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