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
140 Chilled foods Table 6.4 Oxygen and water vapour transmission rates of selected packaging materials film (25m) T 23°C:0%RH 38°:90%RH EVOH 0.2-1.6b 24-120 PVDC 0.8-92 0.3-3.2 MXDE 2.4b 50-100 20-30 PA6 PETG MOPP 1.5-3.0 UPVC 22-35 PAIl PVC 2000-10000° 200 OPP 2000-2500 HDPE 2100 2500-5000 110160 OPS 2500-5000 PP 3000-3700 PC 4300 LDPE 7100 EVA 12000 110-160 Microperforated 0000-2000000 Dependent on pinhole Dependent on moisture and level of plasticiser a technique to improve product image, reduce wastage and extend the quality shelf-life of a wide range of foods(Day 1992). Established chilled products now available in MAP include red meats, fish, seafood, poultry, crustaceans, offal cooked and cured meats and fish, pasta, pizza, kebabs, cheese, cooked and dressed vegetable products, dairy and bakery goods, ready meals, and whole and prepared fresh fruit and vegetables(Day and wiktorowicz 1999, Air Products Plc1995) Gases The gas mixture used in MAP (see Table 6.5)must be chosen to meet the needs of the specific food product, but for nearly all products this will be some combination of carbon dioxide( co2), oxygen(O2)and nitrogen(N2)(Day and wiktorowicz 1999). Carbon dioxide has bacteriostatic and fungistatic properties and will retard the growth of mould and aerobic bacteria. The combined negative effects on various enzymic and biochemical pathways result in an increase in the lag phase and generation time of susceptible spoilage microorganisms. However, CO2 does not retard the growth of all types of
a technique to improve product image, reduce wastage and extend the quality shelf-life of a wide range of foods (Day 1992). Established chilled products now available in MAP include red meats, fish, seafood, poultry, crustaceans, offal, cooked and cured meats and fish, pasta, pizza, kebabs, cheese, cooked and dressed vegetable products, dairy and bakery goods, ready meals, and whole and prepared fresh fruit and vegetables (Day and Wiktorowicz 1999, Air Products Plc 1995). Gases The gas mixture used in MAP (see Table 6.5) must be chosen to meet the needs of the specific food product, but for nearly all products this will be some combination of carbon dioxide (CO2), oxygen (O2) and nitrogen (N2) (Day and Wiktorowicz 1999). Carbon dioxide has bacteriostatic and fungistatic properties and will retard the growth of mould and aerobic bacteria. The combined negative effects on various enzymic and biochemical pathways result in an increase in the lag phase and generation time of susceptible spoilage microorganisms. However, CO2 does not retard the growth of all types of Table 6.4 Oxygen and water vapour transmission rates of selected packaging materials Packaging film Oxygen Water vapour (25 m) Transmission rate transmission rate (cm3 m2 day1 atm1 ) (g m2 day1 ) 23ºC: 0% RH 38ºC: 90% RH Al foil neg.a neg.a EVOH 0.2–1.6b 24–120 PVDC 0.8–9.2 0.3–3.2 MXDE 2.4b 25 PET 50–100 20–30 PA6 80b 200 PETG 100 60 MOPP 100–200 1.5–3.0 UPVC 120–160 22–35 PA11 350b 60 PVC 2000–10 000c 200 OPP 2000–2500 7 HDPE 2100 68 PS 2500–5000 110160 OPS 2500–5000 170 PP 3000–3700 10–12 PC 4300 180 LDPE 7100 16–24 EVA 12 000 110–160 Microperforated 20 000–2 000 000d – a Dependent on pinholes. b Dependent on moisture. c Dependent on moisture and level of plasticiser. d Dependent on degree of microperforation. 140 Chilled foods
Chilled food packaging 141 Table 6.5 Gas-mix guide for MAP of retail chilled food products Chilled food item %o CO, Meat(red 60-85 Meat(cured) Meat(cooked) 25-30 Offal(raw) Poultry(white) Poultry(reddish) 25-35 Fish(oily) 55-65 ceans and molluscs 333 25-35 3 25-35 65-75 25-35 65-75 25-35 65-75 25-35 65-75 Quiche 25-35 65-75 Meat pies 25-35 65-75 Cheese(hard) 25-35 Cheese(mould-ripened) --------1 fruit/vegetable 3-1 tables(cooked) 5-35 microorganisms. For example, the growth of lactic acid bacteria is improved in the presence of CO2 and a low O2 content. CO2 has little effect on the growth of yeast cells. The inhibitory effect of CO2 is increased at low temperatures because of its enhanced solubility in water to form a mild carbonic acid. The practical significance of this is that MAP does not eliminate the need for refrigeration. The absorption of CO2 is highly dependent on the water and fat content of the product. Excess CO2 absorption can reduce the water-holding capacity of meats, resulting in unsightly drip. In addition, some dairy products can be tainted, and fruit and vegetables can suffer physiological damage owing to high CO2 levels. If products absorb excess CO2, the total volume inside the package will be reduced, giving a vacuum package look known as pack collapse In MAP, Oz levels are normally set as low as possible to inhibit the growth of aerobic spoilage microorganisms and to reduce the rate of oxidative deterioration of foods. However, there are exceptions; for example, O2 is needed for fruit and vegetable respiration, colour retention in red meats or to avoid anaerobic conditions in white fish MA packs Nitrogen is effectively an inert gas and has a low solubility in both water and fat. In MAP, N2 is used primarily to displace O2 in order to retard aerobic spoilage and oxidative deterioration. Another role of N2 is to act as a filler gas so as to prevent pack collapse. Other gases such as carbon monoxide, ozone ethylene oxide, nitrous oxide, helium, neon, argon, propylene oxide, ethanol vapour, hydrogen, sulphur dioxide and chlorine have been used experimentally or on a restricted commercial basis to extend the shelf-life of a number of food
