 g of whey powder are dissolved in 100 ml distilled water using a mixer (as for Insolubility index, see section 11.2.). The amount of powder for other products should correspond to their natural concentration. After mixing, the solution should stand for 1 hour. Thereafter follows a gentle mixing and the transfer of 17.6 ml into a white glazed porcelain casserole using a pipette. 0.5 ml 1%-alcoholic phenolphtalein solution is added and titration is conducted using 0.1 N NaOH until a faint pink 215

colour persists for 30 seconds. Titratable acidity is then the consumption of 0.1 N NaOH in ml divided by 20. The conversion Table 11.4 for various expressions for titratable acidity is below (%l.a.=% lactic acid, Th=Thörner, D=Dornick, SH=Soxhlet-Henkel). Table 11.4. - Conversion table for titratable acidities

11.17. Whey Protein Nitrogen Index (WPNI) The procedure for the determination of whey protein nitrogen index (WPNI) was originally introduced by ADMI, exclusively for classifying skim milk powders according to heat treatment. Nowadays it is used also for other products. For instance, knowledge of WPNI is very useful when investigating quality problems of instant whole milk powder. Reconstitute 2 g of skim milk powder in a test tube (25x150 mm) in 20 ml distilled water, add 8 g of NaCl, place in water bath at 37 ± 1°C for 30 minutes while shaking 10 times during the first 15 minutes. Without cooling, shake the mixture and filter through S&S 602 filter paper (or Munktell 20H+110 or S&S 605 or S&S Selecta folding filter 572½), re-filter the first portions if cloudy and collect 6-7 ml of filtrate. Pipette 5 ml into 100 ml Erlenmeyer flask, add 50 ml saturated solution of NaCl and mix slowly. Fill two photometer cuvettes with a known amount of filtrate and add 1 drop of HCl solution (23 ml of concentrated HCl-37% in 77 ml distilled water) per each 5.5 ml into the first cuvette. Close the cuvette and mix by inverting twice. The second cuvette is used for adjusting the spectrophotometer. Set the photometer to wavelength 420 nm and adjust the transmission by means of second cuvette to 100%. 5-10 minutes after adding HCl to the first cuvette, invert it once again and make a duplicate reading of transmission. If the difference between the duplicates is greater than 2% another pair should be analysed. The result is expressed as mg of un-denatured whey protein nitrogen per g powder and is found using the graph in Fig. 11.10.

216

11. Analytical methods

Fig. 11.10. Curve for transformation of % transmission to WPNI (mg un-denatured whey protein per g powder) The WPNI is defined for 1 g of powder without considering the moisture content. When analysing whole milk powder the amount of sample must contain the same amount of non-fatsolids as 2 g of skim milk powder (supposed to contain 4% moisture and 0.5% fat). Thus the amount of whole milk powder for the analysis is calculated as follows:

Amount of sample =

1.91 100 − % F − % M

∗ 100

[11,19]

where: %M= % moisture %F = % fat

11.18. Flowability (GEA Niro [31]) The flowability is an important property of milk powders and it can be easily judged by the naked eye. However, it is not that easy to find a method which could cope with the whole range of products from easy flowable to so-called ‘dead’ powders. For instance the measurement of ‘angle of repose’ or ‘time of flow’ of a given amount of powder through a given funnel may be good enough for a particular powder. However, it may not work with a less flowable product. The equipment for the determination of flowability is a stainless steel drum according to the drawing in Fig. 11.11, attached horizontally to a motor with gear so as to operate at 30 RPM. An amount of powder corresponding to 25 times the bulk density (tapped 100 times, expressed as g/cm3) is poured into the drum. The plastic lid is fastened and the rotation started simultaneously with the stop watch. When all powder has left the drum the time in seconds is recorded. The flowability is an average of three measurements. 217

Fig. 11.11. Stainless steel drum for flowability determination The advantage of the method described here is that it is quite universal with a broad scale. Table 11.4 shows some examples of expected flowabilities of various powders. Table 11.5. An example of flowabilities of various products

11.19. Lecithin content Lecithin in cold water instant products is contained in the surface free fat layer. Thus the principle of the method is extraction of the surface free fat and determination of its phosphorus content gravimetrically. 60 g of sample is extracted by means of 300 ml solvent using the same procedure as described 218

11. Analytical methods

in section 11.10. The solvent is then evaporated to about 30 ml and transferred quantitatively into a pre-dried and pre-weighed quartz dish in which the evaporation is continued. The residue is then dried at 105 ± 2°C for 1 hour and the weight of the residue is recorded. The residue is covered with 2.5g MgO and heated on a Bunsen burner until ignition. The dish is then placed in an oven heated to 800°C and left for 2 hours or until white ash remains. The ash is transferred into 250 ml beaker with 10 ml distilled water. Using 15 ml HNO3-H2SO4mixture (1 l of nitric acid D=1.2 with 30 ml concentrated sulphuric acid), rinse the dish 3 times to transfer the residue quantitatively, heating the dish each time. The solution is heated until everything is dissolved. Using a pipette, 50 ml of ammonium molybdate solution is added (50 ml ammonium sulphate is dissolved in 450 ml concentrated nitric acid d=1.4 and 150 g of ammonium molybdate is dissolved in 400 ml hot water, which is then cooled down, mixed with the previous solution and filled up to 1000 ml). After stirring with a glass rod, the solution is stored in darkness for 36 hours. A Jena A4 glass filter funnel is rinsed with acetone and dried in vacuum at 15 mm Hg for 30 minutes. Weigh the beaker, place it on a suction flask attached to vacuum pump and filter the contents of the 250 ml beaker transferring first the supernatant. Transfer the sediment quantitatively by means of NH4NO3 solution (1 l of 2% ammonium nitrate solution mixed with 5 ml of 20% nitric acid) repeating 6 times this procedure. Suck the A4 glass filter beaker dry, fill it with acetone stirring the sediment with glass spatula and suck it dry again. Repeat the same procedure once more. Dry the A4 beaker in vacuum at 15 mm Hg and record the weight. The calculation is the following:

% free fat on powder =

% lecithin on free fat =

% lecithin on powder =

a ∗ 300 ∗ 100  c − a  ∗ b     0.94  d * 34.762 e

[11,20]



[11,21]

% free fat * % lecithin on free fat 100



[11,22]

where: a = evaporation residue from c ml filtrate b = weight of sample c = ml of filtrate used = weight of sediment e = weight of free fat

11.20. Analytical methods for milk concentrates 11.20.1. Total solids There are two methods available: the oven drying method, which is considered as an exact reference method, and the refractometrical method as a quick routine control procedure. With oven drying, milk concentrate is well mixed with sea sand. 20-30 g of sea sand is dried in a small weighing dish with well-fitting cover with a small spatula for 2 hours at 100°C. After cooling down with the lid on, the dish is weighed first empty (W1) and then again after adding of about 1.5 g of concentrate (W2). 5 ml of distilled water is added and the components are 219

well mixed with the spatula. The excess of water is first evaporated on a water bath for about 20 minutes under occasional stirring and then dried in the oven heated to 100°C for 2 hours. After cooling, the dish is weighed again (W3). The result is: 

[11,24]

The refractometrical method is based on the determination of the index of refraction of a drop of concentrate, and the measurement is conducted according to the instructions for the apparatus in question. It is an advantage to use a refractometer with scale in sugar degrees (°Brix); otherwise it is necessary to use a table for recalculation of the index of refraction to sugar degrees. It is important to keep the refractometer prism clean and grease-free by washing carefully with distilled water after each use and possibly occasionally with petroleum ether. The total solids content of the concentrate is then obtained reading in °Brix multiplied by an empirical factor, which depends on the product composition: for skim milk for whey for whole milk

0.9 0.97 1.0

Modern evaporators are equipped with density meters or mass flow meters. From the density and the corresponding temperature reading the solids content can be calculated with a reasonable accuracy from a re-arranged equation [9,13] and using equations [9,15 – 9,17]:



[11,25]

11.20.2. Insolubility index The solubility problems of milk powder are often considered as a spray drying problem. However, when troubleshooting insolubility index problems, it is a useful practice to occasionally check the insolubility index of the concentrate to find out whether the deterioration of the solubility has taken place prior to spray drying. When sampling the concentrate from the evaporator using a sampling valve, it is important to wash carefully the mouth of the valve to remove any possible dry residues of the concentrate. The method is in principle the same as described for powders in section 11.2. The amount of concentrate for the determination should be such to contain 10 g and 13 g of solids for skim milk and whole milk respectively. The concentrate is diluted with distilled water of 24°C to give 107 ml. The further procedure and precautions are the same as described previously (section 11.2).

11.20.3. Viscosity For routine check of the viscosity of all milk and milk product concentrates the viscosity is the value obtained by Brookfield viscometer type LVT, equipped with spindle no. 2 at 60 RPM and 40°C. 220

11. Analytical methods

The reference temperature of 40°C was chosen in order to enable comparison of viscosities of concentrates obtained under various conditions. Furthermore it is easy to adjust being always lower than the temperature of the concentrate immediately after sampling and because the age thickening at such a temperature is not that fast. The concentrate is filled into a 250 ml beaker to reach the level of the recess on the spindle, and stirred, possibly in a cold water bath, until the correct temperature is reached. The apparatus is adjusted to the correct height of the spindle and started at speed 60 RPM. The result is an average of three readings after the previous readings have stopped decreasing. The value of dynamic viscosity in cP is the reading average multiplied by 5. For concentrates which are supposed to be supplied cold to the spray dryer, as for instance whey concentrates, the measurement is taken at the actual temperature and using a spindle corresponding to expected viscosity. The total solids content and the temperature of the concentrate must be recorded together with the viscosity.

