《肉制品冷冻技术》（英文版） Part 1 Refrigeration and meat quality

Part 1 Refrigeration and meat quality

Part 1 Refrigeration and meat quality

Microbiology of refrigerated meat There are many pertinent texts on the microbiology of meats. The purpose of this chapter is to examine briefly the types of micro-organisms and con- ditions that are of interest in relation to the refrigeration of meat and meat products. In a perfect world, meat would be completely free of pathogenic(food poisoning micro-organisms when produced. However, under normal methods the production of pathogen-free meat cannot be guaranteed. The internal musculature of a healthy animal is essentially sterile after slaughter (Gill, 1979, 1980). However, all meat animals carry large numbers of differ ent micro-organisms on the outer surfaces of the body and in the alimentary tract. Only a few types of bacteria directly affect the safety and quality of the finished carcass. Of particular concern are foodborne pathogens such Campylobacter spp, Clostridium perfringens, pathoge Escherichia coli, Salmonella spp, and Yersinia enterocolitica In general, the presence of small numbers of pathogens is not a problem because meat is normally cooked before consumption. Adequate cooking will substantially reduce the numbers, if not completely eliminate all of the pathogenic organisms present on the meat. Most meat-based food poison- ing is associated with inadequate cooking or subsequent contamination after cooking. The purpose of refrigeration is to reduce or eliminate the growth of pathogens so that they do not reach levels that could cause problems Normally the growths of spoilage organisms limit the shelf-life of meat The spoilage bacteria of meats stored in air under chill conditions include species of Pseudomonas, Brochothrix and Acinetobacter/Moraxella. In general, there is little difference in the microbial spoilage of beef, lamb, pork nd other meat derived from mammals(Varnam and Sutherland, 1995)

1 Microbiology of refrigerated meat There are many pertinent texts on the microbiology of meats. The purpose of this chapter is to examine briefly the types of micro-organisms and con￾ditions that are of interest in relation to the refrigeration of meat and meat products. In a perfect world, meat would be completely free of pathogenic (food poisoning) micro-organisms when produced. However, under normal methods the production of pathogen-free meat cannot be guaranteed. The internal musculature of a healthy animal is essentially sterile after slaughter (Gill, 1979, 1980). However, all meat animals carry large numbers of differ￾ent micro-organisms on the outer surfaces of the body and in the alimentary tract. Only a few types of bacteria directly affect the safety and quality of the finished carcass. Of particular concern are foodborne pathogens such as Campylobacter spp., Clostridium perfringens, pathogenic serotypes of Escherichia coli, Salmonella spp., and Yersinia enterocolitica. In general, the presence of small numbers of pathogens is not a problem because meat is normally cooked before consumption. Adequate cooking will substantially reduce the numbers, if not completely eliminate all of the pathogenic organisms present on the meat. Most meat-based food poison￾ing is associated with inadequate cooking or subsequent contamination after cooking.The purpose of refrigeration is to reduce or eliminate the growth of pathogens so that they do not reach levels that could cause problems. Normally the growths of spoilage organisms limit the shelf-life of meat. The spoilage bacteria of meats stored in air under chill conditions include species of Pseudomonas, Brochothrix and Acinetobacter/Moraxella. In general, there is little difference in the microbial spoilage of beef, lamb, pork and other meat derived from mammals (Varnam and Sutherland, 1995)

4 Meat refrigeration Meat is considered spoiled by bacteria when the products of their meta- bolic activities make the food offensive to the senses of the consumer(Gill 1983). Therefore, the perception of a state of spoilage is essentially a sub- jective evaluation that will vary with consumer expectations. Few, however, would not acknowledge that the appearance of slime, gross discoloration and strong odours constitute spoilage Off odours are due to an accumulation of malodorous metabolic prod ucts, such as esters and thiols. Several estimations have been made of the number of bacteria on meat at the point at which odour or slime becomes evident and the mean is about 3 cm(Shaw, 1972). When active growth occurs, the number of bacteria increases exponentially with time. Therefore, a convenient measure of the growth rate is the time required for doubling of numbers, often called the generation time. If this, for example, were one hour, the number would increase two-fold in 1h, four-fold in 2h, eight-fold in 3h. and so on. The bacterial safety and rate of spoilage depends upon the numbers and types of micro-organisms initially present, the rate of growth of those micro- organisms, the conditions of storage(temperature and gaseous atmosphere) and characteristics(pH, water activity aw )of the meat. Of these factors, tem- perature is by far the most important 1.1 Factors affecting the refrigerated shelf-life of meat 1.1.1 Initial microbial levels l.. Tissue sterility or many years microbiologists believed that the tissues of healthy animals normally contained bacteria(Reith, 1926; Ingram, 1972). These intrinsic bacteria were the cause of phenomena such as ' bone taint. The cause of bone taint is still questioned and will be discussed later. The prevailing view of the majority of textbooks(Banwart, 1989: Varnam and Sutherland, 1995) based in part on the work of Gill(Gill, 1979, 1980)is that the meat of a healthy animal is essentially sterile. Low numbers of specific micro- organisms, which have reached the tissues during the life of the animal, may occur in the viscera and associated lymph nodes from time to time(Gill, 1979: Roberts and Mead, 1986). These are often pathogenic species, such as Salmonella, and clostridia spores. The absence of bacteria appears to be due to the continued functioning of the immune system in slaughtered animals. Experiments with guinea pigs showed that the antibacterial defences of live animals persisted for an hour or more after death and could inactivate bacteria introduced during slaughter (Gill and Penney, 1979). Clearly, if bacteria are thus inactivated there can be no multiplication, in deep tissue, during carcass chilling irrespective of cooling rates

