《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

Microbiology of refrigerated meat 7 5C. The meat developed an off odour by the third day at 20C, the tenth day at 5C and the 20th day at 0C. Similar data has been reported by other workers. They clearly demonstrate the effectiveness of refrigeration in reduc ing the rate of increase in bacterial numbers and extending shelf-life As bacteria generally grow more rapidly than fungi, mould spoilage of meat is thought to develop only when competing bacteria are inhibited. Temperature is usually assumed to be the critical factor, mould spoilage being typically associated with frozen meat. It has been generally accepted that moulds can develop on meat at temperatures as low as -10 or-12C. There is some evidence that this is an exaggeration and that for practica purposes the minimum temperature for mould growth on meat should be taken to be ca. -5C(Lowry and Gill, 1984). It is further thought that surface desiccation, rather that temperature, is the factor that inhibits bac terial growth. If this is the case then mould growth on frozen meats is indica tive of particularly poor temperature control. Many factors influence the growth and survival of micro-organisms in meat during freezing and frozen storage. However, the main factor affect ing the growth of micro-organisms during freezing is the availability of water Until the temperature is reduced below the minimum temperature for growth, some micro-organisms have the potential to multiply. While most of the water in meat is turned to ice during freezing, there is always some free liquid water available, 26% at-5C, 18% at -10C, 14% at -18C, 10% at-40C (Rosset, 1982). The transformation of water into ice significantly modifies the growth environment for micro-organisms, since solutes become concentrated in the remaining free water to the level that microbial growth is inhibited. Below the freezing point of the meat, the water activity is progressively reduced preventing microbial growth(Fig. 1.0 0.9 .8 0.7 Fig. 1.1 Water activities(aw)of meat at various sub-freezing temperatures(source Leistner and Rodel, 1976)

25 °C. The meat developed an off odour by the third day at 20 °C, the tenth day at 5 °C and the 20th day at 0 °C. Similar data has been reported by other workers.They clearly demonstrate the effectiveness of refrigeration in reduc￾ing the rate of increase in bacterial numbers and extending shelf-life. As bacteria generally grow more rapidly than fungi, mould spoilage of meat is thought to develop only when competing bacteria are inhibited. Temperature is usually assumed to be the critical factor, mould spoilage being typically associated with frozen meat. It has been generally accepted that moulds can develop on meat at temperatures as low as -10 or -12 °C. There is some evidence that this is an exaggeration and that for practical purposes the minimum temperature for mould growth on meat should be taken to be ca. -5 °C (Lowry and Gill, 1984). It is further thought that surface desiccation, rather that temperature, is the factor that inhibits bac￾terial growth. If this is the case then mould growth on frozen meats is indica￾tive of particularly poor temperature control. Many factors influence the growth and survival of micro-organisms in meat during freezing and frozen storage. However, the main factor affect￾ing the growth of micro-organisms during freezing is the availability of water. Until the temperature is reduced below the minimum temperature for growth, some micro-organisms have the potential to multiply. While most of the water in meat is turned to ice during freezing, there is always some free liquid water available, 26% at -5 °C, 18% at -10 °C, 14% at -18 °C, 10% at -40 °C (Rosset, 1982). The transformation of water into ice significantly modifies the growth environment for micro-organisms, since solutes become concentrated in the remaining free water to the level that microbial growth is inhibited. Below the freezing point of the meat, the water activity is progressively reduced preventing microbial growth (Fig. Microbiology of refrigerated meat 7 –30 –20 –10 0 Temperature (°C) 0.7 0.8 0.9 1.0 Water activity ( aw) Fig. 1.1 Water activities (aw) of meat at various sub-freezing temperatures (source: Leistner and Rödel, 1976)

