Chilled food packaging B P F. Day, Campden and Chorleywood Food Research Association 6.1 Introduction During recent years there has been a greatly increased consumer demand for perishable chilled foods which are perceived as being fresh, healthy and convenient. The major food retailers have satisfied this consumer demand by providing an ever increasing range of value-added chilled food products. The wide diversity of chilled foods available is accompanied by a huge range of packaging materials and formats which are used to present attractively packaged foods in retail chill cabinets. This chapter overviews the requirements and types of packaging materials and formats which are commonly utilised for a broad variety of chilled food products. In addition, established and emerging packaging technologies for extending chilled food shelf-life, such as modified atmosphere packaging, vacuum packaging, vacuum skin packaging and active e described and new developments are highlighted packaging techniques that rely on heat treatments to achieve extended shelf- lives for chilled food products, such as hot-fill, sous-vide and in-pack pasteurisation, are outside the scope of this chapter, but are described in Chapter 11 6.2 Requirements of chilled food packaging materials Table 6. 1 lists the main requirements of a chilled food package (Turtle 1988) Depending on the type of food packaged, not all of these requirements will need to be satisfied. The packaging material must contain the food without leaking, be non-toxic and have sufficient mechanical strength to protect the food and itself
6.1 Introduction During recent years there has been a greatly increased consumer demand for perishable chilled foods which are perceived as being fresh, healthy and convenient. The major food retailers have satisfied this consumer demand by providing an ever increasing range of value-added chilled food products. The wide diversity of chilled foods available is accompanied by a huge range of packaging materials and formats which are used to present attractively packaged foods in retail chill cabinets. This chapter overviews the requirements and types of packaging materials and formats which are commonly utilised for a broad variety of chilled food products. In addition, established and emerging packaging technologies for extending chilled food shelf-life, such as modified atmosphere packaging, vacuum packaging, vacuum skin packaging and active packaging, are described and new developments are highlighted. Process and packaging techniques that rely on heat treatments to achieve extended shelflives for chilled food products, such as hot-fill, sous-vide and in-pack pasteurisation, are outside the scope of this chapter, but are described in Chapter 11. 6.2 Requirements of chilled food packaging materials Table 6.1 lists the main requirements of a chilled food package (Turtle 1988). Depending on the type of food packaged, not all of these requirements will need to be satisfied. The packaging material must contain the food without leaking, be non-toxic and have sufficient mechanical strength to protect the food and itself 6 Chilled food packaging B. P. F. Day, Campden and Chorleywood Food Research Association
136 Chilled foods Table 6.1 Main requirements of a chilled food package · Contain the product · Seal integrity Be compatible with food Prevent microbial contamination Non-toxic Protect from odours and taints Run smoothly on filling lines e Prevent dirt contamination Withstand packaging processes Resist insect or rodent infestation Handle distribution stresses Prevent physical damage · Have sales appeal Possess appropriate gas permeability Communicate product information Control moisture loss or gain Show evidence of tampering Protect against light · Easily openable Possess antifog properties .Be tolerant to operational temperatures from the stresses of manufacture, storage, distribution and display. Certain packs equire a degree of porosity to allow moisture or gaseous exchange to take place, and packaging materials used in these situations should possess appropriate permeability properties. Alternatively, most modified atmosphere packs require moisture and gases to be retained within the pack and hence the packaging materials used should possess appropriate barrier properties. The specific the type of chilled food product, the packaging material may need to be tolerant of high temperatures experienced during hot filling, in-pack pasteurisation or re- ating prior to consumption. The packaging material, particularly with high- speed continuous factory operations, may need to be compatible with form-fill- seal machines. The pack closure must have seal integrity but at the same time should be easy to open. There may be a need for reclosure during storage after opening in the home. Also, with the increased incidence of malicious contamination, tamperproof or tamper-evident packaging is desirable. The package is the primary means of displaying the contained chilled food and providing product information and point-of-sale advertising. Clarity and printability are two pertinent features that require consideration in the choice of materials. Finally, the packaging must be cost-effective relative to the contained food. For example, a prepared ready meal retailing at a high price can pport a considerably higher packaging cost than a yoghurt dessert selling at fraction of that price 6.3 Chilled food packaging materials Once the requirements of a container for a particular chilled food product have been established, the next step is to ascertain which type of packaging material will provide the necessary properties. The answer is almost certain to be more han one type, Packaging materials consisting of paper, glass, metal or plastic
