SHORTREVIEWS

Controlled atmosphere packaging of chilled meat

C.O. Gill

The storage life of chilled meat can be extended by packaging product under a preservative gaseous environment to inhibit growth of spoilage bacteria. The maximum storage life is attained by packaging in a gas-impermeable pouch under an atmosphere of oxygen-free C02, with the gas added in sufficient quantity to fully saturate the meat at the optimum storage temperature (-1~7°C) and atmospheric pressures. In such controlled atmosphere packaging (CAP), the storage life of meat is between 8 and 15 times that of the same product stored in air. The CAP environment assure retention of good raw meat colour, and develop- ment of good eating qualities in cooked product. CAP packaging is being used commercially for shipment of chilled lamb to distant markets. Continuing studies indicate that it could be applied equally effectively to prolonging the storage life of a wide range of other pei&hable food p&ducts.-

Keywords: Controlled atmosphere package; meat

INTRODUCTION

Growth of spoilage bacteria severely limits the storage of life of raw meats stored in air at chiller temperatures. The growth of spoilage bacteria can be retarded, and storage life thus extended, by packaging meat to provide a preservative gaseous environment for the product.

Four types of preservative packaging can be distin- guished, differentiated by the manner in which spoilage organisms are inhibited. In vacuum packages, growth of aerobic organisms is prevented by removing oxygen from the in-pack environment. Spoilage then develops at a later time as a consequence of the proliferation of slower-growing organisms that tolerate anaerobic con- ditions. In high oxygen modified atmosphere packages (high-O* MAP), growth of aerobic organisms is inhi- bited, but not suppressed, by moderate concentrations of carbon dioxide. In low oxygen modified atmosphere packages (low-O2 MAP), the inhibitory effects of carbon dioxide on the aerobic flora is augmented, to various degrees, by limited availability of oxygen, and the CO2 concentration may be sufficiently high to slow the growth of species tolerant of anaerobic conditions. In controlled atmosphere packages (CAP), oxygen deprivation prevents growth of the aerobic species,

Meat Industry Research Institute of New Zealand (Inc.), PO Box 617, Hamilton, New Zealand

74

while high concentrations of carbon dioxide inhibit growth of the species tolerant of anaerobic conditions (Finne, 1982; Gill and Penney, 1986; Sebranek, 1986; Shay and Egan, 1986).

VACUUM AND MAP PACKAGINGS

Vacuum and MAP packaging are widely used for raw meats, their use being favoured by the substantial extensions of storage life achievable with relatively simple packaging systems. In contrast, CAP packaging of raw meat is a recent development, resulting from the other forms of preservative packaging being inadequate for many commercial purposes (Brody, 1987).

The deficiencies in vacuum and MAP packagings arise because the time at which microbial spoilage develops can depend upon factors related to product composition, as well as the storage temperature and the packaging itself. In addition, non-microbial deteriora- tion that the packaging does not prevent, or may even precipitate, can precede microbial spoilage. These cir- cumstances can be illustrated by consideration of the use of vacuum and MAP packagings with raw red meats.

vacuum padcaging

Vacuum packaging achieves its preservative effect by maintaining an oxygen-deficient environment for the product. In anoxic conditions, potent spoilage bacteria

0956-7135/90/020074-05 @ 1990 Bulieworth-Heinetnann Ltd

Controlled atmosphere packaging of chilled meat: C. 0. GiH

are severely or totally inhibited on low-pH (c5.8) product. However, their growth on high-pH product is only slowed. Consequently, meat with high-pH muscle tissue, or extensive fat cover of inevitably neutral pH, will spoil relatively rapidly in a vacuum pack (Gill, 1986a; Grau, 1983).

The efficacy of vacuum packaging depends on there being close contact between a film of low gas permea- bility and all the product surfaces (Egan, 1984; Rixvi, 1981). If there are vacuities within the pack, these will develop an oxygen-containing atmosphere as gases permeate into the pack during storage. At meat surfaces exposed to such an atmosphere, .bacterial growth will accelerate and product colour will deterio- rate because of oxidation of the muscle pigment, myoglobin (O’Keefe and Hood, 1982; Grau, 1981). Relatively early colour deterioration can also occur when the meat surface area is large relative to the meat mass, so that an extensive film surface is presented for oxygen permeation. Consequently, vacuum packaging is relatively ineffective for preserving products, such as carcasses, whose shapes preclude close application of the packaging film to all surfaces and small cuts of any shape.

