Introduction To Retort Pouch Technology

Introduction to Retort Pouch Technology W. Scott Whiteside, Ph.D. Associate Professor Department of Packaging Science Clemson University

ABSTRACT Retort pouches have been in existence for several decades with the only major market being MRE’s for the military. Since the late 1990’s, retail food products in retort pouches have experienced substantial growth. Much is known about processing shelf stable food products in rigid, metal cans, but the processing technology for retort pouches is different. This presentation will present an overview of current processing technology for retort pouches.

INTRODUCTION Thermal sterilization of low-acid food products (LACF), sometimes referred to as canning or retorting, has been a widely accepted means of food preservation for many years. Retorting produces a shelf stable food product that does not require refrigeration. The retorting process involves sealing a low-acid food product in a hermetically sealed container and applying sufficient heat to render the product commercially sterile according to FDA or USDA regulations. Commercial sterility is defined as the destruction of all viable microorganisms of public health significance as well as those capable of reproducing under normal nonrefrigerated conditions of storage and distribution. The retorting process also extends the shelf life of food products by inactivating many detrimental food enzymes. Prior to World War II, the packaging for these food products was typically rigid metal or glass containers. In the late 1940’s, the initial concept of a flexible package for sterile food products was developed. The United States military was the first major consumer of retort pouches in the form of meals-ready-to-eat (MRE) as a replacement for C-rations which were packaged in a rigid metal cans. FDA and USDA recognize qualified individuals trained in thermal processing techniques as process authorities. Process authorities specify the time, temperature and other factors required to render a food product in a retort pouch commercially sterile. Retort pouch products have a distinct advantage over similar volume products packaged in rigid metal cans or glass jar in that it has quicker heat penetration. Faster product heating allows for improved quality of the food product. Other advantages that retort pouches have compared to rigid containers is their lighter weight and easier opening. Retorting food products in flexible pouches has several key differences when compared to rigid metal cans. The pouch materials, styles, filling, sealing and retort systems.

RETORT POUCH TECHNOLOGY Retort pouch materials, unlike metal cans or glass jars, are multilayer laminations with each layer providing a different function for the retort pouch. Functions needed in a retort pouch are heat seal ability, thermal stability, gas barrier, rigidity and printability. These functions can be provided in either 3 or 4 layer structures depending on the situation. Cast polypropylene provides a good sealant layer and has good heat stability at retort temperatures. There are two main types of barrier layers in retort pouches; foil and nonfoil. Foil retort pouches contain a layer of 0.00035” aluminum foil. Non-foil retort pouches can have several different materials. Currently, the most common non-foil barriers layers of retort pouches are coatings applied to polyester or nylon. These coatings are typically silicon dioxide or aluminum oxide. Both coatings have a potential for cracks to occur in the barrier coating due to flexing that occurs during the retort process or handling. Other non-foil barriers include ethylene vinyl alcohol (EVOH) and polyvinylidene chloride (PVDC). EVOH is susceptible to barrier loss due to exposure to water. Stand-up type, retort pouches require adequate stiffness to function properly when filled with a food product. Retort pouches also need puncture or tear resistance to maintain the hermetic seal. Biaxially oriented nylon or

polyester provides good tear and puncture resistance. Most retort pouches utilize polyester as the printing layer. Retort pouch structures are adhesive laminations, therefore the adhesives used can be a failure point in a retort pouch. Adhesives in retort laminations undergo severe strain. Retort pouches are typically premade and shipped to the final customer for filling, sealing and processing. However, there are some formfill-seal machines being utilized for retort pouches. Retort pouch styles are typically pillow or stand-up. Zippers, sliders, and spout fitments are also being incorporated in the many retort pouches. Consistent pouch filling and sealing are one of the key limitations of retort pouches compared to metal cans or glass jars. Pouch filling for retort pouches can be more problematic and slower when compared to rigid containers due to the fact that pouches have to be opened before they can be filled. The pouch opening operation can reduce filling speeds, also insufficient opening can lead to contamination in the seal area. Retort pouch sealing can be difficult and variable, since contamination from the food product in the seal area compromises the hermetic seal. Double seamed metal cans or lugged metal lids on a glass jars rarely experience these types of sealing problems. Retort pouch seals must maintain a minimum of 1/16” on all side and ends seals for a proper hermetic seal. Retort pouch seals are generally inspected and a nondestructive test conducted every 30 minutes during production as are metal cans or glass jars. A destructive test such as ASTM D882 methods “A” or ”B” must be performed every fours hours. Retort pouch processors also use “burst” testers to verify seal integrity, which inflates the pouch to a particular air pressure to verify seal strength. Other seal test methods can include placing the pouch in a vacuum chamber to check for seal leaks or applying a specified compressive load on a filled pouch for selected time period. Similar to metal cans and glass jars, retort pouches need to be heated above 240oF to begin thermal destruction of clostridium botulium spores, which is the main microorganism of concern in LACF. A retort is the piece of food processing equipment used to produce uniform, sterilizing temperatures above 240oF. Retorts typically utilize pressurized steam as the heat source, though the actual heating medium may change. Due to their flexible nature and their potential for expansion during processing, retort pouches are placed in metal or plastic racks to maintain pouch thickness during the retort process. Retort systems can be classified several different ways. The first classification is either still or agitating. Food containers remain stationary in a still retort system and containers are rotated in an agitating retort system. Container rotation allows for more even heating and often quicker heat transfer. It is typically used for products that are heat sensitive. The next classification is whether the retort system is batch or continuous flow. Typically retort pouch systems are batch systems, meaning the retort is loaded with a specified number of retort pouches and then processed. The retort is then unloaded and the process repeated. Continuous flow retort systems such allow for containers to constantly mover through the retort system. These types of systems are generally used for metal cans or glass jars. Another classification is whether the retort is equipped with overpressure capability. Overpressure capability is required on all retort systems that process retort pouches and is the major difference between retort systems for cans or glass and retort systems for pouches. When a food product is sealed in a container, some residual gas is trapped in the container unless it is vacuum sealed. Trapped residual gas expands as it is heated causing the flexible retort pouch to swell. Residual gas in a rigid container such as a metal can or glass jar expands also but the rigidity of the container does not require air overpressure to maintain container integrity. Without adequate air overpressure, pouch swelling can continue until the retort pouch seals are stressed to the point of failure. The final classification level is related to what type of heating medium is used in the system. There are three basic mediums used for processing retort pouches; steam, full water immersion and water spray. There can be several variations on each of these three basic types. A pure steam retort involves the introduction of pressurized steam through a series of perforated pipes, called steams spreaders, located near the bottom of the retort chamber. The retort is vented through a valve at the top of the retort until the complete chamber volume is saturated in steam and the retort has reached its processing temperature. One variation of this type of retort utilizes a high volume fan located in the rear of the retort chamber. The fan forces the steam throughout the retort chamber providing more even heat distribution. This type of retort is referred to as a steam-air retort. Steam retorts are characteristically difficult to maintain uniform overpressure due to the regular introduction of steam into the vessel.