from the stresses of manufacture, storage, distribution and display. Certain packs require a degree of porosity to allow moisture or gaseous exchange to take place, and packaging materials used in these situations should possess appropriate permeability properties. Alternatively, most modified atmosphere packs require moisture and gases to be retained within the pack and hence the packaging materials used should possess appropriate barrier properties. The specific requirements for modified atmosphere packs are described later. Depending on the type of chilled food product, the packaging material may need to be tolerant of high temperatures experienced during hot filling, in-pack pasteurisation or reheating prior to consumption. The packaging material, particularly with highspeed continuous factory operations, may need to be compatible with form–fill– seal machines. The pack closure must have seal integrity but at the same time should be easy to open. There may be a need for reclosure during storage after opening in the home. Also, with the increased incidence of malicious contamination, tamperproof or tamper-evident packaging is desirable. The package is the primary means of displaying the contained chilled food and providing product information and point-of-sale advertising. Clarity and printability are two pertinent features that require consideration in the choice of materials. Finally, the packaging must be cost-effective relative to the contained food. For example, a prepared ready meal retailing at a high price can support a considerably higher packaging cost than a yoghurt dessert selling at a fraction of that price. 6.3 Chilled food packaging materials Once the requirements of a container for a particular chilled food product have been established, the next step is to ascertain which type of packaging material will provide the necessary properties. The answer is almost certain to be more than one type. Packaging materials consisting of paper, glass, metal or plastic Table 6.1 Main requirements of a chilled food package • Contain the product • Seal integrity • Be compatible with food • Prevent microbial contamination • Non-toxic • Protect from odours and taints • Run smoothly on filling lines • Prevent dirt contamination • Withstand packaging processes • Resist insect or rodent infestation • Handle distribution stresses • Be cost effective • Prevent physical damage • Have sales appeal • Possess appropriate gas permeability • Communicate product information • Control moisture loss or gain • Show evidence of tampering • Protect against light • Easily openable • Possess antifog properties • Be tolerant to operational temperatures 136 Chilled foods
Chilled food packaging 13 Table 6.2 Comparison of chilled food packaging materials Material Main technical advantages Impermeability Lightweight Container axial strength Withstands internal pressure Great variety of paper grades Ease of decoration Adjunct to all other packaging materials 1g Semi-rigid plastics Properties variable with type of plastic Choice of container shap In-house manufacture Lightweight Flexible plastic Properties variable by combination Very lightweight containers Tailor-made sizing Chemical inertness Impermeability Product visibility Container axial strengt withstands internal vacuum pressure have their individual advantages and these should be exploited when making a choice. The main technical advantages of current chilled food packaging materials are compared in Table 6.2, while the principal types of materials(and their abbreviations) are listed in Table 6.3(Turtle 1988). For any particular product, a number of materials can generally be used, either as separate components or in the manufacture of a composite 6.3.1 Paper-based materials Paper and board are widely used in chilled food packaging. They are easy to decorate attractively and are complementary to all other packaging materials in the form of labels, cartons, trays or outer packaging. They are available with coatings such as wax, silicone and polyvinylidene chloride(PVdc)or as laminates with aluminium foil or flexible plastics. Such coating or lamination imparts heat-sealability or improves oxygen, moisture or grease barrier properties. For example, butter is traditionally packed in waxed paper or aluminium laminated paper Dual ovenable trays can be made of paperboard that is extrusion-coated with polyethylene terephthalate(PET). They can resist temperatures up to 220C and
have their individual advantages and these should be exploited when making a choice. The main technical advantages of current chilled food packaging materials are compared in Table 6.2, while the principal types of materials (and their abbreviations) are listed in Table 6.3 (Turtle 1988). For any particular product, a number of materials can generally be used, either as separate components or in the manufacture of a composite. 