microorganisms. For example, the growth of lactic acid bacteria is improved in the presence of CO2 and a low O2 content. CO2 has little effect on the growth of yeast cells. The inhibitory effect of CO2 is increased at low temperatures because of its enhanced solubility in water to form a mild carbonic acid. The practical significance of this is that MAP does not eliminate the need for refrigeration. The absorption of CO2 is highly dependent on the water and fat content of the product. Excess CO2 absorption can reduce the water-holding capacity of meats, resulting in unsightly drip. In addition, some dairy products can be tainted, and fruit and vegetables can suffer physiological damage owing to high CO2 levels. If products absorb excess CO2, the total volume inside the package will be reduced, giving a vacuum package look known as pack collapse. In MAP, O2 levels are normally set as low as possible to inhibit the growth of aerobic spoilage microorganisms and to reduce the rate of oxidative deterioration of foods. However, there are exceptions; for example, O2 is needed for fruit and vegetable respiration, colour retention in red meats or to avoid anaerobic conditions in white fish MA packs. Nitrogen is effectively an inert gas and has a low solubility in both water and fat. In MAP, N2 is used primarily to displace O2 in order to retard aerobic spoilage and oxidative deterioration. Another role of N2 is to act as a filler gas so as to prevent pack collapse. Other gases such as carbon monoxide, ozone, ethylene oxide, nitrous oxide, helium, neon, argon, propylene oxide, ethanol vapour, hydrogen, sulphur dioxide and chlorine have been used experimentally or on a restricted commercial basis to extend the shelf-life of a number of food Table 6.5 Gas-mix guide for MAP of retail chilled food products Chilled food item % CO2 % O2 % N2 Meat (red) 15–40 60–85 0–10 Meat (cured) 20–35 – 65–80 Meat (cooked) 25–30 – 70–75 Offal (raw) 15–25 75–85 – Poultry (white) 20–50 – 50–80 Poultry (reddish) 25–35 65–75 – Fish (white) 35–45 25–35 25–35 Fish (oily) 35–45 – 55–65 Crustaceans and molluscs 35–45 25–35 25–35 Fish (cooked) 25–35 – 65–75 Pasta (fresh) 25–35 – 65–75 Ready meals 25–35 – 65–75 Pizza 25–35 – 65–75 Quiche 25–35 – 65–75 Meat pies 25–35 – 65–75 Cheese (hard) 25–35 – 65–75 Cheese (mould-ripened) – – 100 Cream – – 100 Fresh fruit/vegetables 3–10 210 80–95 Vegetables (cooked) 25–35 – 65–75 Chilled food packaging 141
142 Chilled foods products. For example, carbon monoxide has been shown to be very effective at maintaining the colour of red meats, maintaining the red stripe of salmon and inhibiting plant tissue decay. However, the commercial use of most of these other gases is severely limited owing to safety concerns, regulatory constraints, negative effects on sensory quality or economic factors(Day 1992) Argon(Ar)and nitrous oxide(n20)are classified as miscellaneous additives and are permitted gases for food use in the European Union. Air Liquide s.a (Paris, France)has stimulated recent commercial interest in the potential MAP applications of using Ar and, to a lesser extent, N2O. Air Liquide' s broad range of patents claim that in comparison with N2, Ar can more effectively inhibit enzymic activities, microbial growth and degradative chemical reactions in selected perishable foods. Although Ar is chemically inert, Air Liquide's research has indicated that it does have biochemical effects, probably due to its imilar atomic size to molecular O2 and its higher density and solubility in water compared with N2 and O2(Brody and Thaler 1996). Hence, Ar is probably more effective at displacing O2 from cellular sites and enzymic O2 receptors with the consequence that oxidative deterioration reactions are likely to be inhibited. In ddition, Ar and N,O are thought to sensitise microorganisms to antimicrobial agents. This possible sensitisation is not yet well understood, but may involve alteration of the membrane fluidity of microbial cell walls, with a subsequent influence on cell function and performance(Day 1998). Clearly, more ndependent research is needed to better understand the potential beneficial effects of Ar and n2o Packaging materials Specifically