11.20.4. Degree of crystallization The degree of crystallization of whey concentrates is the amount of lactose in form of -lactose monohydrate present in the total lactose content expressed as percentage. The method is based on two refractometrical readings of the concentrate before and after crystallization. For the initial refractometrical reading before crystallization, it is necessary to take an average of readings taken at regular intervals (at least 10 readings for a batch) of the concentrate leaving the evaporator. The second reading is taken on concentrate from the crystallization tank. For exact determination it is necessary to know the content of lactose and total solids content of the concentrate using methods described in sections 11.15 and 11.20.1, respectively. Then the degree of crystallization is:

[11,26]

The content of lactose in the concentrate is:

L=

% L TS * % TS 100



[11,27]

where: S1 = first refract metrical reading S2 = second refract metrical reading L = total lactose content of the concentrate LTS = total lactose content of whey solids % TS = total solids content of the concentrate For a routine fast determination, it is possible to use for total content of lactose LTS a value, which is usually known in a given factory and given season. For the total solids content of the concentrate the S1-reading is multiplied by 0.97 (see 11.20.1). See also Fig. 10.22.

221

12. Troubleshooting operations The expression troubleshooting may sound quite dramatic. However, what is understood here as troubleshooting is in fact any intervention that changes the operational parameters in order to obtain the desired result. An operator of a spray dryer is exposed every day to situations where he has to decide what to do if, for instance, the final moisture content of the product is suddenly too high. This is also a kind of mini-troubleshooting intervention. On the other hand there are troubleshooting operations requiring great effort in analyzing the situation and proposing a strategy for tackling the problem. This may be the case if one of the more sophisticated properties of the powder is deviating from the specification, due to lack of dryer capacity, formation of serious deposits in the dryer, bacteriological problems etc. It is impossible to give exact instructions for each case. However, the paragraph below presents at least some guidelines for the most typical problems. The first approach to most problems involves: - careful study of the production documentation around the time when the problem appeared, especially: - log-sheets to find out any deviations from usual operating conditions, - maintenance log-books to find out what changes or maintenance work on the dryer or its components have been conducted, - laboratory analysis records to find out if any change of product quality has been detected, - check the calibration of the control instruments which may have some connection to the problem, - check independently the laboratory results, - conduct laboratory analyses of the properties, which may have some connection to the problem, but are not part of the daily routine analyses, - interview the staff individually about observations and opinions.

12.1. Lack of capacity It is not unusual that lack of capacity appears gradually or suddenly on an installation, which has been in successful operation for many years. There is only one reason, which may cause the lack of capacity: the specified amount of heat for evaporation is not available. This again may be caused either by lower T (too low inlet or too high outlet temperature) or lack of drying air. The following diagnostic steps are recommended: - conduct air flow measurement as described in section 6.6, preferably water evaporation test and cyclone pressure drop measurements. If the air flow is too low, check the damper positions and pressure drops over all components in the system, i.e. air filter, air heater, cyclones etc., - check the inlet temperature: the cause of low inlet temperature can be too low steam pressure. If the steam pressure is correct, the problem may be caused by loose fins in the steam heater or by malfunction of condensate traps, - check the outlet temperature: the cause of too high outlet temperature (when obtaining the same moisture content as before) may be either poor atomization or wrong air flow pattern (air disperser adjustment), - poor atomization is due to either malfunction of the atomizing device, or changes of feed properties, 222

12. Troubleshooting operations

- check rpm and direction of rotation of the rotary atomizer and the performance of the liquid distributor, - check atomizing pressure, sizes and condition of nozzle inserts of pressure nozzle system, - check the properties of the feed, especially viscosity. Too high a viscosity can be caused by increased heat treatment (pasteurization) conditions, increase of total protein content of the processed milk, increased solids content or higher homogenization pressure in case of fat-containing products. With rotary atomizer operations, the pressure on the top point of the feed line should be positive - negative pressure can cause pulsation of feed supply. Pressure nozzles require a daily check. Spray nozzle performance has a critical influence on the operating costs as well as on the quality of the product. It is easy to control a single nozzle plant by comparison of atomizing pressures of successive operations, while malfunction of one nozzle in a multi-nozzle assembly is less apparent. A worn nozzle exhibits a dramatic increase of flow rate and consequently larger droplet size. The performance of nozzles can be checked by following methods: - on each start-up on water, adjust the inlet temperature for operation with one nozzle. Under constant feed rate exchange the nozzles one by one and check the pressure. A difference >10% requires attention, - simultaneously with the previous step check the atomizing cloud visually (preferably against the light source). Correct nozzle operations exhibit a uniformly transparent cloud while streaky sections indicate an erosion problem. A difference in spraying angle indicates the same, - clean the nozzle inserts carefully manually after each operation. Do not use hard tools but wood, plastic materials and brushes. Do not use cleaning solutions, especially acid which may cause corrosion, - check the conditions of inserts frequently under the microscope. Many modern plants are nowadays equipped with nozzle test stands, where most of the above mentioned checks can be performed. Some typical examples of capacity troubleshooting actions are given below: Case I: An old dryer, which has been in operation for 20 years, has during the last several months exhibited a gradual capacity decrease, which finally resulted in only 60% of specified capacity. A check of cyclone pressure drop indicated that the dryer had too low an air flow and this was confirmed by a water evaporation test. Further investigation of pressure drops in the system revealed a high pressure drop across the main air filter, and visual inspection confirmed that the filter material was dirty. Mounting of clean filters cured the problem. Case II: After an overhaul a dryer exhibited lack of capacity, as a higher outlet temperature than previously was required to achieve the same moisture content as before. A check of the cyclone pressure drop confirmed that the air flow was as specified. Investigation of feed properties did not give any indication of increased viscosity. Powder properties in comparison with samples of earlier produced powder showed slightly higher mean particle size and higher content of occluded air. Thus the attention was directed to the atomization. It was found that the rotary atomizer which operated with a curved vane wheel was rotating in the wrong direction. During the overhaul, the phases on the atomizer motor were incorrectly connected. 223

Case III: During commissioning of a new dryer, the maximum obtainable inlet air temperature was about 20°C lower than specified. The steam pressure and temperature were correct. Closer investigation indicated that the fins on the steam heater were loose. Case IV: An old dryer suddenly required 6°C higher outlet temperature to obtain the specified powder moisture. Nothing unusual was found during the whole checking procedure as outlined above. The closer inspection of the system revealed that the reason was leaking steam heaters. Generally the lack of capacity due to lack of main air flow is relatively seldom and easy to find and cure. On the other hand, a lack of air appears quite often on fluid bed dryers, both with fluid beds integrated in the dryer chamber base and externally mounted ones. This, however, does not result in a too distinct capacity loss - if any at all - but can cause fluidization problems leading, in the extreme, to plugging of the fluid beds. This problem develops slowly and is caused by a gradual, partial blocking of the holes in the perforated plate in combination with the air flow being controlled by keeping a constant pressure below the plate. Consequently the best way to avoid this problem is to keep a constant air flow. This can be done by controlling the air flow frequently by means of Pitot-tube measurements (see section 6.5) and adjusting the damper accordingly. A better way is an air flow meter installed in the air inlet duct, combined with automatic damper adjustment for constant air flow. Fig. 12.1 shows measured values of static fluid bed air rates and plenum pressures during the period of 10 days between CIP. The air flow was controlled by a flow meter and kept practically constant (56,860-57,380 kg/h), i.e. the fluidizing velocity was also constant (0.955-0.961 m/s). The third curve on the graph shows what the air flow would be if the plenum pressure is kept constant i.e. 152 mm WG as it was after CIP. The fluidizing velocity would gradually drop from 0.957 to 0.848 m/s.

224

12. Troubleshooting operations

Fig. 12.1. Demonstration of the consequences of a dirty plate on fluid bed air rate

Obviously a fluid bed, which is supposed to operate with powder of higher moisture content, is exposed to higher risk of deposits and blockages than a fluid bed operating with dry powder. Therefore an integrated fluid bed is also affected faster by powder deposits than an external one. However, such deposit creations do not take place that much during normal operation, but rather during plant start-up and shut-down.

12.2. Product quality In a factory producing the highest quality products, small adjustments of the plant operating conditions are part of the daily routine in order to compensate for small changes in external factors, mainly air humidity and milk composition. Factors influencing individual properties are described in chapter 10. The most important product properties, i.e. moisture content, insolubility index, scorched particles and bulk density are usually detected by process control in 1 - 2 hour intervals. The key person to decide what action to take to re-establish the desired target powder specification is the operator. The task of the technologist is to follow the variations in incoming milk and powder quality and to recommend the operating conditions for the operators. However, it is not unusual that suddenly, without any obvious reason, quality exhibits a serious deterioration. A more extensive investigation must then be conducted following the guidelines presented in the introduction to this chapter and using the guidelines for all involved individual properties as described in chapter 10. 225

One of the external conditions, which can fluctuate quite extensively and which may have considerable influence on both powder quality and capacity, is the ambient air humidity. It is surprising how little attention is paid in practice to this factor. A humidity-meter placed in the main air intake can eliminate the necessity for a powder moisture adjustment, thus enabling a preventive correction before a deviation appears. Fig. 12.2 shows levels of air humidity during a two week period (February 1992) in New Zealand (North Island). It is interesting to observe that the daily variations are between 3 - 7 g/kg. The maximum humidity in this period was 21 and minimum 11 g/kg. This would mean that for a single stage spray dryer operating at a constant outlet temperature, the moisture fluctuations would be in the range of 0.86% (see Fig. 10.1) or alternatively it would be necessary to adjust the outlet temperature within the range of 4.3°C.