6.3.1 Paper-based materials Paper and board are widely used in chilled food packaging. They are easy to decorate attractively and are complementary to all other packaging materials in the form of labels, cartons, trays or outer packaging. They are available with coatings such as wax, silicone and polyvinylidene chloride (PVDC) or as laminates with aluminium foil or flexible plastics. Such coating or lamination imparts heat-sealability or improves oxygen, moisture or grease barrier properties. For example, butter is traditionally packed in waxed paper or aluminium laminated paper. Dual ovenable trays can be made of paperboard that is extrusion-coated with polyethylene terephthalate (PET). They can resist temperatures up to 220ºC and Table 6.2 Comparison of chilled food packaging materials Material Main technical advantages Aluminium Impermeability Lightweight Container axial strength Withstands internal pressure Paper Great variety of paper grades Ease of decoration Adjunct to all other packaging materials Lightweight Semi-rigid plastics Properties variable with type of plastic Choice of container shape In-house manufacture Lightweight Flexible plastics Properties variable by combination Very lightweight containers Tailor-made sizing Glass Chemical inertness Impermeability Product visibility Container axial strength Withstands internal vacuum pressure Reuse facility Chilled food packaging 137
138 Chilled foods Table 6.3 Chilled food packaging materials EVOH (ethylene-vinyl alcohol) HDPE (high density polyethylene) Cellulose HIPS (high impact polystyrene) Cellulose fibre LDPE (low density polyethylene) Glass LLdPe (linear low density polyethylene) Natural casings MXDE (modified nylon P OPP(orientated polypropylene) Metallised board OPS(orientated polystyrene) Metallised film Pa (polyamide-nylon) Steel PC(polycarbonate) Plastics Pe(polyethylene) ABS (acrylonitrile-butadiene-styrene) PET(polyethylene terephthalate) APET(amorphous PET) PETG (modified PET) CA (cellulose acetate) PP CPET(crystallised PET) Ps(polystyrene) CPP(cast polypropylene) PVc(polyvinyl chloride) EPS (expanded polystyrene) PVDC (polyvinylidene chloride) EVA(ethylene-vinyl acetate) UPVC (unplasticised polyvinyl chloride) hence are suitable for microwave and conventional oven heating of chilled ready meals. Another application of paperboard in chilled food packaging is in the area of microwave susceptors which enable the browning and crisping of meat and dough products, e.g., pizza and pies, during microwave heating. A typical microwave susceptor is constructed of metallised PET film laminated to paperboard Glass jars and bottles are the oldest form of high-barrier packaging and have the advantages of good axial strength, product visibility, recyclability and chemical inertness. Returnable glass bottles are still used extensively for pasteurised milk in the UK. Aluminium caps and closures make opening simple, while tamper evident features such as pop-up buttons provide an important consumer safety factor. Impact breakage of glass containers is a major disadvantage, but new glass technology and plastic sleeving with polyvinyl chloride(PVC) expanded polystyrene(EPS) have helped to reduce glass breakage. 6.3.3 Metal-based materials Pressed aluminium foil trays have a long history of use for prepared frozen meals and hot take-away food. They are also used for many chilled ready meals Their temperature stability makes them ideal for conventional oven heating, but precautions should be taken to prevent arcing if used in microwave ovens Guidelines have been developed for the successful use of foil containers in microwave ovens(Foil Container Bureau 1991).In some circumstances
hence are suitable for microwave and conventional oven heating of chilled ready meals. Another application of paperboard in chilled food packaging is in the area of microwave susceptors which enable the browning and crisping of meat and dough products, e.g., pizza and pies, during microwave heating. A typical microwave susceptor is constructed of metallised PET film laminated to paperboard. 6.3.2 Glass Glass jars and bottles are the oldest form of high-barrier packaging and have the advantages of good axial strength, product visibility, recyclability and chemical inertness. Returnable glass bottles are still used extensively for pasteurised milk in the UK. Aluminium caps and closures make opening simple, while tamperevident features such as pop-up buttons provide an important consumer safety factor. Impact breakage of glass containers is a major disadvantage, but new glass technology and plastic sleeving with polyvinyl chloride (PVC) or expanded polystyrene (EPS) have helped to reduce glass breakage. 6.3.3 Metal-based materials Pressed aluminium foil trays have a long history of use for prepared frozen meals and hot take-away food. They are also used for many chilled ready meals. Their temperature stability makes them ideal for conventional oven heating, but precautions should be taken to prevent arcing if used in microwave ovens. Guidelines have been developed for the successful use of foil containers in microwave ovens (Foil Container Bureau 1991). In some circumstances, Table 6.3 Chilled food packaging materials Aluminium foil EVOH (ethylene-vinyl alcohol) Cardboard HDPE (high density polyethylene) Cellulose HIPS (high impact polystyrene) Cellulose fibre LDPE (low density polyethylene) Glass LLDPE (linear low density polyethylene) Natural casings MXDE (modified nylon) Paper OPP (orientated polypropylene) Metallised board OPS (orientated polystyrene) Metallised film PA (polyamide-nylon) Steel PC (polycarbonate) Plastics PE (polyethylene) ABS (acrylonitrile-butadiene-styrene) PET (polyethylene terephtlalate) APET (amorphous PET) PETG (modified PET) CA (cellulose acetate) PP (polypropylene) CPET (crystallised PET) PS (polystyrene) CPP (cast polypropylene) PVC (polyvinyl chloride) EPS (expanded polystyrene) PVDC (polyvinylidene chloride) EVA (ethylene-vinyl acetate) UPVC (unplasticised polyvinyl chloride) 138 Chilled foods