with regard to MAP, the main characteristics to consider when selecting packaging materials are as follows Gas permeability. In most MAP applications, excluding fresh fruit and vegetables, it is desirable to maintain the atmosphere initially incorporated into the package for as long a period as possible. The correct atmosphere at the start will not serve for long if the packaging material allows it to change too rapidly Consequently, packaging materials used with all forms of MA-packed foods (with the exception of fresh fruit and vegetables) should have barrier properties Typically, the lidding film consists of 15u PVDC-coated PET/60u PE and the tray consists of 350u PVC/PE (see Fig 6.1). Alternatively, PA/PVDC/LDPE, PA/PVDDC/HDPE/EVA, OPP/PVDC/LDPE or PC/EVOH/EVA may be used for the lidding film, and hdPe, Pet/evoH/LdPe Pvc/ EvoH/LDPE or PS/ EVOH/LDPE used for the tray(Air Products Plc 1995) The permeability of a particular packaging material depends on several factors such as the nature of the gas the structure and thickness of the material the temperature and the relative humidity(RH). Although CO2, O2 and N permeate at quite different rates, the order CO2>O2>N2 is al ways maintained and the permeability ratios CO2/O2 and O2/N2 are usually in the range 3 to 5 Hence, it is possible to estimate the permeability of a material to CO2 or N2
products. For example, carbon monoxide has been shown to be very effective at maintaining the colour of red meats, maintaining the red stripe of salmon and inhibiting plant tissue decay. However, the commercial use of most of these other gases is severely limited owing to safety concerns, regulatory constraints, negative effects on sensory quality or economic factors (Day 1992). Argon (Ar) and nitrous oxide (N2O) are classified as miscellaneous additives and are permitted gases for food use in the European Union. Air Liquide S.A. (Paris, France) has stimulated recent commercial interest in the potential MAP applications of using Ar and, to a lesser extent, N2O. Air Liquide’s broad range of patents claim that in comparison with N2, Ar can more effectively inhibit enzymic activities, microbial growth and degradative chemical reactions in selected perishable foods. Although Ar is chemically inert, Air Liquide’s research has indicated that it does have biochemical effects, probably due to its similar atomic size to molecular O2 and its higher density and solubility in water compared with N2 and O2 (Brody and Thaler 1996). Hence, Ar is probably more effective at displacing O2 from cellular sites and enzymic O2 receptors with the consequence that oxidative deterioration reactions are likely to be inhibited. In addition, Ar and N2O are thought to sensitise microorganisms to antimicrobial agents. This possible sensitisation is not yet well understood, but may involve alteration of the membrane fluidity of microbial cell walls, with a subsequent influence on cell function and performance (Day 1998). Clearly, more independent research is needed to better understand the potential beneficial effects of Ar and N2O. Packaging materials Specifically with regard to MAP, the main characteristics to consider when selecting packaging materials are as follows: Gas permeability. In most MAP applications, excluding fresh fruit and vegetables, it is desirable to maintain the atmosphere initially incorporated into the package for as long a period as possible. The correct atmosphere at the start will not serve for long if the packaging material allows it to change too rapidly. Consequently, packaging materials used with all forms of MA-packed foods (with the exception of fresh fruit and vegetables) should have barrier properties. Typically, the lidding film consists of 15 PVDC-coated PET/60 PE and the tray consists of 350 PVC/PE (see Fig. 6.1). Alternatively, PA/PVDC/LDPE, PA/PVDDC/HDPE/EVA, OPP/PVDC/LDPE or PC/EVOH/EVA may be used for the lidding film, and HDPE, PET/EVOH/LDPE, PVC/EVOH/LDPE or PS/ EVOH/LDPE used for the tray (Air Products Plc 1995). The permeability of a particular packaging material depends on several factors such as the nature of the gas, the structure and thickness of the material, the temperature and the relative humidity (RH). Although CO2, O2 and N2 permeate at quite different rates, the order CO2 O2 N2 is always maintained and the permeability ratios CO2/O2 and O2/N2 are usually in the range 3 to 5. Hence, it is possible to estimate the permeability of a material to CO2 or N2 142 Chilled foods