Meat is considered spoiled by bacteria when the products of their meta￾bolic activities make the food offensive to the senses of the consumer (Gill, 1983). Therefore, the perception of a state of spoilage is essentially a sub￾jective evaluation that will vary with consumer expectations. Few, however, would not acknowledge that the appearance of slime, gross discoloration and strong odours constitute spoilage. ‘Off’ odours are due to an accumulation of malodorous metabolic prod￾ucts, such as esters and thiols. Several estimations have been made of the number of bacteria on meat at the point at which odour or slime becomes evident and the mean is about 3 ¥ 107 cm-2 (Shaw, 1972).When active growth occurs, the number of bacteria increases exponentially with time.Therefore, a convenient measure of the growth rate is the time required for doubling of numbers, often called the generation time. If this, for example, were one hour, the number would increase two-fold in 1 h, four-fold in 2 h, eight-fold in 3 h, and so on. The bacterial safety and rate of spoilage depends upon the numbers and types of micro-organisms initially present, the rate of growth of those micro￾organisms, the conditions of storage (temperature and gaseous atmosphere) and characteristics (pH, water activity aw) of the meat. Of these factors, tem￾perature is by far the most important. 1.1 Factors affecting the refrigerated shelf-life of meat 1.1.1 Initial microbial levels 1.1.1.1 Tissue sterility For many years microbiologists believed that the tissues of healthy animals normally contained bacteria (Reith, 1926; Ingram, 1972). These ‘intrinsic’ bacteria were the cause of phenomena such as ‘bone taint’. The cause of bone taint is still questioned and will be discussed later.The prevailing view of the majority of textbooks (Banwart, 1989;Varnam and Sutherland, 1995), based in part on the work of Gill (Gill, 1979, 1980) is that the meat of a healthy animal is essentially sterile. Low numbers of specific micro￾organisms, which have reached the tissues during the life of the animal, may occur in the viscera and associated lymph nodes from time to time (Gill, 1979; Roberts and Mead, 1986). These are often pathogenic species, such as Salmonella, and clostridia spores.The absence of bacteria appears to be due to the continued functioning of the immune system in slaughtered animals. Experiments with guinea pigs showed that the antibacterial defences of live animals persisted for an hour or more after death and could inactivate bacteria introduced during slaughter (Gill and Penney, 1979). Clearly, if bacteria are thus inactivated there can be no multiplication, in deep tissue, during carcass chilling irrespective of cooling rates. 4 Meat refrigeration