Chilled food packaging 139 aluminium foil enables more uniform heating than microwave-transparent trays (Bows and Richardson 1990). Aluminium foil or aluminium laminated paper are also used for many dairy products, such as butter, margarine and cheese Aluminium foil is also used in cartonboard composite containers for chilled fruit juices and dairy beverages. In addition, aluminium or steel aerosol containers are used for chilled creams and processed cheeses 6.3 4 Plasti Plastics are the materials of choice for the majority of chilled foods. Chilled desserts, ready meals, dairy products, meats, seafood, pasta, poultry, fruit and vegetables are all commonly packed in plastics or plastic-based materials. Semi- rigid plastic containers for chilled foods are predominantly made from polyethylene(PE), polyproplyene (PP), polystyrene(PS), PVC, PET and acrylonitrile-butadiene-styrene (ABS). Other plastics such as polycarbonate (PC) are used in small quantities. Containers are available in a wide range of bottle, pot, tray and other shapes and thermoforming, injection moulding and blow moulding techniques give food processors the option of in-house manufacture. Flexible plastics offer the cheapest form of barrier packaging and may be used to pack perishable chilled food under vacuum or modified atmosphere. Multilayer materials are typically made by coextrusion or coating processes, using sandwich layers of PVDC or ethylene-vinyl alcohol (EVOH)to provide an oxygen barrier. Alternatively, plastics such as PE or PP may be metall ised or laminated with foil to provide very high-barrier materials. The required technical properties and pack size and shape may be matched to a desired specification, thereby ensuring cost effectiveness. The oxygen and water vapour transmission rates of aluminium foil and selected monolayer plastic films are compared in Table 6.4 Most polymers used for chilled food packaging are thermoplastics, i.e., they are reversibly softened by the application of heat, provided that no chemical breakdown occurs. PE is derived from the polymerisation of ethylene, whereas thermoplastics such as PP, PVDC, PS, PVC, ethylene-vinyl acetate(EVA) s are similarly polymerised from ethylenic monomers. In contrast, plastics such as polyamide(Pa), PC and PEt are manufactured by condensation reactions. For example, PET film is produced from PET resin, the polycondensation product of ethylene glycol and terephthalic acid, by a stretching process known as biaxial orientation 6.4 Packaging techniques for chilled food 6.4.1 Modified atmosphere packaging(MAP) MAP is an increasingly popular food preservation technique in which the gaseous atmosphere surrounding the food is different from air(Day 1989). The consumer demand for fresh, additive-free foods has led to the growth of MAP
aluminium foil enables more uniform heating than microwave-transparent trays (Bows and Richardson 1990). Aluminium foil or aluminium laminated paper are also used for many dairy products, such as butter, margarine and cheese. Aluminium foil is also used in cartonboard composite containers for chilled fruit juices and dairy beverages. In addition, aluminium or steel aerosol containers are used for chilled creams and processed cheeses. 6.3.4 Plastics Plastics are the materials of choice for the majority of chilled foods. Chilled desserts, ready meals, dairy products, meats, seafood, pasta, poultry, fruit and vegetables are all commonly packed in plastics or plastic-based materials. Semirigid plastic containers for chilled foods are predominantly made from polyethylene (PE), polyproplyene (PP), polystyrene (PS), PVC, PET and acrylonitrile-butadiene-styrene (ABS). Other plastics such as polycarbonate (PC) are used in small quantities. Containers are available in a wide range of bottle, pot, tray and other shapes and thermoforming, injection moulding and blow moulding techniques give food processors the option of in-house manufacture. Flexible plastics offer the cheapest form of barrier packaging and may be used to pack perishable chilled food under vacuum or modified atmosphere. Multilayer materials are typically made by coextrusion or coating processes, using sandwich layers of PVDC or ethylene-vinyl alcohol (EVOH) to provide an oxygen barrier. Alternatively, plastics such as PE or PP may be metallised or laminated with foil to provide very high-barrier materials. The required technical properties and pack size and shape may be matched to a desired specification, thereby ensuring cost effectiveness. The oxygen and water vapour transmission rates of aluminium foil and selected monolayer plastic films are compared in Table 6.4. Most polymers used for chilled food packaging are thermoplastics, i.e., they are reversibly softened by the application of heat, provided that no chemical breakdown occurs. PE is derived from the polymerisation of ethylene, whereas other thermoplastics such as PP, PVDC, PS, PVC, ethylene-vinyl acetate (EVA) and ABS are similarly polymerised from ethylenic monomers. In contrast, plastics such as polyamide (PA), PC and PET are manufactured by condensation reactions. For example, PET film is produced from PET resin, the polycondensation product of ethylene glycol and terephthalic acid, by a stretching process known as biaxial orientation. 6.4 Packaging techniques for chilled food 6.4.1 Modified atmosphere packaging (MAP) MAP is an increasingly popular food preservation technique in which the gaseous atmosphere surrounding the food is different from air (Day 1989). The consumer demand for fresh, additive-free foods has led to the growth of MAP as Chilled food packaging 139