In addition, the anoxic conditions within the vacuum pack result in the muscle tissue having the dull, purple colour of myoglobin rather than the bright red colour of oxymoyoglobin that consumers associate with good quality in meat. Consequently, vacuum packaging is generally considered to be unsuitable for display packs of red meat (Young et al., 1988).

Vacuum packaging can, therefore, extend the stor- age life of primal cuts composed largely of low (normal) pH muscle tissue, such as beef and venison, by about fivefold over that achieved in air (Johnson, 1974; Seman et al., 1988). For other meats and small cuts, only a twofold extension of storage life can be safely anticipated, while the in-pack meat colour is undesirable for most display purposes (Allen, 1989).

High-O2 MAP

High-q MAP systems, which have atmospheres of ==30% CO* and up to 70% 02, are used both to extend the colour stability and to delay microbial spoilage of display-packaged product. The commercial objective is usually to obtain a product life sufficient to permit meat cutting and retail pack preparation at a central facility (Hermansen, 1983; Renerre, 1989).

Both the colour stability and time to spoilage are approximately doubled by high-O* MAP packaging. Howev&r, for many commercial purposes, a doubling of product life is not wholly adequate to allow for distribution as well as display during the time that adequate colour is maintained (Taylor and McDougall, 1973; Cole, 1986). Moreover, because oxygen is respired and CO* is highly soluble in meat, the pack atmnosphere tends to alter as storage proceeds. Adequate stability of the atmosphere can be main: tained only when ratio of the pack volume to the meat volume is about 3 to 1. The excessive space occupied by packaging rather than product is economically dis- advantageous (Holland, 1980; Taylor, 1985).

Therefore, high-O* MAP packaging is not suitable’ for prolonged storage of meat or for bulk packaging, and its suitability for display packaging will depend as

much on the particular commercial circumstances as on the limited preservative capabilities of the packaging.

Low-Or MAP

The colour of raw muscle tissue is dependent on the oxidation state of the purple muscle pigment, myoglobin. Oxygenated myoglobin, oxymyoglobin, is a desirable red; oxidized myoglobin, metmyoglobin, is an undesirable brown. Oxygenation is rapidly rever- sible with changes in the partial pressure of oxygen. Oxidation is only slowly reversible by enxyme- mediated reactions, metmyoglobin reduction activity. The metmyoglobin reduction capacity of muscle tissue is limited and decays during storage. Myoglobin is more susceptible to oxidation than oxymyoglobin, so metmyoglobin forms most rapidly at low oxygen con- centrations (Ledward, 1970; 1985). It follows that in low-Oa MAP packages, when air has been largely displaced by COz, red meats rapidly discolour because of the low oxygen concentrations. Consequently, low-O2 MAP packaging cannot be used with red meats, although storage life extensions exceeding threefold are possible with poultry (Hotchkiss et al., 1985; Bohnsack et al., 1989).

CAP PACKAGING

Because of the discolouration with low-O2 MAP packaging, use of high CO* atmospheres with red meat was generally thought to be impracticable (Taylor, 1985). However, when it was considered that the dis- colouration must be due to oxygen, it was obvious that meat colour would not be adversely affected by high- CO;? atmospheres if oxygen was rigorously excluded from packs. Such a system would be a true CAP pack, i.e. a packaging where a defined pack atmosphere could be established and remain invariant during the life of the package. A packaging system of that type was developed for the purpose of achieving a long storage life for chilled lamb (Gill, 1989; Davenport, 1989).

Lamb must be regarded as a high-pH meat. With usual preparation and chiller storage conditions, vacuum packaged lamb primal cuts have a storage life of =6 weeks. The storage life can be approximately doubled by careful attention to processing hygiene and maintenance of the optimum temperature for storage (Gill and Penney, 1985). However, even 12 weeks storage life can be inadequate when meat is being shipped by sea to markets on the other side of the world. For that purpose, a packaging system capable of giving a reliable storage life of at least 16 weeks was seen to be required (Gill, 1986b).