Fig. 12.2. An example of ambient air humidity fluctuations

Fig. 12.3 demonstrates the average month’s values and maximum month’s values of air humidity on the European continent near Munich. The daily variations cannot be seen on this graph, but the difference between average and maximum values indicates that they are of the same order of magnitude as shown on the previous figure.

226

12. Troubleshooting operations

Fig. 12.3. An example of ambient air humidity fluctuations Another factor often ignored, but with a great influence on the powder quality, is the variation of the composition of the processed milk.

Fig. 12.4. An example of the seasonal variation in protein content

Fig. 12.4 demonstrates the variations of total protein content in non-fat solids of milk in a region on the North Island of New Zealand. Such variations are extreme and seldom seen in other parts of the world. In Europe the maximum value is about 42% protein in nonfat solids and the variations are smaller. The influence of protein variations on viscosity is shown in Fig. 9.7. Generally, milk with up to about 40% protein can be processed by adjusting the operating parameters, mainly the concentrate solids content correspondingly, but above this level, protein standardization is advisable.

12.3. Deposits in the system A build-up of powder deposits in a spray dryer is obviously undesirable due to: - product loss, - downgrading of the powder quality, - jeopardizing a smooth operation, with the need for more frequent dryer shut-down, - requiring more frequent dryer cleaning (washing) and thus increased down-time, - initiating stainless steel corrosion, - increasing risk of spontaneous combustion and fire. 227

Powder deposits can occur in any place in the system, i.e. chamber, fluid bed, ducts, cyclones, bag filters, rotary valves etc. The creation of deposits depends on the combination of four main factors: 1. Poor adjustment of air/spray cloud mixing, usually related to the setting of the air disperser. 2. Too high a moisture content of the particles reaching the wall of the chamber, duct, cyclone, fluid bed etc. Two-stage process is therefore more exposed to the risk of deposits than single stage drying, and the higher the moisture content from the primary drying stage, the higher the danger of product build-up. 3. Certain chemical and physical properties of the product, i.e. hygroscopic and fat-containing powders tend to create more wall build-up than non-fat and non-sticky products. 4. Product type, i.e. non-agglomerated products especially in combination with two stage process are more likely to form deposits than agglomerated powders, due to their poor flowability. Dry powder deposits occurring in relatively cool (i.e. close to outlet temperature) places in the system and reaching just a few mm in layer thickness during operation and disappearing during plant shut-down can be considered of no importance regarding powder quality or risk of fire. However, thick wet deposits on the chamber walls and cone present a potential risk of developing heat by the Maillard reaction, leading, under certain circumstances, to fire. Also deposits occurring around the hot edge of the air disperser, if discoloured and burnt, exhibit a high potential risk. Fire risk is discussed further in section 12.4. If increased amounts of deposits occur in the drying chamber with a product, which has previously been produced without difficulties, it is advisable to compare all the present operation parameters, including moisture, with those from earlier operations and, if no difference or fluctuations are found, the calibration of the measuring instruments should be checked. The feed conditions have also to be checked whereby special attention must be paid to the viscosity and acidity. Excessive homogenisation pressure on fat containing products can also be the reason for high viscosity. For a new spray dryer the first approach to solve the deposit problem in the chamber is to check whether the feed and atomization systems are operating correctly and to inspect the atomizer cloud (see also section 12.1). An air disperser for a rotary atomizer must have adjustment possibilities. Usually these are (see 4.2.4): a) the height of the guide cone - lowering the cone increases the gap and thus the amount of inner air, passing down over the atomizer wheel edge, b) outer vanes on the guide cone - controlling the rotation of the outer air and swirl in the drying chamber, c) inner vanes on the guide cone - controlling the rotation of the inner air. The spray cloud can best be observed if portholes are situated at the level of the atomizer facing a light source. It can be seen clearly on fat-containing or agglomerated products, but is more difficult with dusty powders. Under specified feed rate conditions the cloud should have a shape of a broad umbrella without any back flow (eddies). The water cloud is much steeper with the diameter of the lower part about three wheel diameters.

228

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Adjusting air dispersers requires experience and should preferably be conducted by the equipment manufacturer. A question often asked is whether it is possible to have a deposit-free system. Before answering this question, let us define the meaning of a deposit-free system and what the limitations are. A suitable definition is an installation, which can operate for a long period of time without the necessity of any cleaning between programmed shut-downs, and operates without any adverse effects on powder quality or safety. The achievement of such a goal requires elimination of the factors responsible for formation of powder deposits: 1. The deposits, which are observed after the operation, i.e. usually after 20 hours run, can have been created during just a few minutes of operation when operating conditions are out of control. 2. In most cases, the deposits are not created under normal operating conditions, but during the start and shut-down of the dryer. These are the most critical phases and very difficult to control even by experienced operators, if the plant is equipped only with simple control instruments, which just indicate inlet and outlet temperatures and amperes for the load of an atomizer or nozzle pressure. Powder deposits can be created even during start-up on water because normally there is powder from the previous run circulating, which will be wetted. Such deposits, when touched, feel like sandpaper. The most critical phase of an operation is the switching from product to water under a shutdown operation. At the moment when water reaches the atomizer, the feed rate must be cut down to about half. It happens very often that the water reaches the atomizer at a rate equivalent to the full product rate, and consequently wetting of the walls is the result. This cannot be seen immediately on the outlet air temperature because, due to air retention time in the chamber, the reaction is first seen with about half a minute’s delay. Moreover, part of this water reaches the walls without being evaporated and thus does not influence the outlet air temperature. A very helpful procedure for tracing the conditions during this critical phase is to place a thermometer close to the wall (at about half of the cylinder height). The temperature measured is very often well below the outlet temperature as measured by the thermometer in the control room. An example of such investigation is shown in Fig. 12.5, where the manually measured wall temperature is indicated by symbols, while the other parameters were obtained from the dryer’s computer. The shut-down procedure was in this case automatic using the outlet air temperature to control feed rate. The best way to solve start-up and shut-down problems is automatic control of feed rate based on in-line measurement of the total solids content of the feed.

229

Fig. 12.5. An example of poor shut-down procedure 3. A similar effect can occur due to variations of ambient air humidity, as shown in Fig. 12.2. It can rise during a couple of hours quite considerably resulting in higher powder moisture if the outlet air temperature is kept constant. Measurement of the ambient air humidity is helpful to correct the outlet temperature before any problem will appear. 4. Type of drying process is of course of primary importance. Two stage drying has many advantages, mainly better powder quality, higher powder output and better economy. On the other hand, there is a disadvantage of higher tendency to powder build-up. Optimizing a two stage process as to proper outlet temperature and powder moisture is not exactly easy and requires trial and error. This means that the extent of two stage drying process has to be chosen with respect to the product in question, mainly to its moisture/stickiness relationship. 5. Type of product plays a dominating role and here is no alternative. In some cases, however, it is possible to influence product sticking behaviour. For instance, pre-crystallization of lactose helps enormously when drying whey products, agglomeration generally improves the dryer performance, and dosing of free-flowing agent through the chamber ceiling helps to overcome deposit problems with high fat products. 6. Drafts of cold air along the chamber and other dryer components, especially cyclones, may cause deposits. Pressure relief doors or panels, which according to the safety directives lead to the outside free area and have heavy frames, may create cold bridges that generate deposits. Thus any draft of cold air in direct contact with the crucial parts of the installation has to be avoided. Good results have been obtained by shielding the pressure relief doors or panels from the outside atmosphere with plastic foil or polystyrene panels, and heating the space next to the doors to about 60 - 70°C. 7. If the surrounding temperature around the dryer is too low, deposits may appear on the supporting structure of the chamber. Deposits do not arise only by accumulation of wet, 230

12. Troubleshooting operations

sticky material on the walls. Also dry powder can create powder-drifts on the leeward side of previous deposits. It is not unusual that, in order to provide a pleasant atmosphere for the personnel, the whole dryer area is air-conditioned. Apart from such an installation requiring relatively high both initial and operating costs, it is often a source of problems when drafts of cool air flow across vital parts of the installation. Besides, with the well instrumented and operated plant, there is no reason for operators to stay long in the dryer area. 8. The air disperser is one of the critical points where a dangerous deposit can occur around the hot edge. The hot edge is normally cooled by blowing relatively cold air around it or by forcing cold air to pass a space between the hot air disperser parts and the chamber ceiling. If this air is too cold, condensation may cause an annular deposit around the air inlet. The inner edge of such deposit is often burnt. It can be solved by heating the air to not less than 40°C. 9. A very important factor is also the temperature profile in the hot air duct prior to the air disperser. Temperature stratification can be caused by malfunction of the steam air heater, i.e. condensation pots or leaking finned tubes, or by an uneven air velocity profile into the heater. 10. The reason for deposits can be poor atomization due to worn-off nozzles. It is advisable to frequently check the state of nozzle inserts under the microscope. For rotary atomizers poor atomization can take place after an overhaul, if the wheel is rotating in the wrong direction. However this has a minor effect with a straight vane wheel, but can be a serious problem with a curved vane wheel. It has been mentioned above that deposits can also cause corrosion of stainless steel. This can happen with powders containing chloride ions, e.g. hydrochloric acid whey. Apparently innocent powders can also initiate corrosion. Corrosion has been shown to occur when drying coffee whitener having glucose syrup, manufactured by the acid conversion process instead of the enzymatic. Such pit-corrosion does not appear during normal operation, but during the down-time period if the dryer is allowed to stand cold, with powder deposits, for longer periods of time. The best way to avoid start-up and shut-down deposits is logically to avoid these procedures. The dryer can operate continuously for long periods with many products, and the only reason for interrupting the operation is the necessity of CIP, mainly of the evaporator and the feed system including the atomizing device. The problem of the evaporator can be solved by installing a second evaporator which will feed the dryer while the first is under CIP. As to the atomizing device, nozzles have an advantage being interchangeable. Another possibility is installing a second feed system involving feed pump, feed line and nozzles and to replace nozzles of system I by nozzles of system II during operation. A number of plants equipped for continuous operation are now in operation. Very helpful means for approaching a deposit-free chamber is Computational Fluid Dynamics (CFD) program. The use of CFD design concepts minimizes deposit formation and removes potential ignition sources. This is a great benefit when designing new plants. An example of a CFD-diagram illustrating the air flow in a Multi-Stage-Dryer is shown in Fig. 12.6.

