Part III Novel packaging and particular products 17.1 Introduction Preservative packagings for fresh meats should maintain acceptable appearance odour and flavour for product, while allowing the development of desirable characteristics associated with ageing, and retarding the onset of microbial spoilage (Taylor, 1985). Such effects can be achieved by packaging meats under various atmospheres of oxygen, carbon dioxide, carbon monoxide and/or nitrogen. The atmosphere within a pack may alter during storage, because of reactions between components of the atmosphere and the product, and/or because of transmission of gases into or out of the pack through the packaging film (Stiles, 1991). Packagings of that type are termed Modified Atmosphere Packs (MAP), which are distinguished from Controlled Atmosphere Packs (CAP) within which invariant atmospheres are maintained throughout the time of storage (Brody, 1996). Both MAP and CAP can take various forms, depending on the type of meat that is packaged, the form of the meat, and the commercial uses for the product. Obviously, a commercial user of preservative packagings would usually seek the simplest, and presumably least expensive packaging that would give a storage life and organoleptic quality suitable to the trading envisaged for a particular product. Thus, the optimum packaging for a product can be decided only with knowledge of how the qualities of the particular meat are affected by the various atmospheres to which it might be exposed, and the conditions the packaged product will have to tolerate during commercial storage, distribution and display. 17 Active packaging in practice: meat C.O. Gill, Agriculture and Agri-Food Canada 17.2 Control of product appearance The appearance of raw meat has major effects on the purchasing decisions of consumers (Cornforth, 1994). For red meats, consumers much prefer bright, red muscle tissue and white rather than yellow fat. When bone is present in a retail cut, consumers prefer that any exposed spongy bone appears bright red also. For poultry, bright, white flesh and skin are preferred. The colour of muscle tissue in red meat is determined by the quantity and chemical state of the muscle pigment myoglobin (Fig. 17.1). The deoxy form is a dull, purple colour that consumers consider unattractive. The function of myoglobin is to transfer oxygen from blood to the muscle tissue cells. Myoglobin therefore reacts rapidly and reversibly with oxygen to give the bright red form oxymyoglobin. The fraction of pigment in the oxymyoglobin form is dependent on the partial pressure of oxygen to which the pigment is exposed (Livingston and Brown, 1981). Myoglobin can also react with oxygen to give the stable, oxidised form metmyoglobin (Faustman and Cassens, 1990). Meat with the dull, brown colour of metmyoglobin is considered undesirable by most consumers (Renerre, 1990). Although metmyoglobin is stable, it is slowly reduced to deoxymyoglobin by enzymic reactions involving reduced co-enzymes (Echevarne et al., 1990). Those reactions are termed metmyoglobin reduction activity. Muscle tissue with high metmyoglobin reduction activity can generally maintain a bright red colour when exposed to oxygen for longer than tissue with little or none of the activity, although high respiratory activity tends to accelerate discolouration (O’Keefe and Hood, 1982). Different muscles vary considerably in their metmyoglobin reduction and respiratory activates, and so vary in their colour stabilities during Fig. 17.1 Reactions of myoglobin with oxygen and carbon monoxide. 366 Novel food packaging techniques the first days after slaughter. For example, the longissimus dorsi usually has good colour stability while the colour stability of the psoas major is poor (Hood, 1980). However, enzymic activities in muscle tissue decay with time, so after storage for several days all muscle tissue has similar, low colour stability (Ledward, 1985). The colour stability of ground meat is similarly low because both respiratory and metmyoglobin reduction activates are rapidly lost when meat is ground (Madhavi and Carpenter, 1993). Both deoxymyoglobin and oxymyoglobin can oxidise to metmyoglobin. However, the rate of the oxidation reaction is considerably faster with deoxy- than with oxymyoglobin (Ledward, 1970). Consequently, when oxygen tensions are low, and most of the myoglobin is in the deoxy form, oxidation of the pigment occurs rapidly; while oxidation is retarded when the oxygen tension is high and most of the pigment is in the oxy form. Haemoglobin visible in cut, spongy bone reacts similarly with oxygen. Thus, increasing the oxygen in a pack atmosphere above atmospheric concentrations will stabilise the desirable red colours of muscle tissue and cut spongy bone surfaces. In addition, high concentrations of oxygen will increase the depth of the oxymyoglobin layer at the tissue surface, and so enhance the red colour of