8 Meat refrigeration Table 1.1 Minimum and optimum growth temperatures for pathogens associated with red meats Optimun (°C) C Clostridia perfringens Pathogenic escherichia coli strains Salmonella spp Listeria monocytogenes 027502 43-47 35-40 30-37 Yersinia enterocolitica Source: Mead and Hinton. 1996. 1. 1). The greatest reduction in the microbial load occurs during, or shortly after, freezing itself. During frozen storage, the numbers are gradually reduced further 1.2.1 Pathogenic organisms A number of bacterial pathogens capable of causing food poisoning in humans are known to contaminate red meat. Those of most importance are Campylobacter spp, Clostridium perfringenS, pathogenic serotypes of Escherichia coli (principally E coli o157: H7), Salmonella spp and Yersinia enterocolitica(Nottingham, 1982; Anon, 1993; Mead and Hinton, 1996).Lis- teria monocytogenes is commonly associated with meat, but its public health significance in relation to raw meat is unclear (Mead and Hinton, 1996) The essential characteristics of pathogenic micro-organisms can be found In numerous texts Minimum and optimum growth temperatures for pathogens commonly associated with red meat are show in Table 1.1. Some pathogens, such as L. monocytogens nes,are capable of growth at chill temperatures below 5C. These are often cited as being of particular concern in relation to refriger ated meats since refrigeration can not be relied on to prevent growth (Doyle, 1987). On the other hand, psychrotrophic pathogens are not par ticularly heat resistant and adequate cooking should be sufficient to destroy any such pathogens. Illnesses caused by L. monocytogenes and E coli are often due to inadequate cooking before ingestion 1.2.2 Spoilage organisms The number of types of micro-organisms capable of causing food spoilage is very large and it is not possible to discuss them in any detail in this text Depending on the initial microflora and the growth environment, only a few species of the genera Pseudomonas, Acinetobacter, Moraxella Lactobacillus, Brochothrix and Alteromonas, and of the family

1.1). The greatest reduction in the microbial load occurs during, or shortly after, freezing itself. During frozen storage, the numbers are gradually reduced further. 1.1.2.1 Pathogenic organisms A number of bacterial pathogens capable of causing food poisoning in humans are known to contaminate red meat. Those of most importance are Campylobacter spp., Clostridium perfringens, pathogenic serotypes of Escherichia coli (principally E. coli O157:H7), Salmonella spp. and Yersinia enterocolitica (Nottingham, 1982; Anon, 1993; Mead and Hinton, 1996). Lis￾teria monocytogenes is commonly associated with meat, but its public health significance in relation to raw meat is unclear (Mead and Hinton, 1996). The essential characteristics of pathogenic micro-organisms can be found in numerous texts. Minimum and optimum growth temperatures for pathogens commonly associated with red meat are show in Table 1.1. Some pathogens, such as L. monocytogenes, are capable of growth at chill temperatures below 5 °C. These are often cited as being of particular concern in relation to refriger￾ated meats since refrigeration can not be relied on to prevent growth (Doyle, 1987). On the other hand, psychrotrophic pathogens are not par￾ticularly heat resistant and adequate cooking should be sufficient to destroy any such pathogens. Illnesses caused by L. monocytogenes and E. coli are often due to inadequate cooking before ingestion. 1.1.2.2 Spoilage organisms The number of types of micro-organisms capable of causing food spoilage is very large and it is not possible to discuss them in any detail in this text. Depending on the initial microflora and the growth environment, only a few species of the genera Pseudomonas, Acinetobacter, Moraxella, Lactobacillus, Brochothrix and Alteromonas, and of the family 8 Meat refrigeration Table 1.1 Minimum and optimum growth temperatures for pathogens associated with red meats Minimum temperature Optimum temperature (°C) (°C) Campylobacter spp. 30 42–43 Clostridia perfringens 12 43–47 Pathogenic Escherichia 7 35–40 coli strains Salmonella spp. 5 35–43 Listeria monocytogenes 0 30–37 Yersinia enterocolitica -2 28–29 Source: Mead and Hinton, 1996