231

Fig. 12.6. Example of CFD diagrams. A: Air velocity profile. B. Evaporation rate. C. Temperature profile.

12.4. Fire precaution Fires in milk powder plants have probably been experienced from the beginning of using dryers, but the first publications on this subject appeared around 1970’s by Písecký [40] and Sapryngin & Kiselejev [41]. The main reason for these accidents was that the potential danger had not been recognized at that time. Two main factors were responsible for a series of fires during that period: starting production of high-fat milk powders and introducing a new process element, i.e. a fluid bed, into the system. An explosion may also occur under certain circumstances. In order to be able to evaluate the risk of fires and explosions it is important to know all characteristics of the products produced. Today many plants are multi-product plants operated with frequent changes of products. Plants are often pushed to their capacity limit, increasing the risk of fires and explosions. It is important to realise that only the product produced that can ignite, not the plant.

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Fires and explosions can be ignited in many ways and in EU Directive ATEX 1999/92 and EN 1127 a list of 13 potential ignition sources that have to be listed. Some of these may not be relevant for spray drying or for the products produced in the milk industry though. Every product should have a MSDS (Material Safety Data Sheet) with the following data: LEL = Lower explosion limit MIT = Minimum ignition temperature of dust cloud or gas LIT = Minimum layer temperature (5 mm dust layer) MIE = Minimum ignition energy MAIT = Minimum auto-ignition temperature (Grewer) LOC = Limiting oxygen concentration K st = Maximum rate of pressure increase (1 m3 vessel with dust) = Maximum rate of pressure increase (gas) Kg Pmax = Maximum explosion pressure BZ = Burning behaviour (Brennzahl) Experience has shown that the main reasons for fires are the following: 1. Deposits in the system leading to heat development and spontaneous combustion in the powder deposit layer due to the exothermic Maillard reaction between carbohydrates and protein. 2. Deposits on areas exposed directly to the incoming hot air and becoming overheated, burnt, and eventually glowing. 3. Particles in the supply air getting hot in the air heater. 4. Friction of metal parts, causing local areas of heating. 5. Incidents caused by inappropriate dryer operation. Self-ignition within deposits is the most important cause of fires in milk powder plants. The exothermic reaction develops heat due to reaction between carbohydrates, usually lactose, and proteins. The rate of this reaction depends upon the thickness and porosity of the layer, surrounding air temperature and composition of the product. The time necessary for developing a smouldering or glowing core within a deposit varies between one and many hours (see Table 12.1).

233

Table 12.1. Critical conditions for fire development. (For whole milk powder)

Therefore, the most hazardous powders are theoretically those consisting mainly of lactose and proteins, like skim milk powder. However, the heat created by the exothermic reaction partly dissipates out of the layer due to ventilation and heat conduction. The rate of heat dissipation depends on the layer porosity. Skim milk powder deposits are usually porous so that the generated heat can easily dissipate. On the other hand, fat containing powders, especially high-fat products can create compact non-porous deposits in which the fat seals the pores, preventing heat release. The particle size of powder deposits plays an important role too. Fines of particle size less than 50 μm, having high specific surface area and little interstitial air have a higher fire risk than a powder with particle size above 100 μm. This explains many of the incidents in bag filters. The critical self-ignition temperature depends on the product composition. It is lower with fat containing powders and especially when using fat, containing a high amount of unsaturated fatty acids. Higher moisture content will accelerate the selfignition process as well. Apart from the hot spots around the air inlet, the temperature in the system is seldom higher than 100°C. The exothermic reaction under these conditions cannot take place in layers thinner than 50 mm. However, heat development can reach critical levels within a few hours with layer thicknesses above 150 mm. Agglomeration helps to increase layer porosity and thus reduce the temperature rise. If all the conditions for positive heat and temperature development are available, a glowing core will form inside the layer. The exothermic reaction leading to spontaneous combustion can take place also in powder deposits in other components than the chamber itself, i.e. cyclones, bag filters etc. The reaction rate depends also on the specific surface area of the product and therefore may proceed faster in deposits consisting of fines. Deposits on hot spots exposed directly to the hot drying air become burnt and glowing on their surface. Such spots can occur around the air disperser and the atomizing device. The most frequent cause in pressure nozzle dryers is concentrate leakage at the nozzle assembly. 234

12. Troubleshooting operations

A wheel of a rotary atomizer can generate heat due to friction with the atomizer skirt or liquid distributor if incorrectly assembled or with the deposits if the wheel is not properly cleaned. Generally, a flooding sensor to detect concentrate leakage is standard component of both nozzle and rotary atomizers and the operation must be stopped whenever flooding occurs, in order to avoid concentrate entering into the air disperser. The appearance of glowing material inside the dryer does not always leads to a fire. There have been cases where a glowing particle or lump has left the system without causing any harm. If, on the other hand, such a particle or glowing lump falls into a fluid bed and mixes with fine fluidized or elutriated powder particles, then a dust explosion with subsequent fire can take place. Another cause of ignition can be impurities in the air supply system both prior to and after the air heater and inside the air disperser. Reasons of such contamination of the heating element or hot surfaces may be: - absence or not properly working air filters, - splashing of concentrate into the air disperser, - CIP-device spraying into an air disperser, which is not properly shielded, - natural draft immediately after plant shut-down, causing fine particles to flow back into the air disperser, - fines return system causing fines to blow into the air disperser. A light source in front of a chamber porthole (inspection port) should never be installed without a timer switch as permanent operation can cause overheating the deposits on the glass up to ignition temperature. The return of fines to the atomizer cloud for agglomeration requires special attention as this operation takes place close to the hottest parts of the dryer. The reason for any deposits appearing round these hottest parts must be found and removed. The outlet air passing the cyclones is relatively cold so that there is no danger of direct overheating. However when wall deposits and cyclone cone blockages occur exothermic reactions can develop heat and cause smouldering even at that low temperature after sufficient time. Therefore cyclones are components to which extra attention should be paid frequently during operation. Measuring the cyclone tip surface temperature is a useful and inexpensive method to detect a blocked cyclone. The surface temperature of a blocked cyclone is considerably lower than under normal running conditions. There are known cases where fire has been caused by welding works close to an operating dryer. The consequences of flame sterilization of a spoon for microbiology samples at the fluid bed sampling porthole have been experienced as well. Such happenings may sound an extreme. However, many cases of fires, the cause of which has never been found, may very well belong to this category. A person involved in such cases and surviving the accident with shock is not always willing to disclose the facts. However, this emphasizes the necessity of training and education of the entire staff in order to avoid such accidents. The simplest procedure to detect the beginning of heat discolouring of the powder is the scorched particle test (see section 11.5) conducted frequently in at least hourly intervals. Many, if not most, recorded accidents could have been avoided if this test had been conducted and consequent action taken, i.e. the installation stopped immediately. No statistics are available on the number of fires appearing after a positive scorched particle test, when operation was 235

allowed to continue just to empty the evaporator. However, the number would probably not be negligible. Nowadays temperature surveillance of nozzles and areas around them can be done by means of infrared cameras (GEA Niro SPRAYEYE™). A dust explosion occurs when air-borne, finely dispersed combustible solids are exposed to an ignition source, and requires the following conditions: - sufficient concentration of an exposable air-borne dust, - source of ignition of sufficient strength, - presence of oxygen in the surrounding atmosphere. Powdered milk products, in general, are not considered as particularly hazardous powders. The minimum explosion level of dust concentration for milk powders is considered to be 50 g/m3. The average concentration of the milk powder in spray dryers is also around this figure. However, not all of this can be considered as exposable dust and not all regions of an installation have this critical concentration. Particle size or specific powder surface area also plays an important role. Therefore coarse agglomerated powders are considerably less hazardous than non-agglomerated products with small mean particle size. To minimize the consequences of a dust explosion and to protect both personnel and equipment, the initial explosion must be contained, suppressed or vented. The containment method means construction of a dryer as a pressure vessel, which is strong enough to withstand an explosion without rupturing. This method is suitable only for small laboratory scale units due to fabrication costs. Explosion suppression requires detection of an explosion in its very early stage, activating an instantaneous injection of a chemical suppressant to extinguish the flame before an overpressure develops. An explosion is detected in milliseconds by a pressure or infrared sensor. The most applied method is using explosion vents in a form of hinged doors or bursting panels that are ducted to the outside of the building. The vent duct should be as short as possible, preferably < 3 m. An example of a sanitary explosion vent module is the GEA Niro DRIVENT TM. Many publications on this subject are available. The most important are issued by VDI 3673 [42] (Verein Deutscher Ingenieure, 1979), ABPMM (Association of British Preserved Milk Manufacturers, 1987) [43] and IDF (International Dairy Federation, Bulletin No 219/1987) [44]. Generally it has been accepted to use the venting area as recommended by VDI 3673, i.e.: Venting area, m2 = K * V2/3 

