Part lI Novel packaging and particular products
Part III Novel packaging and particular products
7 Active packaging in practice: meat C.O. Gill, Agriculture and Agri-Food Canada 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
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
366 Novel food packaging techniques MYOGLOBIN high O, Oxymyoglobin (bright red) MRA (cherry red (brown) Fig. 17.1 Reactions of myoglobin with oxygen and carbon monoxide. 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 nuscle 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 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 Although metmyoglobin is stable, it is slowly reduced to deoxymyoglobin by enzymic reactions involving reduced co-enzymes(Echevarne et al., 1990) high metmyoglobin reduction activity can generally maintain a bright red cao Those reactions are termed metmyoglobin reduction activity. Muscle tissue wit 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
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
Active packaging in practice: meat 367 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 stabil edward, 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 muscle and bone tissues can be maintained by exposing the tissues to carbon
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 deoxythan 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
368 Novel food packaging techniques Table 17.1 Fractions of metmyoglobin in the muscle pigment of beef steak surfaces after display in air for I h, following storage at-1.5%C under N2, CO2 or, 67%O2+33% CO,(Gill and Jones, 1994a) Met (days) O,+ co N75308007 42606600 47039 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 oultry 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 uscle 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
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 ÿ1.5ºC under N2, CO2 or, 67% O2+33% CO2 (Gill and Jones, 1994a) Storage Metmyoglobin (%) time Storage atmosphere (days) N2 CO2 O2 + CO2 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