To achieve a CAP package, it is necessary to have equipment that reliably removes all but traces of oxygen from the package. When development started, use was made of existing machines that evacuated pouches and then filled them with gas through snorkels passing between pads that sealed the pouch opening during evacuation and gassing. Although packs with atmospheres adequately low in oxygen could be pre- pared using such snorkel machines, their preparation required operator skill and multiple flushing cycles. Even then, oxygen was not adequately removed from some packs, because air was retained in parts of the pack that were locked-off as the pack collapsed during

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Controlled atmosphere packaging of chilled meat: C. 0. Gill

evacuation. Also, pouch collapse under atmospheric pressure obviously stressed the packaging, and the time required for pack preparation was considered exces- sive .

Consequently, a high vacuum chamber plus snorkel machine with programmable logic control (PLC) was built, in which a pouch is evacuated under reducing external pressure, to maintain free pathways for exhaustion of air. The pouch is then collapsed by mild external pressure, to expel residual air, and subse- quently inflated under increasing external pressure, to minimize stress during gassing of the pack. With this type of machine, residual oxygen levels of ~0.05% can be reliably attained with cycle times of ~30s.

Pouches have to be constructed of a material that is totally impermeable to gases, because during long storage times even a low gas transmission rate allows oxygen to enter a pack in a quantity sufficient to degrade meat colour. Unfortunately, all plastics have some degree of permeability to gases. To present a total barrier to gas transmission, the pouch material would have to incorporate a gas-impermeable layer of metal. An aluminium foil laminate with good sealing and puncture resistance properties was therefore used for fabricating pouches.

The machine and the pouch are the critical elements of the system, but considerable effort was also expended on developing in-pouch packagings for indi- vidual pieces of meat. Such packagings include an absorbent plus bone-guard material, for use with primal cuts and whole lamb carcasses, and display packs for consumer-ready cuts. Outer cases appro- priate to the different types of product were also designed, to simplify handling of product when using the system and to optimize packaging densities (Rennie, 1988).

With this CAP system, the initial objective of obtain- ing a very long storage life for chilled lamb primal cuts was readily achieved (Warburton, 1988). Further, con- tinuing work has shown that the system can be applied equally effectively to a wide range of products. These studies have also shown why the system works so well, and revealed advantages that were not even considered when the development was started.

Control of Spoilage

The high partial pressure of oxygen-free CO* extends the lag phase and slows the growth rate of all spoilage organisms. The types that cause putrid spoilage are either totally inhibited or subjected to a long lag, at least 12 weeks at -1°C. The only potent spoilage organism not fully controlled is the species Brochothrix thermosphacta. That organism is a common cause of spoilage of vacuum packaged pork and processed meats despite its being readily eliminated by proper attention to processing hygiene. However, in CAP packaging, the minimum growth temperature of the organism is raised from below the freezing point of meat to >o”C. Consequently, even heavily contaminated high-pH meats stored at -1°C will remain unspoiled for at least 3 months. At the same near optimum storage tempera- ture, high-pH products prepared to a good hygienic standard will last for 6 months, while low-pH products may last 9 months (Gill and Penney, 1986b, 1988; Gill and Harrison, 1989; Gill et al., 1990).

Contrul of Pathogens A number of moderately pathogenic species of bacteria can grow in vacuum packs at chiller temperatures. In addition, potent mesophilic pathogens can grow at abusive temperatures. As with spoilage organisms in CAP packaging, the lag phases of pathogens are extended, the growth rates are reduced and the minimum temperatures for growth are increased. Pathogen proliferation at 40°C is thus severely restricted and at <5”C, entirely prevented. The same is not true of vacuum or MAP packaged products. The possible risk to public health from CAP packaged product is therefore very much lower than from products in other types of preservative packaging or, indeed, in unpackaged products (Doyle, 1983; Gray et al., 1984; Gill and Reichel, 1989).