[12,1]

where: V is the vessel volume in m3 K is a constant defining critical volume ratio. The recommendation not to use the whole volume of the dryer, but only a part, expressed by the constant K, is based on the consideration that the critical dust concentrations of 50 g/m3 in dryers commonly used in dairy industry are only present in the conical part of the chamber or even just a part of the cone. This means that for the calculation [12,1] only 25% of the chamber volume, or even less, is used. 236

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Explosion vents can prevent severe injury and damage to both personnel and equipment. However, a fire, even a short one, can cause irreversible damage to the stainless steel. Therefore it is important to cool the walls as quickly as possible. A fire extinguishing system must be able to deliver quickly 20-60 m3 of water and to ensure effective wetting of the walls. For this purpose, a number of spray nozzles are installed in all parts of the installation (chamber, ducts, fluid beds, cyclones, bag filters). The extinguishing system is controlled by a separate control panel with separate sensors placed throughout the plant. Control is usually based on the outlet temperature with a two level alarm system: Level 1 is a visual and audible warning, and Level 2 automatically activates the fire extinguishing system. Preventing the possibility of fire or explosion in the dryer requires: 1. To follow strictly the operating instructions for the dryer and operating parameters for the product in question. 2. To follow strictly the instructions for dry cleaning and washing of the installation and checking all parts for deposits after each operation and cleaning. It is very important to completely dry out the plant after wet cleaning. 3. To follow strictly the operating parameters during the operation and check the relation between the amount of feed and the amount of final powder to ensure that the product leaves the plant continuously. 4. To frequently check for presence of scorched particles in the product, using the scorched particle test or by visual control by sifting about 1 kg of powder. 5. To check smell. Exhaust air exhibiting a typical acrolein smell is also a quite useful way of detection, especially as it appears in the very early stages of the combustion. Once dried milk smoulders, it produces also significant amounts of carbon monoxide. A modern method of tracing early stages of smouldering by the detection of CO was suggested by Steenbergen et al. [53]. Based on this principle, a fire detection system was developed, consisting of an air sampling and air sample preparation unit together with a sensitive CO analyzer. CO emission sources in the neighbourhood of the drying plant may cause interference, but the system can compensate for this. CO detection systems are installed in many new dryers nowadays. 6. Last but not least: education and training of the personnel enabling them to be able to make a fast and qualified decision to stop the plant immediately when an indication of danger occurs. Should, in spite of all precautions, a fire occur the following basic procedures are recommended: 1. Stop all fans. 2. Stop all heat supplies. 3. If fire extinguishing equipment has not started automatically, activate it manually. 4. Change from feed to water with highest possible rate. 5. Call the fire brigade. Experience has shown that only a few percent of the registered fires are accompanied by explosions and that the present standard of explosion vents and fire extinguishing systems is safe enough to avoid damage to both personnel and equipment. Apart from the proper 237

functioning of the pressure relief venting and fire extinguishing equipment, which must be checked regularly, the most important aspects are qualified training of the personnel and the following of the elementary rules. Many of the fires registered could have been avoided if these two conditions had been met.

12.5. Principles of good manufacturing practice The vast amount of products produced by the milk powder industry is used for human consumption and a significant part for baby nutrition. Therefore milk powder factories have to be designed and operated to ensure both safe and wholesome manufacture, and avoid risks associated especially with pathogenic microorganisms. Several General Codes of Hygienic Practice are available (IDF Document 123-1980 [45] and 178-1984 [46], FAO/WHO Codex Alimentarius Commission’s Document CAC/RCP 31 1983 [47], ABPMM publication 1987 [48]) and IDF Recommendations for the Hygienic Manufacture of Spray Dried Milk Powders (IDF Bulletin No.267/1991 [49]). All these documents give advisory guidelines requiring that each factory elaborates its own Code of Practice adapting the general rules to local conditions. Processing equipment, from a hygiene point of view, is safe due to the design being based upon many years of experience by recognized manufacturers. Designing a building and assembling the individual processing units into a production line often requires compromises due to local conditions and requires close cooperation of the architect with the specialists from the equipment manufacturer and dairy company in order to avoid costly errors. The most important principles are briefly discussed below. Powder plants should be located in areas free of any air-borne pollution such as industrial exhaust and flue gas, agricultural odours and dust from heavy traffic roads. Service roads and yards must be hard surfaced with wear-resistant material such as asphalt or concrete, and with good drainage and maintained in good condition free of cracks. The drainage system must be designed to handle peak volumes of rainwater. Buildings must be of a type which will prevent contamination of the whole internal environment. Pitched roofs with an angle greater than 20° are preferred over flat roofs. Walls and floors must be waterproof, non-absorbent with smooth surface to allow easy cleaning. Ceilings must be executed to avoid condensation. The maintenance of a positive air pressure in the buildings and an air flow from the processing and packaging areas to the raw material area by means of an air conditioning system is recommendable. However, the temperatures must not be kept below normal room temperature levels and direct flow of any cool air onto any critical parts of the dryer must be avoided. Potable water must be available for hygiene and processing operations and must meet the requirements of WHO-standard for drinking water. The main drying air must be taken both from the outside and top of the building. Drying air of temperatures higher than 120°C requires only coarse filtration corresponding to EU1-EU4 class. All air coming in contact with the product with temperatures less than 120°C must be filtered to at least Eurovent EU7 class. The use of more effective filtration, up to EU13, may be required, depending on the product. Air filters must be cleaned when necessary (as indicated by pressure drop). 238

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All seals, gaskets and sleeves, especially on units operating under partial vacuum, require special attention and maintenance to avoid that the surrounding air is sucked into the dryer. Cooling water for process air conditioning must be recirculated under partial vacuum to avoid penetration into air ducts in case of leaks. Air de-humidifying units must allow for cleaning and maintenance of all heat exchange surfaces. The effluent waste water system must be designed to handle peak load conditions and must be maintained to the highest operational standards at all times. The changing areas, showers and toilets with hand washing and sanitizing facilities must be provided with good ventilation and without direct open access to the processing areas. There must not be any other access to the operation rooms than through the changing area and the protective working clothing must be taken on or off whenever passing this area. In processing areas, hand washing, sanitation and hand drying facilities must be available. Paper towels are preferred. Visitors and contractors must observe the same rules as production personnel. All material fed to the spray dryer processing area must receive a heat treatment corresponding to pasteurization. The pasteurizer must be provided by flow-diversion valve and pasteurization temperature must be measured and recorded. The thermometer must be checked at least once a week. For concentrated milk handling, parallel systems of balance tanks, which are switched and cleaned every four hours, are recommended to reduce potential bacterial growth. Concentrate reheating facilities prior to spray drying is recommendable to avoid the bacterial growth in the feed line. Spray dryer insulation must be constructed to avoid invasion by water and should preferably be removable for inspection. Washing facilities for the rotary atomizer or nozzle units must be constructed to avoid wetting the top of the dryer. The walls of the spray dryer must be checked at least once a year for cracks. The fire water sprinklers must be checked frequently for leakage. The water from fire sprinklers must be drained after each use. The pressure relief doors leading directly to the outside can in some climates cause excessive cooling of external chamber surfaces causing condensation on the internal surfaces. Enclosing the venting duct with a plastic membrane and possibly heating the air between the membrane and pressure relief doors is recommendable. Not only the chamber with removable insulation panels (see Fig. 4.3), but also all the other components of the installation have to be designed to be cleaning-friendly and of hygienic construction such as fully welded fluid beds etc. The spray dryer building interior must be maintained in dry conditions, and dry cleaning by means of vacuum cleaner has to be preferred. Wet cleaning if and when required must be conducted in such a way to avoid excessive wetting. Any water remaining on the floors should be wiped off and dried with a cloth. Operators of a spray drying installation are responsible for cleaning and disinfection of the equipment. The evaporator, spray dryer feed line and the atomizing unit must be wet cleaned and disinfected after each operation. The spray drying chamber, internal and external fluid beds, cyclones, ducts and powder transport lines should be dry cleaned when necessary. Wet 239

cleaning should be applied as a consequence of product accumulation, discoloration or inferior product quality. The wet cleaning procedure has to be done according to the instructions from the equipment manufacturer. An important step in the wet cleaning procedure is an inspection to check whether all powder residues have been completely removed. All water remaining in the dryer after wet cleaning has to be completely drained and dried by circulation of hot air through the plant. The plant must be completely dry before starting a new production run and the dryer has to be inspected with special attention to critical wet spots as manholes, sleeves, gaskets, fluid bed plenum etc. An important element in maintaining high hygienic standards in a milk powder factory is regular education and training. The responsibility for arranging training of all personnel remains with the management. For this purpose, management should elaborate their own, detailed Code of Hygienic Practice based on publications mentioned in the introduction to this section. The presence and the extent of bacterial contamination of dried milk products originates from: a) microorganisms surviving the process, b) microorganisms surviving and growing during the processing, c) microorganisms contaminating the product during or after processing. To the first two groups belong primarily the spore-forming bacteria which are heat-resistant and may survive relatively high heat treatment conditions. The presence of spore-forming bacteria may be an indication of poor raw milk quality. Some non-spore-forming bacteria may be also relatively heat resistant.