the muscle tissue (Young et al., 1988). Although high oxygen concentrations will retard pigment oxidation they do not prevent it. Pigment oxidation is prevented only if oxygen is stripped from the pack atmosphere and subsequently prevented from entering the pack (Gill, 1989). When a pack is first filled with a gas or gases other than oxygen, at least some traces of oxygen will be present in the atmosphere (Penney and Bell, 1993). The residual oxygen will react with the muscle pigment to form metmyoglobin. However, provided that the metmyoglobin reduction capacity of the muscle tissue is not exceeded, the metmyoglobin will be reconverted to deoxymyoglobin during the first few days of storage (Gill and Jones, 1994a). After that, the pigment will remain in the deoxy form until it is exposed to air or a high oxygen atmosphere (Table 17.1). Then, the tissue will bloom to the bright red colour of freshly cut meat as oxymyoglobin is rapidly formed at tissue surfaces. Such a desirable colour will, however, be maintained for a relatively short time if the tissues have little if any metmyoglobin reduction activity to counteract the unavoidable oxidation of the pigment. In addition to the discolouration of the muscle tissue, exposed spongy bone in cuts that have been stored under anoxic atmospheres tend to darken and finally blacken relatively rapidly when the cuts are exposed to air. That intense discolouration appears to be due to the accumulation of haemoglobin at cut bone surfaces during storage (Gill, 1990). In air, the pigment oxidises as it would in freshly cut tissue, but because the amount of pigment is so much greater, the final colour is dark brown or black, rather than the lighter brown colours that spongy bone will develop after meat is cut when fresh. As an alternative to using high oxygen concentrations to stabilise meat colour, or oxygen depleted atmospheres to prevent discolouration, red colours for muscle and bone tissues can be maintained by exposing the tissues to carbon Active packaging in practice: meat 367 monoxide. Carbon monoxide reacts with myoglobin to form the cherry red pigment carboxymyoglobin, which is stable and oxidises only slowly (Lanier et al., 1978). Therefore, exposure of meat to low concentrations of carbon monoxide in a pack atmosphere will result in the tissues developing persistent red colours. The above comments about the colour of red meats are not wholly applicable to poultry muscle. Poultry muscle generally has low concentrations of myoglobin and high rates of oxygen consumption. Consequently, little oxymyoglobin is formed when poultry muscle is exposed to air and consumers are accustomed to the tones imparted to poultry meat by muscle pigment in the deoxy- and metmyoglobin forms (Millar et al., 1994). Therefore, the colour of poultry meat is not enhanced by storage under high oxygen atmospheres, while the appearance of the meat is not grossly degraded by its exposure to low concentrations of oxygen that would rapidly discolour red meats. 17.3 Control of flavour, texture and other characteristics Other undesirable, non-microbiological changes that can occur during the storage of raw meats are oxidation of lipids that impart stale and rancid odours and flavours to the product; loss of exudate from the muscle tissue; and loss of texture and development of liver-like flavours as results of the breakdown of proteins. A desirable change is the increase of tenderness with ageing of the muscle tissue. In the absence of oxygen, lipids will not oxidise. Thus, rancidity does not develop when meat is packaged under an oxygen depleted atmosphere. Oxidation will occur with meat in air or oxygen enriched atmospheres. Although it would be expected that the rates of lipid oxidation would increase Table 17.1 Fractions of metmyoglobin in the muscle pigment of beef steak surfaces after display in air for 1 h, following storage at 1.5oC under N 2 , CO 2 or, 67% O 2 +33% CO 2 (Gill and Jones, 1994a) Storage Metmyoglobin (%) time Storage atmosphere (days) N 2 CO 2 O 2 + CO 2 1 7 60 4 2 25 25 7 4 23 14 0 6 0 2 3 8 8 6 9 12 0 0 17 16 0 6 10 20 7 6 23 24 8 0 42 60 0 0 – 368 Novel food packaging techniques with increasing oxygen concentration, it has been reported that rates of oxidation in air and oxygen enriched atmospheres are similar (Ordonez and Ledward, 1977). Antioxidants naturally present in or added to raw meats will retard the development of rancidity and oxidation of myoglobin, but grinding of meat can greatly accelerate lipid oxidation (Sanchez-Escalante et al., 2001). Lipid oxidation is also accelerated by iron and iron containing compounds. Consequently, when mechanically separated meats, which contain relatively large amounts of iron, are included in comminuted products the oxidative stability of the products is greatly reduced (Cross et al., 1987). Loss of exudate from meat is undesirable, because of ill-effects upon the appearance