Control of dour deterioration Colour deterioration is a result of oxidation reactions. In the absence of oxygen, such reactions cannot occur. Products in CAP packaging are therefore colour stable. In the case of raw red meats, the colour is actually improved, by enzymic reduction of pigment that oxidized before the meat was packaged. However, the stability of raw meat colour on exposure to air decreases as the storage life increases. After 12 weeks storage, the colour stability at display reaches a minimum value. After that, acceptable meat colour at display is maintained for 40% of the time that fresh meat retains acceptable colour. That is an inevitable consequence of prolonged chill temperature storage, not an efffect of the packaging itself (Moore and Gill, 1987).

Reduced colour stability may seem to present a problem for retailers, but any such difficulty is avoided when meat is prepared in consumer packs and the CAP packaging is broken only when the product is required for display. The colour of the CAP-packaged product will begin to deteriorate only from that time, whereas deterioration of the colour of fresh meat will begin from the time that the meat surface is first exposed to air. Consequently, when CAP packaging is used appropriately, the display life of an aged product can in practice be comparable to that of locally produced meat (Gill and Penney, 1989).

The peculiar problem of colour stability in raw red meat is not encountered with processed meats or other products.

Odour and flavour stability Many foods develop stale or rancid odours and/or flavours as a result of oxidation of components of the food. These undesirable changes may be slow, as in the development of rancidity in fats, or rapid, as in development of warmed-over flavour in cooked meats (Asghar et al., 1988). All such changes are prevented in CAP packaging by the absence of oxygen.

The CO* atmosphere also tends to strip volatile odour and flavour components from products. With many products, this is inconsequential, while with raw meat it seems to be a distinct advantage in that species and fodder odours and flavours, which some consumers find offensive, are removed from the product (Gill and Penney, 1989). Only when volatile odour or flavour elements are a desirable component of a product can CAP packaging have possibly undesirable effects on those characteristics of a product.

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Controlled atmosphere packaging of chilled meat: C. 0. Gill

Product form and pack volume Because the pack atmosphere is truly controlled without dependence on product activity, the shapes and sizes of product pieces are of no consequence for pack function. Moreover, pieces of product within a master barrier pack can be packaged individually or in groups in a variety of ways, providing that the packaging for such individual units does not trap air and also allows adequate exposure of the meat to the CO2 atmosphere. A wide range of product forms and individual unit packagings are therefore possible, and both product and packaging can be tailored to meet the specific requirements of particular markets and customers.

The minimum volume of the barrier pouch is determined by the need to add CO2 in sufficient quantity to fully saturate product at atmospheric pressure. Carbon dioxide is highly soluble in both water and fat. With raw meat, CO2 at ~1.51 kg-’ meat is required to saturate the product without the packaging collapsing tightly around the contents (Gill, 1988). The necessary initial large gas volume can result in pouches at first overfilling the cartons in which they are contained. The gas dissolves sufficiently rapidly, however, for the pack volume to fall to its final value during overnight storage.

Exudation from raw meat Exudation is not a problem with most food products, but it is an important factor, both economically and aesthetically, with raw meats.

Exudate losses from raw meat in CAP packaging are quantitatively similar to those in other packagings. However, the high CO2 concentration causes solubili- zation of additional protein, giving the exudate a greater viscosity than is usual. The viscous exudate can give an appearance of slime and can stain fat surfaces. When such appearances are aesthetically undesirable, they can be prevented by appropriate use of absor- bents. However, after very long storage periods, the fat tissue of red meats can become discoloured through- out by imbibed exudate and this form of fat discoloura- tion cannot be readily controlled (Gill and Penney, 1989).

The protein solubilization caused by CO2 saturation has no other effect on the appearance of raw meat, but when the meat is cooked, visible spaces and channels, such as occur in cooked, cured meats, appear in the muscle tissue. This does not affect the eating qualities of the product and is apparently unnoticed by con- sumers. However, if the changed appearance of the cooked muscle is of consequence in some products or markets, it can be avoided, albeit with a substantially reduced time to microbial spoilage, by using a CO2 + N2 atmosphere that, at equilibrium, maintains an atmosphere containing no more than 30% CO2 (Gill and Penney, unpublished results).