12.6. The use of computer for quality control and trouble-shooting Every milk powder factory should have in the organization a function to deal with troubleshooting. This function represents a link between production and quality control. The person who carries out this function - a technologist - should have a thorough knowledge of milk powder technology, and also have the daily responsibility to generate a list of the set points of technological parameters for the next operation. To be able to carry this responsibility the technologist must collect information of the operating conditions and product quality data from each production, analyze the data and draw conclusions, based on statistical evaluation of the information collected, and decide which technological parameters (or combination of parameters) are optimum for achieving the desired quality properties of the final product. Achieving first class powder quality is the prime goal. As earlier emphasized, efforts for improving or just maintaining this quality can be considered as a kind of troubleshooting. It would not be quite correct to think that with the present state of the art of equipment and technology, the optimum operating conditions are fixed. The performance of each spray dryer is influenced by a number of factors, and the optimum operating conditions to achieve top product quality have to be found individually for each plant and for each product specification. They must be checked daily and revised at relatively short intervals. Factors that cause variations in final product quality are, for example: raw milk quality, composition and seasonal fluctuations, plant location, building execution inclusive of air flow and temperatures around the plant, type and layout of the installation and its individual components, climate and air condition variations, especially humidity, during operation, etc. Computer control is now universally applied for spray drying installations. The whole operation, including start-up and shut-down, is controlled by a computer, and during production the 240

12. Troubleshooting operations

operational parameters can be printed out at regular intervals as a table or a screen-dump showing the numerical values on a flow-sheet of the installation. The advantage of this system is reliability of both time interval and accuracy of information. The disadvantage is that the computer produces information on many pages of paper. The computer stores the information as so-called historical trends, which remain in the computer’s memory for a certain time depending on storage capacity, and can be visually studied as graphical trends of individual or combinations of parameters on the screen and printed out as screen-dumps. Again, the information collected in this way may be spread over a number of pages. Needless to say, for the technologist it is much easier to collect the information needed from a one page log sheet. Besides, computer print-outs should never be a replacement for manually written log sheets. There is a simple reason for this, namely that operators will lose the feeling for and contact with the technological parameters if they are not forced to read and write down the data every hour, and consequently become totally familiarized with them. Control computer systems record the data at pre-programmed intervals and store them in memory for a period of time depending on the data storage capacity. The system described here makes it possible to collect the operating parameters at the end of each operation directly from the process control computer and to save them on other data storage media. Such a file contains usually much more information than those which can be found on the manually written log sheets. From there they can be further processed and converted, depending on their format, either directly or after using the translation facility of the spreadsheet system used into a spreadsheet file. The use of a computer with spreadsheet program for this purpose was described by Písecký [50]. The analytical results from the laboratory, both in-process and final control, can be entered into these log sheets, which are then saved for future use. The information can also be used to generate a survey of production quantity for each day, or a given period of time, energy consumption, production economy, etc. The possibilities for utilizing this information are in fact almost unlimited. The system is supposed to be a tool for the technologist and production supervisor, enabling them to follow, control and optimize the quality of the product and the production rate.

241

References   1. Schlünder,E.U.:Dissertation Techn.Hochschule Darmstadt D 17, 1962.  2.  Masters, K.: Spray Drying. An introduction to principles, operational practice and applications. Leonard Hill Books, London, 1972.   3. King, N.: Dairy Sci.Abstr., 27, 91, 1965.   4. Snoeren, T.H.M, Damman A.J. Klok, H.J., van Mil P.J.J.M.: Effect of droplet size on the properties of spray-dried whole milk, Kyoto Int.Conf., Kyoto, 1984.   5. Snoeren, T.H.M, Damman A.J. Klok, H.J.: De invloed van de voorverhitting van de melk op enkele eigenschappen van ondermelkpoeder, NIZO-nieuws nr.12, 1982.   6. Snoeren, T.H.M, Damman A.J. Klok, H.J.: De viscositeit van ondermelk-concentrat, NIZOnieuws nr.9, 1981.   7. Snoeren, T.H.M, Damman A.J. Klok, H.J.: Het nadikken van ondermelk-concentrat, NIZOnieuws nr.11, 1981.   8. Snoeren, T.H.M, Damman A.J. Klok, H.J.: The viscosity of skim-milk concentrates,Neth.Milk & Dairy J.,36, 305-316, 1982.   9. Eilers, H.: Die Viskosität von Emulsionen hochviskoser Stoffe als Funktion der Konzentration, Kolloid-Z., 97, 313, 1941. 10. Torssel, H., Sandberg, U., Thureson, L.E.: Changes in viscosity and conductivity during concentration of milk. XII.Int.Dairy Congr.Proceedings, 2, 246, 1949. 11. Free, K.: Sweet cream viscosity. Quoted in: Physical properties of dairy products (Wood., P.W.), Ministry of Agriculture and Fisheries, Hamilton New Zealand, 1982. 12. H  unziker, O.F.: Condensed milk and milk powder, La Grange, Illinois, 7.ed., 1949. 13. Gosselin, D.: Le séchage de la gouttelette de concentré, Cours de Formation à Hendecourt, 1985. 14. Iglesias, H.A., Chirife, J.: Handbook of food isotherms: Water sorption parameters for foods and food components, Academic press New York, 1982. 15. H  alsey, G. J.: J.Chem.Phys. 16, 931, 1948 16. B  erlin, E., Anderson, B.A., Pallansch, M.J.: J.Dairy Sci., 53, 146, 1970. 17. P  ísecký, J.: Water activity of milk powders, Milchwissenschaft, 47, 1, 3, 1992. 18.  Waite, R., White, J.C.D.: The composition of the soluble and insoluble portions of reconstituted milk powders., J.Dairy Res. 16,3, 379, 1949. 19. Howat, G.R., Wright, N.C.: Factors affecting the solubility of milk powders, J.Dairy Res., 4, 265, 1933. 20. W  right, N.C.: Factors affecting the solubility of milk powders, J.Dairy Res., 4, 123, 1932. 21. Mol, J.J.,: The milk fat globule membrane and the solubility of whole milk powder, Neth. Milk and Dairy J., 29, 212, 1975. 22. Mol, J.J.,: De invloed die de voorverhitting van melk heeft op enkele eigenschappen van melkpoeder, NIZO-nieuws 4, 1976. 23. Westergaard, V.: Milk powder technology, Evaporation and Spray drying., Niro Atomizer, Copenhagen, 1994. 24. Buma, T.J.: A correlation between free fat content and moisture content of whole milk spray powders, Neth.Milk & Dairy J., 22, 1968. 25. Buma, T.J.: Free fat in spray-dried whole milk. 10.A final report with a physical model for free fat in spray-dried milk., Neth.Milk & Dairy J., 25, 159, 1971. 26. Wewala, A.R.: Manipulation of water activity: An important aspect of extending the shelf life of whole milk powder., NZDRI Palmerston North 1991. 27. Ruyck, H. de: Avoidance of white flecks during the manufacture of instant dried milk, Revue de l’Agriculture, 44, 4, 751, 1991. 28. P  allansch, M.: Drying of acid whey, Proceedings of Whey Products Conf., Chicago 1968. 29. Kirchmeyer, O., El-Shobery, M., Kamal, N.M.: Milcherhitzung und SH-Gruppen- Entwicklung, Milchwissenschaft 39, 12, 1984. 242