and handling qualities of cuts, and because of loss of saleable weight when cuts must be divided and repackaged. Exudate losses are unavoidable, but tend to be less with muscle tissue of higher than normal pH. Exudate losses are exacerbated by cutting of meat to smaller portions and pressure on the product (Offer and Knight, 1988). Therefore, in practice, the only options for containing the adverse effects of exudate loss are the avoidance of pressure on product and the inclusion in packs of absorbent pads or wraps of sufficient capacity to hold all the exudate that may be released. Unlike most changes that occur in meat with age, increased tenderness is generally desirable (Jeremiah et al., 1993). The rate of tenderisation declines approximately exponentially with time of storage. For beef stored at 2oC, 80% and 100% of maximum tenderisation have been reported to be achieved after about 9 and 17 days, respectively (Dransfield et al., 1992). Rates of tenderisation are seemingly affected little if at all by the compositions of pack atmospheres, and with most meats, as with beef, tenderising apparently does not continue indefinitely. Even so, the breakdown of proteins can continue, with the accumulation of peptides and free amino acids that impart liver-like flavours to the meat (Rhodes and Lea, 1961). Consumers may find such flavours objectionable (Gill, 1988a). With lamb it has been observed that tenderising can continue until the fibrous texture of muscle tissues is lost. That undesirable loss of texture and development of liver-like flavours do not occur when lamb is stored under an atmosphere of carbon dioxide (Gill, 1989). No other effects of carbon dioxide on tenderising processes have been reported. 17.4 Delaying microbial spoilage Spoilage bacteria will grow on meat that is not frozen under both aerobic and anaerobic conditions (Lowry and Gill, 1984). When the initial numbers of bacteria are relatively low, the spoilage flora will be dominated by those species of bacteria that grow most rapidly in the environment provided by the meat and the surrounding atmosphere (Gill, 1986). When initial numbers are high, slower growing species may persist as substantial fractions of a flora, as the maximum numbers may be approached before they are overgrown by the usually dominant species. The meat will be spoiled when the metabolic activities of the spoilage bacteria cause changes Active packaging in practice: meat 369 in the appearance, odour or flavour of the product that are unacceptable to the consumers (Gill, 1981). The stage of development of the spoilage flora at which such changes occur depends on both the composition of the spoilage flora and the intrinsic qualities of the tissues on which the bacteria are growing. When fresh meat is stored in air, the spoilage flora is dominated by species of Pseudomonas, which are strictly aerobic (Gill and Newton, 1977). Those organisms preferentially utilise glucose, which is present in small quantities in muscle tissue of normal pH (5.5) and usual higher values. When glucose is exhausted the bacteria metabolise amino acids and produce offensive by- products such as ammonia, amines and organic sulphides (Nychas et al., 1988). Thus, on normal pH muscle tissue the onset of aerobic spoilage occurs abruptly when bacterial numbers are about 10 8 /cm 2 . However, on muscle tissue of high pH (> 6.0) and moist fat tissue, little or no glucose may be available (Gill and Newton, 1980). Then, aerobic spoilage will occur when bacterial numbers are about 10 6 /cm 2 . The pseudomonads grow at their maximum rates when oxygen concentration in the atmosphere is as low as 1% (Clark and Burki, 1972). Therefore, increasing the oxygen concentration in a pack atmosphere to preserve meat colour does not accelerate microbial spoilage. However, if the storage life of meat is to be extended the rapid growth of pseudomonads must be suppressed. Growth of pseudomonads is inhibited by carbon dioxide. The growth rate decreases with increasing concentrations of carbon dioxide in the atmosphere up to about 20% (Gill and Tan, 1980). Further increases in carbon dioxide concentration do little more to slow the rate of growth. Thus, with an aerobic atmosphere, a doubling of the time before the onset of microbial spoilage is the most that can be achieved by the inclusion of carbon dioxide in a pack atmosphere. When growth of pseudomonads is inhibited by carbon dioxide, the flora of meat in an aerobic atmosphere is usually dominated by lactic acid bacteria, with more or less large fractions of strict aerobes, such as pseudomonads and acinetobacteria, and facultative anaerobes, such as Brochothrix thermosphacta and enterobacteria (Gill and Jones, 1996). If meat is held in air after storage under a modified atmosphere the lactic acid bacteria, which are of low spoilage potential, may continue to predominate in the flora. However, the fractions of the strict