Chilled food packaging 143 PET film PVDC coating PE film Fig. 6. 1 Construction of a typical tray and lidding film Ma pack. hen only the Oz permeability is known. As a general rule, packaging materials withO2 transmission rates less than 100 cmmday-atm are used in MAP ckaging materials are usually laminated or coextruded in order to have the necessary barrier properties required(roberts 1990) In contrast to other perishable foods that are packed in MA systems, fresh fruit nd vegetables continue to respire after harvest and any packaging must take this into account. The depletion of O2 and the accumulation of CO2 are natural consequences of the progress of respiration when fresh fruit or vegetables are stored in a sealed package. Such modification of the atmospheric composition results in a decrease in the respiration rate, with a consequent extension in the shelf-life of fresh produce. However, packaging film of the correct permeability must be chosen to realise the full benefits of MAP of fresh produce(Day 1998) Typically, the key to successful MAP of fresh produce is to maintain an equilibrium MA(EMA) containing 2-10%O2/CO2 within the package. For highly respiring produce such as mushrooms, beansprouts, leeks, peas and broccoli, traditional films like LDPE, PVC, EVA, OPP and cellulose acetate(CA) are not sufficiently permeable. Such highly respiring produce is most suitably packed in highly permeable microperforated films. However, micrope ow fo films are relatively expensive, permit moisture and odour loss, and may allow for the ingress of microorganisms into sealed packs during wet handling situations Day 1998). A very interesting development for the packing of fresh prepared produce involves the use of high O2(70-100%)MAP which has been recently shown to overcome the many disadvantages of current air packing and low O MAP. High O2 MAP has been demonstrated to inhibit enzymic discolorations prevent anaerobic fermentation reactions and inhibit microbial growth with the result of exten Ice shelf-life(Day, 199 Water vapour transmission rate. Water vapour transmission rates are quoted in gm- day at a given temperature and rh. Similar to gas permeabilities, there
when only the O2 permeability is known. As a general rule, packaging materials with O2 transmission rates less than 100 cm3 m2 day1 atm1 are used in MAP. Packaging materials are usually laminated or coextruded in order to have the necessary barrier properties required (Roberts 1990). In contrast to other perishable foods that are packed in MA systems, fresh fruit and vegetables continue to respire after harvest and any packaging must take this into account. The depletion of O2 and the accumulation of CO2 are natural consequences of the progress of respiration when fresh fruit or vegetables are stored in a sealed package. Such modification of the atmospheric composition results in a decrease in the respiration rate, with a consequent extension in the shelf-life of fresh produce. However, packaging film of the correct permeability must be chosen to realise the full benefits of MAP of fresh produce (Day 1998). Typically, the key to successful MAP of fresh produce is to maintain an equilibrium MA (EMA) containing 2–10% O2/CO2 within the package. For highly respiring produce such as mushrooms, beansprouts, leeks, peas and broccoli, traditional films like LDPE, PVC, EVA, OPP and cellulose acetate (CA) are not sufficiently permeable. Such highly respiring produce is most suitably packed in highly permeable microperforated films. However, microperforated films are relatively expensive, permit moisture and odour loss, and may allow for the ingress of microorganisms into sealed packs during wet handling situations (Day 1998). A very interesting development for the packing of fresh prepared produce involves the use of high O2 (70–100%) MAP which has been recently shown to overcome the many disadvantages of current air packing and low O2 MAP. High O2 MAP has been demonstrated to inhibit enzymic discolorations, prevent anaerobic fermentation reactions and inhibit microbial growth with the result of extending prepared produce shelf-life (Day, 1998; 1999a). Water vapour transmission rate. Water vapour transmission rates are quoted in g m2 day1 at a given temperature and RH. Similar to gas permeabilities, there Fig. 6.1 Construction of a typical tray and lidding film MA pack. Chilled food packaging 143
144 Chilled foods is a wide variation between different packaging materials(see Table 6.4) However, there is no correlation between what is a good barrier to gas and what is a good barrier to water. A further complication is that some materials(e.g nylons and EVOH)are moisture-sensitive and their gas permeabilities are dependent on RH (Day 1992) Mechanical properties. Packaging materials used for MAP must have sufficient strength to resist puncture, withstand repeated flexing and endure the mechanical stresses encountered during handling and distribution. Additionally if trays are to be thermoformed, the web must draw evenly and not thin excessively on the corners. Poor mechanical properties can lead to pack damage and leakage(Day 1992) Sealing reliability. It is essential that an integral seal is formed in order to maintain the correct atmosphere within a Ma