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30. L abuza, T.P.: Sorption phenomena in foods. Food Technol. 22, , 3, 263, 1968. 31. Analytical methods for dry milk products, 4.ed., A/S Niro Atomizer, Copenhagen 1978. 32. IDF Standard 26:1964: Determination of water content of dried milk. 33. IDF Standard 129A:1988: Dried milk and dried milk products - Determination of insolubility index. 34. IDF Standard 134:1986: Dried milk and dried milk products - Determination of bulk density. 35. British Standard Method 1743:1968. 36. Analytical Methods for Dry Milk Products, American Dry Milk Institute Inc., Chicago, Ill., 1971. 37. IDF Standard 87:1979: Determination of the dispersibility and wettability of instant dried milk. 38. IDF Standard 123A:1988: Milk-based instant foods - Determination of fat content - RöseGottlieb method. 39. IDF Standard 86:1978: Drier milk - Determination of titratable acidity. 40. Písecký, J.: Příčiny explose sušeného mléka (Causes of explosions in dried milk), Průmysl potravin 19, 7, 1, 1968. 41.  Sapryngin, G.,Kiselejev,J.A.:Inflammation spontanée du lait en poudre, La technique laitière, XI, 537, 15-18, 1966. 42.  VDI 3673: Richtlinien fur Druckentlastung von Staubexplosionen. Verein Deutscher Ingenieure, Dusseldorf, 1979. 43. A sssociation of British Preserved Milk Manufacturers: Prevention of fire and explosion in spray drying plant., London ABPMM, 1987. 44. IDF Bulletin no.219/1987: Recommendations for fire prevention in spray drying of milk powder. 45. IDF Document 123-1980. 46. IDF Document 178-1984. 47. F  AO/WHO Codex Alimentarius Commission’s Document CAC/RCP 31 1983. 48. A  sssociation of British Preserved Milk Manufacturers: London 1987. 49. IDF Bulletin no.267/1991: Recommendations for the hygienic manufacture ofspray dried milk powders. 50. Písecký, J.: Computerized logsheet keeping and trouble shooting,J.Soc.Dairy Technol. 46, 4, 1993. 51. Refstrup, E.:Begrænsning af støvemission fra spraytørringsanlæg,Mælkeritidende, 6, 138, 1991. 52. Schrøder-Hansen, E.:Anwendung der Membranfiltration in der Milchindustrie, Deutsche Molkerei Zeitung, 113. 39, 1992. 53. Steenbergen, A. E., Houwelingen, G. Van, Straatsma, J.: System for early detection of fire in a spray drier. Journal of the Society of Dairy Technology (1991), 44, (3), 76-79 (En, 5 ref.), NIZO (Netherlands Institute for Dairy Research). 54. Warburton, S., Pixton, S.W.: The moisture relations of spray dried skimmed milk, J.stored Prod.Res., 14, 143. 55. P  ísecký, J.:Milk droplets: Their creation and drying. World Galaxy, 5, November 1974. 56. Vuataz, G.: The phase diagram of milk: a new tool for optimising the drying process. Lait, 82, 495, 2002. 57.  Thomsen, M.K., L. Jespersen, K. Sjøstrøm, J. Risbo, L. H. Skibsted. Water activity – Temperature state diagram of amorphous lactose. J. Agric. Food Chem., 53, 9182, 2005 58. Thomsen, M.K., L. Lauridsen, L. H. Skibsted, J. Risbo. Temperature effect on lactose crystallization, Maillard reactions, and lipid oxidation in whole milk powder. J. Agric. Food Chem., 53, 7082, 2005

243

Index A ABPMM238 code of hygienic practice 238 absolute air humidity 33 acid butter milk powder 125 acid whey powder noncaking132 ordinary131 acidity152 conversion factors 153, 216 Dornick 153, 216 SoxhletHenkel 153, 216 Thörner 153, 216 “as lactic acid” 153, 216 adiabatic conditions 33 ADMI determination of acidity 215 scorched particles 204 adsorption 156, 158 after-crystallization 96, 130, 154 age thickening 30, 145 agglomeration174 agglomerate type grape176 onion176 raspberry176 agglomerated fat filled milk powder 129 skim milk powder 127 whole milk powder 127 agglomeration174 compact176 efficiency174 forced174 loose176 primary174 secondary174 spontaneous174 air absolute humidity 33 ambient36 incorporation of 42, 65, 170 relative humidity 34 supply57 air density, calculation of 35 air disperser 60 adjustment of 228 downwards flow 61 DDD61 rotational flow 61 244

air filters 57 air flow 55 rotary55 straight55 air flow measurement 103 air velocity 103 amount of condensate 104 amount of heat 104 cyclone pressure drop 104 water evaporation rate 105 gas or oil consumption 105 electricity consumption 105 air flow pattern 55 horizontal55 rotary downwards 55, 60 vertical 55, 61 air heating 58 direct59 electrical60 hot oil 59 indirect58 steam58 air humidity fluctuations 226 air supply system 57 air velocity in ducts measurement 103 air/spray mixing 55 concurrent55 countercurrent55 a-lactose 143, 214 ambient air 36 amorphous lactose 143 analytical methods 199 a-lactose214 bulk density 201 cakiness213 coffee test 208 concentrate crystallization 221 concentrate insolubility index 220 concentrate total solids 219 concentrate viscosity 220 degree of caking 213 dispersibility205 flowability217 free fat content 210 free moisture 200 hot water sediment 208 hot water test 208 hygroscopicity212 insolubility index 200 lecithin content 218

Index

mechanical stability 212 moisture199 particle density 202 particle size distribution 211 scorched particles 204 SDP208 slowly dispersible particles 208 sludge207 titratable acidity 215 total fat content 209 total lactose 214 total moisture 200 water of crystallization 200 wettability204 whey protein nitrogen index 216 white flecks number 209 Assoc.of Brit.Pres.Milk Man.  238 code of hygienic practice 238 atomization64 atomizing device 64 rotary wheel 65 pressure nozzle 68 attrition 168, 175 automation84 B baby food 135 baby food powder 135 bag filter 72 baggingoff82 barometric distribution law 106 belt96 crystallization132 timing132 b-lactose143 bigbag system 82, 127 blowthrough valve 72, 76, 78 boiling point elevation 18 bulk density 166 British standard 201 influence of various factors 168 influence of WPNI 169 LEDOUX201 Niro201 regular skim milk powder 123 bustle89 butter milk powder 125 acid125 sweet125 butter powder 139

C caking191 calandria13 calcium caseinate 137 calculation of air density 35 cyclone diameter 109 dry air rate 102 evaporation output 99 evaporation rate 99 evaporator feed rate 99 heating of air 100 mixing air streams 101 product output 99 product rate 99 water vapour rate 102 capacity - lack of 222 capillary fat 179 carbondioxide - solubility in water 172 caseinate powder 137 calcium137 sodium137 caseinates - fluidizing velocity 48 CDI92 CDP91 Cencomixer201 CFD231 characteristics of homogenous spray 65 cheese powder 139 checklist for a spray dryer 115 cocoamilksugar138 composition139 code of hygienic practice 238 coffee creamer 137 coffee test 190, 208 coffee whitener 137 composition, examples 138 commissioning110 91, 92 CompactTM Dryer component of free fat hydrophilic178 lipophilic178 water repellent 178 composition of cows milk - gross 140 of human milk 135 computational fluid dynamics 231 concentrate crystallization 221 concentrate density calculation of 150 skim milk 151 245

whey152 whole milk  151 concentrate properties 30 age-thickening30 insolubility index 220 total solids 219 viscosity 30, 145 control room 84 cream powder 121 crystallization degree of 221 of lactose 153 crystallization belt 96 cyclone separator 71 cyclone - pressure drop 104 cyclone diameter - calculation of 109 D degree of caking 213 density air35 a-lactose monohydrate 37 amorphous lactose 37 milk components 37 milk fat 37 milk proteins 37 nonfat milk solids 37 of milk concentrates 36, 150 of whey concentrates 152 skim milk concentrate 151 sugar37 water37 whey solids 37 whole milk concentrate 151 deposits227 desorption157 determination of a-lactose214 bulk density 201 cakiness213 coffee test 208 concentrate crystallization 221 concentrate insolubility index 220 concentrate total solids 219 concentrate viscosity 220 degree of caking 213 dispersibility205 flowability217 free fat content 210 free moisture 200 hot water sediment 208 hot water test 208 246

hygroscopicity212 insolubility index 200 lecithin content 218 mechanical stability 212 moisture199 particle density 202 particle size distribution 211 scorched particles 204 SDP208 slowly dispersible particles 208 sludge207 titratable acidity 215 total fat content 209 total lactose 214 total moisture 200 water of crystallization 205 wettability209 whey protein nitrogen index 221 white flecks number 214 dew point temperature 33 direct contact regenerative preheaters 7 direct steam injection 8, 9 dispersibility183 IDF 87:1979 205 NZDB207 dissolution fat 179 distribution systems 11 dynamic11 static11 dried milk products 121 dryer deposits 227 dryer components 51 droplet number65 size65 surface area 65 temperature41 dry air rate, calculation of 102 dry bulb temperature 33 drying chamber 54 shape55 box type 55 tall form 55 drying air characteristics 33 drying rate 41 constant41 falling41 first41 second41 duplex preheating system 8

Index

E enthalpy35 evaporation3 latent heat of 35 ratio13 output99 test222 evaporator boiling point elevation 18 boiling temperature 26 combined MVR/TVR 19 condensers21 cooling tower 23 coverage coefficient 25 de-superheating, MVR 19 direct contact regen. preheaters 7 direct steam injection 9 distribution systems 11 duplex preheating system 8 energy consumption 14 energy consump., comparison 20 flash cooler 22 heat transfer coefficient 24 heating surface 24 holding tubes 11 incondensable gases 14, 22 main components, evaporator 4 mechanical vapour recompr. 17 mixing condenser 21 MVR17 multi-effect17 one-effect MVR 18 pasteurizing systems 9 preheater5 sealing water 23 separators, tang. vapour inlet 15 separators, wrap-around 15 spiral-tube preheaters 5 spore forming bacteria 7, 8 spliteffect26 surface condenser 21 thermal vapour recompression 16 TVR16 vacuum pump 14, 19, 22 vapour recompression systems 16 with two MVR fans 20 wrap-around separator 15 F fall velocity FAO Codex 207-1999 FAO/WHO Codex Alimentarius

50 185 238

fat filled milk 125, 129 standardization of fat 125 fat filled milk powder 125, 129 fat filled whey powder 133 feed supply systems 61 filter64 concentrate heater 63 line64 pump 62, 64 tank62 FILTERMAT™ integrated belt dryer 95 fines return system 76 to pressure nozzles 77 to rotary wheel 77 fines diverter valve 76 fire development - critical conditions 234 fire extinguishing 237 fire precaution 232 flash cooler 22 flowability 177, 217 fluid bed systems 79 drying48 vibrating48 fluidization79 fluidizing velocity 48, 79 caseinate48 high fat powder 48 integrated circular fluid bed 49 integrated annular fluid bed 49 skim milk powder 48 whole milk powder 48 FMD95 forced agglomeration 174 free fat capillary fat 178 dissolution fat 179 outer layer fat 179 surface fat 179 free fat content 179, 210 free moisture 200 FSDTM91 full cream milk powder 121 G gases incondensable glass transition glass transition temperature of different food components H half cream milk powder heat of