aerobes and facultative anaerobes will usually increase as the flora proliferates; and spoilage will develop as a result of the activities of those latter organisms (Gill and Jones, 1994b). Under anaerobic conditions, the strictly aerobic pseudomonads cannot grow and again the spoilage flora is usually dominated by lactic acid bacteria (Egan, 1983). Those bacteria can grow to maximum numbers about 10 8 /cm 2 without spoilage of the meat. Thereafter, spoilage will develop only slowly as the by- products of the lactic acid bacteria’s metabolism impart acid, dairy flavours to the meat (Dainty et al., 1979). The spoilage process can differ if the tissue pH is >5.8 or the atmosphere contains traces of oxygen. Then, facultative anaerobes such as B. thermosphacta, enterobacteria and Shewanella putrefaciens may grow to spoil the meat as the flora approaches maximum numbers (Blickstad, 1983; 370 Novel food packaging techniques Grau, 1983). However, in a controlled atmosphere of carbon dioxide alone, the growth of some of those organisms is inhibited or prevented when temperatures are at the lower end of the chill temperature range (Gill and Harrison, 1989). Inclusion of small amounts of carbon monoxide in anaerobic atmospheres does not affect development of the spoilage flora (S?rheim et al., 1999). If meat is held in air after storage under an anaerobic atmosphere, spoilage by facultative anaerobes or strictly aerobic organisms is likely to occur although lactic acid bacteria continue to predominate in the flora (Gill and Jones, 1996). 17.5 The effects of temperature on storage life All changes that occur in chilled meat during storage are likely to be accelerated by increasing temperature. As most changes are deleterious, it follows that the optimum temperature for storing chilled meats is the minimum that can be maintained indefinitely without freezing the muscle tissue. In practice, that temperature is found to be 1.5 0.5oC (Gill et al., 1988). When red meats are displayed in aerobic atmospheres, discolouration rather than microbial spoilage is likely to limit the useful life of the product. The rate at which discolouration develops in muscle tissue exposed to air appears to increase linearly with temperature for all muscles, but the rate of increase differs between muscles (Hood, 1980). The rate of increase seems to be less for colour stable than for colour unstable muscles, as discolouration of the colour stable longissimus dorsi and the colour unstable psoas major muscles are reported to be, respectively, twice and five times as rapid at 10oC than at 0oC. The effect of temperature on the rate of discolouration of meat stored in modified atmospheres rich in oxygen does not appear to be well identified in the literature, but it seems likely that discolouration with increasing temperature accelerates much as for meat stored in air. When meat is stored anaerobically, the colour stability of muscle tissue increases at first, and then declines (O’Keefe and Hood, 1980–81). The initial increase of stability is probably related to the relatively rapid loss of respiratory activity, while the subsequent decrease in stability reflects the decay of metmyoglobin reduction activities. The rate at which colour stability degrades is reported to be twice as fast at 5oC and four times as fast at 10oC as at 0oC (O’Keefe and Hood, 1982). Rates of lipid oxidation in air and oxygen enriched atmospheres are apparently similar, but the effect of storage temperature on the rate of development of rancidity does not seem to have been established. Exudate losses are reported to be about 30% and 100% more, respectively, at 5oC and 10oC than at 0oC (O’Keefe and Hood, 1980–81). The rate at which muscle tenderises is over twice as fast at 10C as at 0C (Dransfield, 1994). Spoilage bacteria will grow on meat that is not frozen at temperatures down to 3oC under both aerobic and anaerobic conditions. Thus, storage at chiller temperatures can delay but not prevent the ultimate onset of microbial spoilage. Active packaging in practice: meat 371 Although the rates of growth of different species of spoilage bacteria differ considerably the rates of all increase rapidly with small increases in temperature above the optimum for storage of chilled meat (Gill and Jones, 1992; Gill et al., 1995). The proportional loss of storage life for the same increase in storage temperature is then broadly similar for all types of spoilage flora. Thus, it is found that the storage life of meat in any or no packaging at 0, 2 and 5oC is about 70, 50 and 30%, respectively, of the storage life that would be obtained for the product stored at 1.5oC (Fig. 17.2). In view of the substantial effects of small increases in temperature on rates of discolouration and bacterial growth, it is apparent that any storage life ascribed to a fresh meat product must be accompanied by a statement of the storage temperature if the storage stability of the product is to be properly understood. 