A full water immersion retort has a large chamber located directly over the processing vessel. The top vessel preheats the processing water to a temperature of above 270oF and discharges this water into the processing vessel. The initial discharge of pressurized, superheated water from the top heating vessel can be very damaging to the retort pouches located on the top racks of the retort. Also retort pouches can float and move during processing in a full immersion retort system causing potential damage. Full water immersion systems are also equipped with a circulation pump that ensures proper circulation of the process water during the retort process. Proper water circulation is essential for proper heat distribution in a full water immersion retort. A water-spray retort utilizes a pump that forces super heated water, greater than 250oF, through spray nozzles located at the top and sides of the retort chamber. This water spray design provides a very uniform heating pattern around the retort pouches. The water spray retort is the newest type of retort processing technology for retort pouches. Water spray systems are recognized for improved control of the processing temperature and overpressure during the retorting process because the process water is heated and cooled through a plate heat exchanger instead of direct steam injection. Given that the process water is pumped through the heat exchanger, sufficient heat distribution is typically present in a water spray retort. A cascading water retort is similar to a water spray retort except the superheated water is cascaded over the top of the retort pouches instead of being sprayed from the top and sides.

REFERENCES Gavin, A. and Wedding, L. M., Canned Foods: Principles of Thermal Process Control, Acidification and Container Closure Evaluation, The Food Processors Institute, Washington, D.C., 1995. Hernandez, R. J., Selke, S. E. M. and Culter, J. D., Plastics Packaging, Hanser Gardner Publications, Inc. Cinncinnati, Ohio, 2000. National Food Processors Association, Bulletin 41-L, Flexible Package Integrity. NFPA, Washington, D.C., 1989. National Food Processors Association, Guidelines for Thermal Process Development for Foods Packaged in Flexible Containers. NFPA, Washington, D.C., 1985. Robertson, G.L. Food Packaging: Principles and Practice, Marcel Dekker, Inc. New York, NY, 1993.

2005 PLACE Conference September 27-29 Las Vegas, Nevada

Introduction to Retort Pouch Technology Presented by:

W. Scott Whiteside, Ph.D. Associate Professor Department of Packaging Science Clemson University

What is Retorting? – A method of food preservation • food and it’s container are rendered commercially sterile by the application of heat, alone or in combination with pH and/or water activity or other chemicals.

– Commercial sterility • C. Botulium

– Hermetically sealed container – “Canning” – Process Authority

Retort Pouch Advantages • • • •

Lighter weight Safer Microwavable Faster heating

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Retort Pouch History • First Concept - 1940’s • U.S. Military (MRE’s) early 70’s • Pet Food – late 90’s • Tuna – late 90’s • Various products - 2005

Retort Pouch Materials • Multilayer lamination – Functional layers

• Barrier Materials – Foil – Non-foil • SiOx • AlOx • PVDC • Besela • EVOH

• Styles – Pillow, stand-up, shaped

• Features – Fitments, Zippers, Peg holes, Sliders

Retort Pouch Materials – Title 21 CFR 177.1390 • Resistance to penetration/migration

– Permeability • O2 less than 1 cc/100in2 /24 hrs. atm. • WVTR less than 0.05 g/100 in2/ 24 hrs. atm.

– Resistant to processing temperatures • 250 – 270oF (121 – 132oC) • 30 – 60 psi

– Physical strength • Puncture • Abrasion

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4 Ply-MRE Polyester Ink Adhesive

Nylon Adhesive

Aluminum Foil Adhesive

Cast Polypropylene

3 Ply- Non Foil Pouch Polyester (barrier coated) Ink Adhesive

Nylon Adhesive

Cast Polypropylene

Pouch Filling/Sealing – Filling • Pouch opening • Sealing issues

– Seal testing • Visual inspection at start-up and every 30 minutes – Seals widths, leaks, delamination

• Physical tests every four hours • Various methods

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Pouch Retorts –All retorts are not created equal –Categories • Still or agitating • Batch or continuous • Overpressure • Process medium • 250oF = 15 psi –10 tons of force

Why Overpressure? Air pressure

Heat Expansion

Heating Mediums • Steam • Water Spray • Full Water Immersion

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Racks/Crates

Overpressure Retort

Full Immersion Retort

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Thank You PRESENTED BY

W. Scott Whiteside, Ph.D. Associate Professor Department of Packaging Science Clemson University [email protected]

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