14, 22 159 160 161

121 247

cooling air 107 evaporation 35, 107 fines107 fines transport air 107 product solids 107 radiation107 heat balance 106 heat capacity 34 of milk fat 37 of nonfat milk solids 37 of air 34 of milk components 37 of water 35 of water vapour 35 heat classification 28 heat exchanger 5 heat stability 184 heat transfer coefficient 24 heating of air 100 high-heat 195, 217 high-heat heat-stable 29 holding tubes 11 homogenizer64 hot water sediment 190, 208 hot water test 190, 208 human milk composition 135 humanization135 humanized milk powder 135 humidity absolute33 relative34 h-x diagram 37 hydrocyclone, self-cleaning 12 hydrolyzed whey powder 133 hygroscopicity 191, 212 I incondensable gases 14, 22 industrial production principles 110 infant formula 135 insolubility index 165, 200 instant fat filled milk powder 129 instant properties 180 coffee test 190 dispersibility183 heat stability 184 hot water sediment 190 hot water test 190 sinkability181 slowly dispersible particles 187 sludge183 wettability181 248

white fleck number 190 instant skim milk powder 127 instant whole milk powder 128 instrumentation84 integrated annular fluid bed 90 fluidizing velocity 49 Integrated Belt Dryer 95 integrated circular fluid bed 90 fluidizing velocity 49 i-x diagram 37 J jet velocity

181

K -casein9 L lactose143 a-143 amorphous 143, 153 b-143 configuration143 crystallization153 density144 glass 143, 153 heat of crystallization 144 melting point 144 mutarotation 143, 153 optical rotation 144 relationships between the forms 144 solubility144 sweetness144 transformation velocity 154 latent heat of evaporation 35 lecithin treatment system 81 lecithination81 liquid distributor 67 hole67 swirl67 volute67 logsheet for a spray dryer 114 for an evaporator 113 LONOX burner 59 low-heat 195, 217 M Maillard reaction maintenance logbook MALVERN manufacturing practice

156, 163, 198, 228 115 211, 212 238

Index

mean particle size 39 measurement air flow 103 mechanical stability 176, 212 mechanical vapour recompression 17 medium-heat 195, 217 membrane processes 1 microphotos of whole milk powder instant 129 lactose crystals 155 microscopic counting 211 milk acidity152 boiling point 152 composition140 minerals145 physical properties 145 quality140 redoxpotential153 viscosity145 milk components variations 141 milk fat 143 milk minerals 145 milk powder 121 butter milk  125 fat filled milk 121, 125 full cream milk 121 half cream milk 121 heat classified 28 high-heat heat-stable 29 regular121 skim milk 122 skim milk agglomerated 127 skim milk regular 122 whole milk 123 whole milk agglomerated 127 whole milk instant 128 whole milk regular 123 whole milkhigh free fat 124 milk proteins 142 variations142 milk solids components 142 lactose143 fat143 proteins142 minerals145 milk sugar see lactose mixing air streams 101 mixing condenser 21 moisture - determination of 199 moisture content 36, 163 final163 influence of various factors 164

intermediate163 on dry basis 36 Mollier diagram 37 momentum176 mother liquor powder 134 MSDTM93 multi-effect evaporator 17 Multi Stage Dryer 93 mutarotation143 MVR17 N nanofiltration1 noncaking acid whey powder 132 noncaking sweet whey powder 132 non-condensable gases 14, 22 NZDBmethods coffee test 208 dispersibility207 hot water sediment 208 slowly dispersible particles 208 sludge207 O occluded air ordinary acid whey powder ordinary sweet whey powder outer layer fat

166 131 130 179

P Particle volume 42 particle density 166, 202 particle diameter arithmetic mean 39 geometric mean 39 median39 most frequent 39 Sauter39 volume/surface39 particle size distribution 38, 211 pasteurisation, effect on powder 27 pasteurizing systems 9 direct9 indirect9 perforated plates 81 permeate powder 134 physical properties 145 age thickening 30 boiling point 152 density150 redoxpotential153 viscosity145 249

water activity 156 whey protein nitrogen index 146 WPNI146 plug flow 79, 94 powder aftertreatment 55, 78 fluid bed 79 pneumatic conveying 78 powder conveying 82 powder emission 73 powder recovery system 70 powder separators bag filter 72 combinations of 75 cyclone71 wet scrubber 75 powder sieve 82 powder storage 82 powdering techniques 124, 133 preheaters5 direct contact regenerative 7 duplex system 8 spiral-tube5 straight-tube6 vapour cooler/preheater 5 pressure nozzle flow rate calculation 69 with grooved core 68 with swirl chamber 68 primary agglomeration 174 product discharge 55 product flow 55 product output 99 product properties 163 product quality 225 specification117 product rate 99 production documentation111 log-sheet112 protein content variations 227 protein standardization 185, 195, 227 psychrometric chart 37 Q quality control 117 final119 process117 product119 use of computer for 240 R radiation loss 250

107, 108

rate of drying 41 raw milk 140 composition variations 140 quality140 readsorption 157, 158 redox-potential153 relative humidity  34 removable insulation 56 reverse osmosis 1 RÖSE-GOTTLIEB209 rotary valve 72 rotary wheel atomization 65 S saturation point 34 73 SANICIPTM bag filter Scanning Electronic Microscopy 45 scorched particles 204 secondary agglomeration 174 seeding130 seeding material 130 SEM45 Separators15 tangential vapour inlet 15 wrap-around15 separation efficiency 76, 175 shelf life 196 SH-groups 29, 196 single-stage drying 43 sieving test 211 skim milk concentrate whipping ability 170 skim milk powder agglomerated127 instant127 regular122 slowly dispersible particles 187, 208 sludge 183, 207 sodium caseinate 137 solubility index, see insolubility index specific gravity of milk components 36 of milk products - calculation 36 specification operational parameters 117 product quality 117 spiral-tube preheaters 5 spontaneous agglomeration 174 spore forming bacteria 7, 8 spray dryer shutdown 230 spray dryer components 51 spray dryer types 86

CDI92 CDP91 choice of 98 COMPACT DRYERTM91 Fluidizer Spray Dryer 91 FMD95 IFDTM94 integrated belt dryer 95 MSDTM93 MSDTM-PF95 Multi Stage Dryer 93 Multi Stage Dryer with PF bed 95 single stage systems 86 TALL FORM DRYERTM89 three stage 91 two stage 88 with aftercrystallization belt 96 with annular fluid bed 91 with circular fluid bed 91 with cooling bed 87 with fluid bed afterdrying 88 with integrated filter & fluid bed 94 with integrated fluid bed 90 with pneumatic conveying 87 without any aftertreatment 86 spray drying fundamentals 33 principles33 terms33 spray fluidization 176 Stampfvolumeter202 Stickiness159 sticking point160 temperature160 zone160 storage, powder 82 straight-tube preheaters 6 sulphhydryl groups 29, 196 surface condenser 21 surface fat 179 sweet butter milk powder 125 sweet whey powder noncaking132 ordinary130 T temperature ambient36 dew point 33 droplet41 dry bulb 33 glass transition 159

inlet36 outlet36 sticking44 wet bulb 33 thermal vapour recompression 16 thermocompressor, principle of 16 thermoplastic products 44, 96, 131 thermoplasticity159 thixotropy 97, 131, 145 TIXOTHERM™ dryer 97 timing belt 96 titratable acidity 152, 215 conversion factors 153, 216 Dornick 153, 216 SoxhletHenkel 153, 216 Thörner 153, 216 “as lactic acid” 153, 216 total fat content 209 total lactose 214 total moisture 199 total solids content 36 totebin system 82 transport system - vacuum 83 troubleshooting110 use of computer for 240 operations222 TVR16 twofluid nozzle 70 with external mixing 70 with internal mixing 70 two-stage drying 44 extended47 U UHT treatment

9

V vacuum pump 14, 19, 22 steam jet 22 vanes inner228 outer228 vapour recompression 16 mechanical, MVR 17 thermal, TVR 16 VDI 3673 236 velocity fall 50 fluidizing50 suspension50 Verein Deutscher Ingenieure 236 vibrating fluid bed 79 viscosity 251

calculation of 148 cream149 dynamic145 influence of heat treatment 146 influence of homogenization 147 influence of protein content 147 influence of temperature 147 skim milk concentrates 146, 147 whole milk concentrates 147 W wall sweep 55 water content 36 water activity 49, 156 water activity calculation 157, 158 water evaporation test 222 water of crystallization 200 water vapour rate - calculation of 102 wet bulb temperature 33 wet scrubber 75 wettability 181, 204 IDF 87:1979 204 NIRO204 wetting agent 81, 181 wheel atomizer 65 factors influencing particle size 66 for high bulk density 67 twotier68 ventilation67 with bushings 66 with curved vanes 66 with straight vanes 66 whey powder composition 130 whey powder noncaking 132 whey products 129 whey protein nitrogen index 28, 169, 216 whey protein powder 133 white fleck number 190, 209 whitening power 138 whole milk powder agglomerated127 coffee stability 30 instant128 keeping quality 29 regular123 with high free fat content 124 WPNI 28, 169, 216

252

Index

253