17.6 MAP technology for meat products Modified atmosphere packagings may be used for bulk or retail ready product. Several trays of retail ready product may be placed in a master pack which is filled with the modified atmosphere, or individual, sealed trays may contain the modified atmosphere. Modified atmospheres invariably contain substantial fractions of carbon dioxide to retard the growth of aerobic spoilage organisms. In addition, atmospheres used with red meats will usually contain a high concentration of oxygen to preserve the meat colour or the initial atmosphere may contain a small amount of carbon monoxide to impart a stable red colour to Fig. 17.2 Effects of storage temperature on the storage life of chilled meat limited by microbial spoilage. 372 Novel food packaging techniques the product. An atmosphere may also contain a more or less substantial fraction of nitrogen, to prevent pack collapse. The materials used to form modified atmosphere packs must provide a barrier to the exchange of gases between the pack and the ambient atmosphere. However, the gas barrier properties of the packaging materials differ for different types of packaging and differing commercial functions of the packs. Bulk and master packagings which are expected to contain product for only a day or two are often laminates composed of a strong material with limited gas barrier properties, such as nylon, and a sealable layer of a material such as polyethylene. Such materials may have nominal oxygen transmission rates of more than 100 cc/m 2 /24h/atm under stated conditions of humidity and temperature. However, films used for modified atmosphere packs usually have oxygen transmission rates between 10 and 100cc O 2 /m 2 /24h/atm, while packagings designed to contain product for the longest possible times are likely to be composed of materials with oxygen transmission rates less than 10 cc/m 2 / 24/atm (Jenkins and Harrington, 1991). Carbon dioxide, the essential component of any effective modified atmosphere for meat is highly soluble in both muscle and fat tissues (Gill, 1988b). The solubility in muscle tissue decreases with decreasing pH and increasing temperature but, within the chill temperature range, solubility in fat increases with increasing temperatures (Fig. 17.3). Because of the dissolution of carbon dioxide in the product, the initial atmosphere in a pack should contain a higher concentration of carbon dioxide than the 20% that it is desirable to maintain after equilibration for maximum inhabitation of the aerobic spoilage bacteria. The smaller the volume of the atmosphere in relation to the product mass, the higher the carbon dioxide concentration needed in the input gas, and the greater the decrease in the volume of the atmosphere as carbon dioxide dissolves in the tissues after the pack is sealed. Fig. 17.3 Effects of temperature on the solubility of carbon dioxide in normal pH muscle tissue (a72) and fat tissue (?) of beef. Active packaging in practice: meat 373 Unlike carbon dioxide, the solubility of oxygen in muscle and fat tissues is low. However, oxygen is converted to carbon dioxide by the respiratory activities of both muscle tissue and bacteria. Although both gases are lost through packaging films when both are at concentrations above those of air, the carbon dioxide dissolved in tissue buffers decreases in carbon dioxide concentrations. Thus, with modified atmospheres rich in oxygen it is usually found that oxygen concentrations decline with time of storage, but that carbon dioxide concentrations alter little after the initial dissolution of the gas in the tissues (Nortje and Shaw, 1989). If packs with oxygen-rich atmospheres are to be stored for relatively long times, the volume of the pack atmosphere should be about three times the volume of the product, to avoid excessive decreases of oxygen concentrations (Holland, 1980). The solubility of nitrogen in tissues is low, and the gas is metabolically inert. Thus, the only function of nitrogen in a pack atmosphere is to buffer against changes in the volume of the atmosphere that could lead to pack collapse, with crushing of the contained product. If carbon monoxide is included in a pack atmosphere it is at concentrations less than 1%. The gas will be removed from the pack atmospheres as it reacts rapidly and essentially irreversibly with myoglobin. The changes in pack atmosphere volumes as a result of the binding of carbon monoxide are trivial in comparison with the volume decreases arising from dissolution of carbon dioxide. Modified atmosphere packagings for bulk meats are usually intended only to enhance stability for short times during the distribution of product from slaughtering or carcass breaking facilities to retail packing facilities. Protection of the product from crushing by its being in a pillow-pack is often considered to be as important as any