Storage life of products in CAP packaging The storage life of a product may be limited by microbial spoilage or non-microbial deterioration. As well as factors intrinsic to each type of product, the time required for microbial spoilage to develop will be affected by the initial microbiological state of the product and the temperatures it experiences during storage. Non-microbial deteriorations will also acceler- ate with increasing temperature. Any storage life

‘?& 1 The storage lite of CAP packaged chilled products, during which all product remained acceptable

Product

Storage life (weeks)

Initial hygienic condition -1°C + 2°C

Venison, primal cuts Beef, primal cuts, low pH Beef, prime cuts, high pH Beef, consumer cuts (display

packed) Lamb, carcasses Lamb, primal cuts Lamb, consumer cuts (display

packed) Pork, loin pieces Chicken, carcasses Lamb liver Fish Roast beef (whole roast) Roast, stuffed lamb (sliced) Blanched vegetables (carrots,

broccoli, cauliflower, sparagus, mushrooms)

Cooked/chilled meals (meat, sauce, green vegetables, potatoes)

Very good Good Good Satisfactory

Very ~3d 24 14 Satisfactory 20 10 Satisfactory 20 10

Poor Poor Satisfactory Uncertain Good Good Good

Good

30+ 24 20 18”

18 14 12’

24 24 12+=

12+

15 15 12 10

4b 6 6’ 4+d 14 14 12+’

12+

“Rejection due to fat discolouration; bspoilage by Brochothrir themtosphacta; ‘rejection due to texture deterioration; drejection due to eye dulling in fresh schnapper; ‘no deterioration at current latest sampling time; no superscript. rejection due to putrid flavours

ascribed to a product can be, therefore, grossly misleading, unless the reason for rejection, the initial hygienic status and storage temperature of product are specified. Those factors are included in Table 1, which summarizes the storage lives so far observed for CAP packaged products in commercial trials. In that table, the storage life is taken as the last time at which all product was considered acceptable, not as the time at which spoilage was unequivocally evident.

Potential uses of the CAP system The system was initially developed to allow convenient sea shipment of chilled lamb primal cuts and whole lamb carcasses from New Zealand to northern hemis- phere markets. It is being increasingly used for these commercial purposes, fulfilling the original objective of the development. In addition, its use for prolonged storage of shelf ready consumer packs of beef and lamb has been established by consumer trials in the UK using product prepared in New Zealand. Other preliminary trials have demonstrated a prolonged storage life for pork, poultry, venison, offal, fish, cooked meats, blanched vegetable and complete dishes (Table I). Detailed work on some products is now in progress.

Current commercial interest is clearly focused on applying the packagings developed for lamb commod- ities to meats from other species. The immediate commercial objectives are the supply of chilled meat to distant markets that have little, or reducing, interest in frozen product, and uncoupling the availability of chilled product from the seasonal availability of stock for slaughter. Such uses are likely to promote trading of chilled meat commodities to distant markets, in part at the expense of existing frozen meat trades.

Application of the system to the storage and trans- port of shelf-ready consumer packs presents wider

food Control - April 1990 77

Controlled atmosphere packaging of chilled meat: C. 0. Gill

possibilities in the longer term. At present, raw meats must be prepared for consumer display at points in time and space relatively close to the retail outlet. Use of a CAP packaging system would remove such constraints on the production and distribution of consumer-ready items, as the display life when product is removed from a CAP master pack equals that of meat prepared locally, and this display life is not degraded by long periods of storage in the master pack. Consequently, branded consumer items could be produced at a time and place convenient to producers, but handled by retailers as stable, chilled items until the master pack was broken. The system should therefore have applica- tion in local as well as distant trading of shelf-ready chilled raw meats.

It is further apparent that the system can be applied equally effectively to a wide range of perishable food products. Further investigation is, however, required because, although control of microbial spoilage can be assumed, the best means of preserving desirable pro- duct characteristics will have to be ascertained for each product type.

The CAP system can therefore be used to give perishable products a storage life that has previously been attainable only by freezing, while preserving the desirable characteristics associated with non-frozen products. We are just beginning to appreciate some- thing of the system’s potential, but radical effects of its use on at least some areas of meat trading can be confidently anticipated.

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Food Control - April 7990