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
Active packaging in practice: meat 369 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 all the exudate that may be release ads or wraps of sufficient capacity to hold the inclusion in packs of absorbent pac Unlike most changes that occur in meat with age, increased tenderness 1 generally desirable (Jeremiah et al., 1993). The rate of tenderisation declines approximately exponentially with time of storage For beef stored at 2C,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
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
370 Novel food packaging techniques in the appear 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 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 /cm2. 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 about10°m 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 nd 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%/cm 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
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 byproducts 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 108 /cm2 . 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 106 /cm2 . 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 108 /cm2 without spoilage of the meat. Thereafter, spoilage will develop only slowly as the byproducts 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
Active packaging in practice: meat 371 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 ( Sorheim 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 + 0.5C(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 ooC. 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 see kely that discolouration with increasing temperature accelerates much as for meat stored in air When meat is stored anaerobically, the colour stability of muscle tissue ncreases 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 5C and four times as fast at 10C 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 orted to be about 30%and 100% more, respectively, at 5C and 10C than at 0C(OKeefe and Hood, 1980-81). The rate at which muscle tenderise is over twice as fast at 10C as at OC (Dransfield, 1994) Spoilage bacteria will grow on meat that is not frozen at temperatures down to -3C under both aerobic and anaerobic conditions. Thus, storage at chiller temperatures can delay but not prevent the ultimate onset of microbial spoilage
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 (Sørheim 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 ÿ1.5 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 ÿ3ºC 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
372 Novel food packaging techniques l00 ig. 17.2 Effects of storage temperature on the storage life of chilled meat limited by 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 5.C is about 70, 50 and 30%, respectively, of the storage life that would be obtained for the product stored at -15C(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
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 ÿ1.5ºC (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
Active packaging in practice: meat 373 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 prov to the exchange of gases between the pack and the ambient atmosphere lowever, the gas barrier properties of the packaging materials differ for H 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/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 O2/m/24h/atm, while packagings designed to contain product for the longest possible times are likel to be composed of materials with oxygen transmission rates less than 10 cc/m/ 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 ∽3 Temperature(C) ig. 17.3 Effects of ten scle tissue o and fat时mmn
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/m2 /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 O2/m2 /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/m2 / 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 (❍) and fat tissue (•) of beef. Active packaging in practice: meat 373