effects of the atmosphere on the colour or microbiological condition of the product. Bulk packs are usually formed using equipment with two flattened tubes (snorkels) that are inserted into the mouth of each bag. Sprung guides at each side of the mouth prevent bunching. The mouth is held closed around the snorkels by padded jaws. Air is evacuated from the bag through the snorkels. The evacuation may be timed, or terminated when the pressure within the snorkels falls to a pre-set value. If evacuation is controlled by pressure and the snorkel orifices are not sealed by the bag collapsing around them, then the bag will collapse around the product and residual air in the pack will be minimised. After evacuation, the pack is filled for a set time with the selected gas mixture. The evacuation and gassing cycle may be repeated if it is considered that the pack atmosphere may be excessively contaminated with residual air after a single cycle. When the bag has finally been filled with gas, the snorkels are withdrawn from between the closed pads, and the bag is heat sealed. With poultry meats the input gas may be a carbon dioxide/nitrogen mixture with the former gas at concentrations between 40 and 60%. However, 5% oxygen may be included in a mixture because of concerns about the possible growth of Clostridium botulinum if the atmosphere should become anaerobic. In fact, the inclusion of oxygen in the atmosphere will not prevent the growth of botulinum organisms, as anaerobic niches that could permit the growth of such 374 Novel food packaging techniques organisms exist in any package of raw meat, irrespective of the surrounding atmospheres (Lambert et al., 1991). For red meats the input gas would preferably be 70% oxygen and 30% carbon dioxide. However, nitrogen is often included in a mixture although that gas will serve no useful function when, as in these circumstances, the pack is flexible and the volume variable, and any undesirable pack collapse may be countered by simply increasing the volume of input gas. Snorkel type equipment is also used for master packaging of retail ready product, with master packs being filled with the same gas mixtures that are used with bulk product. Retail ready product that is master packaged is usually in conventional, expanded polystyrene trays, which are overwrapped with a clinging film of oxygen permeability between 5,000 and 10,000 cc/m 2 /24h/atm. The trays usually contain plastic covered paper pads, to absorb exudate from the meat. Because collapse of the bag around the trays when the master pack bag is evacuated could easily lead to crushing of the trays, evacuation is usually timed. Evacuation of the bag is then highly uncertain, as the amount of air in the bag when the mouth is closed around the snorkels can vary greatly. Moreover, the overwrapped trays will contain more or less large amounts of air that cannot be removed during evacuation. Consequently, the master pack atmospheres are diluted with air to varying extents. Carbon dioxide and oxygen concentrations in master pack atmospheres are then often much below the concentrations optimal for preservation of the product. However, irrespective of the gas atmosphere, master packs provide mechanical protection for filled trays during their distribution from central cutting facilities to retail outlets. Various types of equipment have been developed for preparing different forms of lidded trays that each contain a modified atmosphere. The atmosphere used for such trays is typically 60% oxygen, 30% carbon dioxide and 10% nitrogen. Storage/display lives of up to two weeks are often claimed for product in such trays. However, to attain such useful life, temperature during display as well as during storage must be well controlled, and the volume of the pack atmosphere must be large in relation to the amount of product in the pack. Control of product temperatures during display is often uncertain (B?gh- S?rensen and Olsson, 1990), and many retailers consider that small quantities of product in large packs are unattractive to consumers. Therefore, retailers often select modified atmosphere packs to provide an attractive packing in which a high concentration of oxygen, and thus an enhanced meat colour, are maintained for a limited time. Adequate display stability for the product is obtained by control of product temperatures near 1.5oC and by frequent, often daily delivery of freshly packaged product to retail outlets (Gill et al., 2002a). The success of many current distribution systems for master packed product is achieved similarly. The use of carbon monoxide in modified atmosphere is not permitted in most countries, because of the highly poisonous nature of that gas. Despite that, the risks to consumers from the presence of small amounts of carboxymyoglobin in Active packaging in practice: meat 375 raw meat appear to be small, and carbon monoxide is a common component of the