pack. Therefore, it is important to select the correct heat-sealable packaging materials and to control the sealing operation. For example, in high-speed form-fill-seal operations, it is important to consider the hot tack of the material. Additionally, there is often a requirement for a peelable seal so that the consumer can gain easy access to the contents. However, the balance between peelability and integrity of the seal must be determined (Day 1992) Transparency. For most MA packed foods, a transparent package is desirable so that the product is clearly visible to the consumer. However, high-moisture foods stored at chilled temperatures have the tendency to create a fog on the inside of the package, thereby obscuring the product. Consequently, many MAP films are treated with coatings or additives to impart antifog properties so as to improve visibility. These treatments only affect the wetability of the film and have no effect on the permeability properties of the film(Roberts 1990) For some MA packed foods(e.g. green pasta and cured meats), it may be desirable to exclude light in order to reduce undesirable light-induced oxidation reactions. In these cases, light barriers such as colour-printed or metallised films may be used. Another influence of light is the possibility of a greenhouse effect' causing a temperature rise within chilled food packs(Malton 1976) However, Gill(1987)concluded that this effect was not an important factor in increasing temperatures of products displayed in chilled cabinets Type of package. The type of package used will depend on whether the product is destined for the retail or the catering trade. Popular options include flexible pillow packs,"bag-in-box'and semi-rigid tray and lidding film systems(see Fg.61) Microwavability. The ability of MAP materials to withstand microwave heating is important, particularly in the case of ready-to-eat food products. For example, the low softening point of PVc makes the popular PVC/LDPE thermoformed
is a wide variation between different packaging materials (see Table 6.4). However, there is no correlation between what is a good barrier to gas and what is a good barrier to water. A further complication is that some materials (e.g. nylons and EVOH) are moisture-sensitive and their gas permeabilities are dependent on RH (Day 1992). Mechanical properties. Packaging materials used for MAP must have sufficient strength to resist puncture, withstand repeated flexing and endure the mechanical stresses encountered during handling and distribution. Additionally, if trays are to be thermoformed, the web must draw evenly and not thin excessively on the corners. Poor mechanical properties can lead to pack damage and leakage (Day 1992). Sealing reliability. It is essential that an integral seal is formed in order to maintain the correct atmosphere within a MA pack. Therefore, it is important to select the correct heat-sealable packaging materials and to control the sealing operation. For example, in high-speed form–fill–seal operations, it is important to consider the hot tack of the material. Additionally, there is often a requirement for a peelable seal so that the consumer can gain easy access to the contents. However, the balance between peelability and integrity of the seal must be determined (Day 1992). Transparency. For most MA packed foods, a transparent package is desirable so that the product is clearly visible to the consumer. However, high-moisture foods stored at chilled temperatures have the tendency to create a fog on the inside of the package, thereby obscuring the product. Consequently, many MAP films are treated with coatings or additives to impart antifog properties so as to improve visibility. These treatments only affect the wetability of the film and have no effect on the permeability properties of the film (Roberts 1990). For some MA packed foods (e.g. green pasta and cured meats), it may be desirable to exclude light in order to reduce undesirable light-induced oxidation reactions. In these cases, light barriers such as colour-printed or metallised films may be used. Another influence of light is the possibility of a ‘greenhouse effect’ causing a temperature rise within chilled food packs (Malton 1976). However, Gill (1987) concluded that this effect was not an important factor in increasing temperatures of products displayed in chilled cabinets. Type of package. The type of package used will depend on whether the product is destined for the retail or the catering trade. Popular options include flexible ‘pillow packs’, ‘bag-in-box’ and semi-rigid tray and lidding film systems (see Fig. 6.1). Microwavability. The ability of MAP materials to withstand microwave heating is important, particularly in the case of ready-to-eat food products. For example, the low softening point of PVC makes the popular PVC/LDPE thermoformed 144 Chilled foods