Microbiology of refrigerated meat 5 1. 1.1.2 Rigor mortis The way in which animals are handled before slaughter will effect the bio- chemical processes that occur before and during rigor mortis. The resulting metabolites influence the growth of micro-organisms on meat. During the onset of rigor mortis, which may take up to 24h, oxygen stored in the muscle is depleted and the redox potential falls from above +250mv to -150mV Such a low redox value combined with the initial muscle temperature of 38C provides ideal growth conditions for meso philic micro-organisms. Stress and excitement caused to the animal before slaughter will cause the redox potential to fall rapidly, possibly allowin proliferation of such micro-organisms before cooling(Dainty, 1971) Concurrent with the fall in redox potential is a fall in pH from an initial value in life of around 7 to a stable value around 5.5, theultimate ph. This is due to the breakdown of glycogen, a polysaccharide, to lactic acid in the muscle tissue. Lactic acid cannot be removed by the circulation nor oxi- dised, so it accumulates and the pH falls until the glycogen is all used or the breakdown stops. The pH has an important role in the growth of micro- organisms, the nearer the pH is to the ultimate value, the more growth is inhibited(Dainty, 1971). Stress or exercise before slaughter can deplete an animals glycogen reserves, consequently producing meat with less lactic acid and a relatively high ultimate pH, this gives the meat a dark, firm, dry (DFD)appearance. Alternative terms are ' dark cuttingand 'high-pH meat. The condition occurs in pork, beef and mutton, but is of little eco- nomic importance in the latter(Newton and Gill, 1981). DFD meat pro vides conditions that are more favourable for microbial growth than in normal meat. The microbiology of DFD meat has been comprehensively reviewed by Newton and Gill (1981) Glucose is the preferred substrate for growth of pseudomonads, the dominant bacteria in meat stored in air at refrigerated temperatures. Only when glucose is exhausted do they break down amino acids, producing the ammonia and sulphur compounds that are detectable as spoilage odours and flavours In meat containing no glucose, as is the case with some DFD meat,amino acids are broken down immediately and spoilage becomes evident at cell densities of 6loglo cfu cm(colony forming units per cen timetre squared). This is lower than in normal meat, where spoilage becomes apparent when numbers reach ca. 8logocfucm-. Thus, given the same storage conditions, DFD meat spoils more rapidly than normal-pH meat. There is no evidence that the spoilage of pale, soft, exuding(Pse meat is any different to that of normal meat(Gill, 1982). There is little sig nificant difference in pH or chemical composition between PSE and normal 1.1.1.3 Surface contamination Initial numbers of spoilage bacteria on carcasses significantly affect shelf life. With higher numbers, fewer doublings are required to reach a spoilage

1.1.1.2 Rigor mortis The way in which animals are handled before slaughter will effect the bio￾chemical processes that occur before and during rigor mortis. The resulting metabolites influence the growth of micro-organisms on meat. During the onset of rigor mortis, which may take up to 24 h, oxygen stored in the muscle is depleted and the redox potential falls from above +250 mV to -150 mV. Such a low redox value combined with the initial muscle temperature of 38 °C provides ideal growth conditions for meso￾philic micro-organisms. Stress and excitement caused to the animal before slaughter will cause the redox potential to fall rapidly, possibly allowing proliferation of such micro-organisms before cooling (Dainty, 1971). Concurrent with the fall in redox potential is a fall in pH from an initial value in life of around 7 to a stable value around 5.5, the ‘ultimate pH’. This is due to the breakdown of glycogen, a polysaccharide, to lactic acid in the muscle tissue. Lactic acid cannot be removed by the circulation nor oxi￾dised, so it accumulates and the pH falls until the glycogen is all used or the breakdown stops. The pH has an important role in the growth of micro￾organisms, the nearer the pH is to the ultimate value, the more growth is inhibited (Dainty, 1971). Stress or exercise before slaughter can deplete an animal’s glycogen reserves, consequently producing meat with less lactic acid and a relatively high ultimate pH, this gives the meat a dark, firm, dry (DFD) appearance. Alternative terms are ‘dark cutting’ and ‘high-pH meat’. The condition occurs in pork, beef and mutton, but is of little eco￾nomic importance in the latter (Newton and Gill, 1981). DFD meat pro￾vides conditions that are more favourable for microbial growth than in normal meat. The microbiology of DFD meat has been comprehensively reviewed by Newton and Gill (1981). Glucose is the preferred substrate for growth of pseudomonads, the dominant bacteria in meat stored in air at refrigerated temperatures. Only when glucose is exhausted do they break down amino acids, producing the ammonia and sulphur compounds that are detectable as spoilage odours and flavours. In meat containing no glucose, as is the case with some DFD meat, amino acids are broken down immediately and spoilage becomes evident at cell densities of 6 log10 cfucm-2 (colony forming units per cen￾timetre squared). This is lower than in normal meat, where spoilage becomes apparent when numbers reach ca. 8 log10 cfu cm-2 . Thus, given the same storage conditions, DFD meat spoils more rapidly than normal-pH meat. There is no evidence that the spoilage of pale, soft, exuding (PSE) meat is any different to that of normal meat (Gill, 1982). There is little sig￾nificant difference in pH or chemical composition between PSE and normal meat. 1.1.1.3 Surface contamination Initial numbers of spoilage bacteria on carcasses significantly affect shelf￾life. With higher numbers, fewer doublings are required to reach a spoilage Microbiology of refrigerated meat 5