modified atmospheres used with raw meats in Norway (S?rheim et al., 1997). As carboxymyoglobin confers a red colour on meat irrespective of the presence of oxygen, a modified atmosphere with carbon monoxide need contain no oxygen. The input gas then typically contains 60% carbon dioxide and 40% nitrogen, with carbon monoxide at 0.3 to 0.5%. The major components of the input gas are at concentrations that will give the maximum carbon dioxide concentration after equilibration without the risk of pack collapse. Thus, the carbon dioxide concentration can be maintained at levels above that required for maximum inhibition of aerobic spoilage organisms for relatively long times, without resort to volumes of atmosphere much greater than the volumes of product. Therefore carbon monoxide/high carbon dioxide atmospheres stabilise meat colour and delay microbial spoilage, and so preserve the product in an acceptable condition even when delivery is relatively infrequent and display is prolonged. 17.7 Controlled atmosphere packaging for meat products The only types of controlled atmosphere packagings currently used with raw meats are those in which an anaerobic atmosphere is maintained indefinitely. Controlled atmosphere packagings may be used for bulk product or items of irregular shape, such as whole lamb carcasses, or as master packs for retail-ready product. Controlled atmosphere packaging is not suitable for individual trays of retail-ready product because of the undesirable colour of anoxic meat, and because packaging materials that are impermeable to gases are mostly opaque. Readily available films that are essentially gas impermeable are laminates that incorporate a layer of aluminum foil, laminates with two layers of a metallised film, or laminates with unusually thick layers of plastics with high barrier properties (Kelly, 1989). Controlled atmospheres may be of carbon dioxide or nitrogen, or mixtures of the two gases. Nitrogen can provide an anaerobic atmosphere, but does not otherwise affect the muscle tissue or the microflora. Thus, the storage life of meats in a controlled atmosphere of nitrogen is similar to that of meats in vacuum pack; although in a gas impermeable, controlled atmosphere pack there is no oxidation of myoglobin in exudate or muscle tissue, which eventually become evident with meat in vacuum packs as the result of small quantities of oxygen permeating the packaging films (Jeremiah et al., 1992). Atmospheres of carbon dioxide have inhibitory effects on some organisms of the anaerobic spoilage flora, and can apparently retard the excessive tenderising of at least lamb. The inhibiting effects of carbon dioxide on the microflora appear to reduce rapidly with reducing concentrations of carbon dioxide in the atmosphere, so an atmosphere of or near 100% carbon dioxide is required if the storage stability of the product is to be substantially increased over that attainable with a nitrogen atmosphere (Gill and Penney, 1988). When an 376 Novel food packaging techniques atmosphere rich in carbon dioxide is used, the high solubility of the gas in meat tissues must be taken into account. In an atmosphere of 100% carbon dioxide, meat will absorb approximately its own volume of the gas. Thus, the initial gas volume must exceed the required final volume by the volume of the enclosed meat. Specialised equipment for forming controlled atmosphere packs is available. With such equipment, snorkels are inserted in the mouth of a filled bag, and the mouth is closed around them; then a hood is placed over the bag, with enclosure of the snorkels and bag sealing elements of the equipment. Air is withdrawn from the bag through the snorkels while the hood is simultaneously evacuated. The bag inflates in the evacuated hood, which ensures that no part of the bag collapses to entrap air. Some air is then admitted into the hood to give a low pressure which will collapse that bag around the product without crushing it. Thus, the volume of the bag is minimised before it is filled with gas. A pack may be flushed with the input gas one or more times before it is sealed. That relatively elaborate filling procedure is adopted to minimise the amount of residual oxygen in the pack. Even so, residual oxygen concentrations after pack sealing are usually about 100ppm (Penney and Bell, 1993). Snorkel equipment without a hood and even tray gassing equipment have been used, at least experimentally, for the production of controlled atmosphere packs. The residual oxygen in such packs is apparently often about 1%, which can have grossly adverse affects upon the colour of red meats. Even 100 ppm of oxygen can result in discolouration of product. However, in those latter circumstances discolouration is usually transient, as the metmyoglobin is reduced to myoglobin, usually within four days, as anoxic conditions are established and maintained (Gill and Jones, 1994a). Various studies have been