6 Meat refrigeration level of ca. 10 organisms cm-. For example, starting with one organism cm, 27 doublings would be needed; for 10 organisms cm-2 initially, the number of doublings is reduced to 17 Contamination of carcasses may occur at virtually every stage of slaugh- tering and processing, particularly during flaying and evisceration of red- meat animals and scalding, and mainly affects the surface of the carcass. Sources of contamination have been reviewed by James et aL.(1999) Hygienic handling practices should ensure that total viable counts on the finished carcass are consistently 105-10 organisms cm or lower for red meats. Bad practices can cause counts to exceed 10" organisms cm with red meats, carcasses of good microbial quality are obtained by 1 preventing contamination from the hide 2 avoiding gut breakage 3 the adoption of good production practices that include more humane practices throughout the slaughtering system. The effectiveness of chemical and physical decontamination systems for meat carcasses has been reviewed by James and James, (1997) and James et al. (1997). Commercial systems using steam have been introduced into the usa and are claimed to reduce the number of bacteria on the surface of beef carcasses to below 1 loglo"(Phebus et al., 1997) 1.1.2 Temperature Micro-organisms are broadly classified into three arbitrary groups(psy chrophiles, mesophiles and thermophiles)according to the range of tem- peratures within which they may grow. Each group is characterised by three values: the minimum, optimum and maximum temperatures of growth Reduction in temperature below the optimum causes an increase in genera- tion time, i.e. the time required for a doubling in number. It is an accepted crude approximation that bacterial growth rates can be expected to double with every 10C rise in temperature(Gill, 1986). Below 10C, however, this effect is more pronounced and chilled storage life is halved for each 2-3C rise in temperature. Thus the generation time for a pseudomonad(a common form of spoilage bacteria)might be lh at 20C, 2.5h at 10C, 5h at 5C, 8h at 2C or 1lh at 0C. In the usual temperature range for chilled meat,-1.5-+5C, there can be as much as an eight-fold increase in growth rate between the lower and upper temperature Storage of chilled meat at 1.5+0.5C would attain the maximum storage life without any surface freezing Meat stored above its freezing point, ca. -2C, will inevitably be spoiled by bacteria. Obviously, the nearer the storage temperature of meat approaches the optimum for bacterial growth(20-40'C for most bacteria) the more rapidly the meat will spoil Work of Ayres(1960)compared the rate of increase in bacterial number on sliced beef stored at 0.5.10. 15.20 and

level of ca. 108 organisms cm-2 . For example, starting with one organism cm-2 , 27 doublings would be needed; for 103 organisms cm-2 initially, the number of doublings is reduced to 17. Contamination of carcasses may occur at virtually every stage of slaugh￾tering and processing, particularly during flaying and evisceration of red￾meat animals and scalding, and mainly affects the surface of the carcass. Sources of contamination have been reviewed by James et al. (1999). Hygienic handling practices should ensure that total viable counts on the finished carcass are consistently 103 –104 organisms cm-2 or lower for red meats. Bad practices can cause counts to exceed 106 organisms cm-2 . With red meats, carcasses of good microbial quality are obtained by 1 preventing contamination from the hide; 2 avoiding gut breakage; 3 the adoption of good production practices that include more humane practices throughout the slaughtering system. The effectiveness of chemical and physical decontamination systems for meat carcasses has been reviewed by James and James, (1997) and James et al. (1997). Commercial systems using steam have been introduced into the USA and are claimed to reduce the number of bacteria on the surface of beef carcasses to below 1 log10 cfu cm-2 (Phebus et al., 1997). 1.1.2 Temperature Micro-organisms are broadly classified into three arbitrary groups (psy￾chrophiles, mesophiles and thermophiles) according to the range of tem￾peratures within which they may grow. Each group is characterised by three values: the minimum, optimum and maximum temperatures of growth. Reduction in temperature below the optimum causes an increase in genera￾tion time, i.e. the time required for a doubling in number. It is an accepted crude approximation that bacterial growth rates can be expected to double with every 10 °C rise in temperature (Gill, 1986). Below 10°C, however, this effect is more pronounced and chilled storage life is halved for each 2–3°C rise in temperature. Thus the generation time for a pseudomonad (a common form of spoilage bacteria) might be 1h at 20°C, 2.5 h at 10°C, 5 h at 5 °C, 8 h at 2 °C or 11 h at 0 °C. In the usual temperature range for chilled meat, -1.5–+5 °C, there can be as much as an eight-fold increase in growth rate between the lower and upper temperature. Storage of chilled meat at -1.5 ± 0.5 °C would attain the maximum storage life without any surface freezing. Meat stored above its freezing point, ca. -2 °C, will inevitably be spoiled by bacteria. Obviously, the nearer the storage temperature of meat approaches the optimum for bacterial growth (20–40 °C for most bacteria) the more rapidly the meat will spoil.Work of Ayres (1960) compared the rate of increase in bacterial number on sliced beef stored at 0, 5, 10, 15, 20 and 6 Meat refrigeration