conducted to determine if oxygen scavengers might be used to prevent permanent discolouration of red meats in atmospheres with initial concentration about 1%, or transient discolouration of meats in atmospheres with very low concentrations of residual oxygen. Although some success with the atmospheres of the former type have been reported (Doherty and Allen, 1998), the general utility of such an approach must be doubted because the muscle tissue itself acts as a very efficient oxygen scavenger (Table 17.2). Certainly, findings with the use of oxygen scavengers in atmospheres of very low initial oxygen concentration have been that numerous, fast reacting oxygen scaverages must be employed if transient browning is to be prevented (Tewari et al., 2002). 17.8 Future trends in active packagings for raw meats In most developed countries, sales of raw meat at supermarkets have tended to increase at the expense of sales at specialised butchers’ stores (Mannion, 1995). The maintenance of butchering facilities at supermarkets is increasingly seen as undesirable, both because of the use of costly floor space that might otherwise Active packaging in practice: meat 377 be used for selling foods, and because of difficulties with obtaining staff skilled in butchery. Therefore, supermarket operators have been for some years generally inclined to move toward the preparation of display ready product at central butchering facilities (Lazar, 2001). Although modified atmosphere packagings of various types have been used with mixed results in central cutting operations, most successful operations now rely on the frequent preparation of retail packs, with frequent and speedy delivery of product held at temperatures near the optimum for chilled meat, rather than the preservative capabilities of modified atmospheres. Simultaneously there has been a trend towards consolidation of slaughtering facilities so that in some regions, such as North America, most animals are now slaughtered at relatively few large plants. Given the move towards central preparation of retail ready meat, it would seem economically advantageous to prepare retail packs at slaughtering plants. That would avoid the double handling, and double packaging of product that now occurs with the consignment of vacuum packaged primal cuts or bulk packed product from slaughtering plants to central cutting facilities. The retail packaging of product of compromised colour stability, and loss of product weight as exudate after prolonged storage in vacuum pack could also be avoided. Although retail preparation of product at slaughtering plants is increasing, particularly with poultry, the need for frequent and speedy delivery limits the area of distribution. Thus, for the largest plants from which product is widely distributed preparation of retail product is a minor activity at most. A general conflation of slaughtering with preparation of retail-ready product at a few large plants would seem to be practicable only if the useful life of retail- ready product reliably exceeds the storage and display times usual in current commercial practice (Fig. 17.4). When meat is stored and displayed at normal commercial temperatures, such storage stability can be attained by master packaging meat under controlled, anoxic atmospheres, as has been demonstrated with commercial systems for the distribution of lamb in the USA or by use of carbon monoxide in modified atmospheres, as has been demonstrated with commercial systems in Norway. Table 17.2 Half life of oxygen in packs containing 4L of atmospheres with < 1% oxygen when packs contained either four trays of ground beef or 32 oxygen scavengers each with a capacity of 200 mlg oxygen (Gill and McGinnis, 1995) Temperature O 2 half life (h) (C) With meat With O 2 scavengers 1.5 4.7 0.6 0 3.8 0.6 2 2.9 0.5 5 1.4 0.5 10 1.6 0.5 378 Novel food packaging techniques Despite the commercial advantages of, and the trivial risks associated with, the use of carbon monoxide, it is unlikely that many countries will sanction meat being treated with a recognised poison. A trend towards increasing use of controlled atmosphere packaging for retail ready product might then be anticipated. However, the complexities of meat trading are likely to ensure that such a trend develops only slowly. Although controlled atmosphere packaging could be used for continental distribution of retail-ready meat, it is unlikely to be used for global distribution of such product. Storage life would not necessarily constrain global distribution but the low packing density of retail packed product as compared with bulk product could render shipment of meat by sea uneconomical. Thus, controlled atmosphere packing is unlikely to replace vacuum packing in trading of chilled meats to distant markets, unless there is a move to retail portioning but not retail packaging at exporting plants. 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