Microbiology of refrigerated meat 9 Enterobacteriaceae are significantly represented in most microflora of chilled meats(Bell and Gill, 1986) The micro-organisms that usually spoil meat are psychrotrophs, i.e. they are capable of growth close to 0C. Only a small proportion of the initial microflora on meat will be psychrotrophs: the majority of micro-organisms present are incapable of growth at low temperatures As storage tempera- ture rises the number of species capable of growth will increase. 1. 1.2.2.1 Spoilage of chilled meat The spoilage of chilled meat stored in air is dominated by Gram-negative, psychrotrophic, aerobic rod-shaped bacteria. Although a wide range of genera are present on meat, only Pseudomonas, Acinetobacter and Psychrobacter species are normally of importance(Dainty and Mackey, 1992).Of these, species of Pseudomonas are of greatest importance (Gill, 1986).Pseudomonas spp. typically account for >50% of the flora and some- times up to 90%(Dainty and Mackey, 1992) Other bacteria are present in small numbers and may occasionally form significant part of the microflora. Brochothrix thermosphacta appears to be of more importance on pork and lamb than on beef especially on fat where the pH value is generally higher, and at temperatures above 5'C (Gill, 1983: Varnam and Sutherland, 1995) Species of both Micrococcus and Staphylococcus are present on meat stored in air but their significance is generally considered limited under refrigerated storage. Psychrotrophic members of the Enterobacteriaceae, ncluding Serratia liquefaciens, Enterobacter agglomerans and Hafnia alv are also common at low levels(Dainty and Mackey, 1992).These organisms become of greater importance at temperatures of 6-10oC, but Pseudo monas spp. usually remain dominant(Varnam and Sutherland, 1995) Yeasts and moulds are considered by many to be of limited importance modern practice(Varnam and Sutherland, 1995). Moulds were of historic importance on carcass meat stored for extended periods at temperatures just above freezing. 1. 1.2.2.2 Spoilage of chilled packaged meat Large vacuum packs usually contain ca 1%O2 that in theory will support the growth of pseudomonads(Varnam and Sutherland, 1995). Continuing respiration, however, by the meat rapidly depletes oxygen(O2)and increases the carbon dioxide(CO2)concentration to ca. 20%. Pseudomonas spp are usually unable to grow under such conditions. In general conditions vacuum packs favour lactic acid bacteria (LAB), although there may also be significant growth of Br. thermosphacta, Shewanella putrefaciens'(for- mally Altermonas putrefaciens)and the Enterobacteriaceae. Under anaer- obic conditions LAB have a considerable advantage in growth rate over competing species of facultative anaerobes(Fig. 1. 2). The predominant LAB are homofermentative species of Lactobacillus, Carnobacterium and

Enterobacteriaceae are significantly represented in most spoilage microflora of chilled meats (Bell and Gill, 1986). The micro-organisms that usually spoil meat are psychrotrophs, i.e. they are capable of growth close to 0 °C. Only a small proportion of the initial microflora on meat will be psychrotrophs; the majority of micro-organisms present are incapable of growth at low temperatures. As storage tempera￾ture rises the number of species capable of growth will increase. 1.1.2.2.1 Spoilage of chilled meat The spoilage of chilled meat stored in air is dominated by Gram-negative, psychrotrophic, aerobic rod-shaped bacteria. Although a wide range of genera are present on meat, only Pseudomonas, Acinetobacter and Psychrobacter species are normally of importance (Dainty and Mackey, 1992). Of these, species of Pseudomonas are of greatest importance (Gill, 1986). Pseudomonas spp. typically account for >50% of the flora and some￾times up to 90% (Dainty and Mackey, 1992). Other bacteria are present in small numbers and may occasionally form a significant part of the microflora. Brochothrix thermosphacta appears to be of more importance on pork and lamb than on beef especially on fat where the pH value is generally higher, and at temperatures above 5 °C (Gill, 1983; Varnam and Sutherland, 1995). Species of both Micrococcus and Staphylococcus are present on meat stored in air but their significance is generally considered limited under refrigerated storage. Psychrotrophic members of the Enterobacteriaceae, including Serratia liquefaciens, Enterobacter agglomerans and Hafnia alvei are also common at low levels (Dainty and Mackey, 1992). These organisms become of greater importance at temperatures of 6–10°C, but Pseudo￾monas spp. usually remain dominant (Varnam and Sutherland, 1995). Yeasts and moulds are considered by many to be of limited importance in modern practice (Varnam and Sutherland, 1995). Moulds were of historic importance on carcass meat stored for extended periods at temperatures just above freezing. 1.1.2.2.2 Spoilage of chilled packaged meat Large vacuum packs usually contain ca. 1% O2 that in theory will support the growth of pseudomonads (Varnam and Sutherland, 1995). Continuing respiration, however, by the meat rapidly depletes oxygen (O2) and increases the carbon dioxide (CO2) concentration to ca. 20%.Pseudomonas spp. are usually unable to grow under such conditions. In general conditions vacuum packs favour lactic acid bacteria (LAB), although there may also be significant growth of Br. thermosphacta, ‘Shewanella putrefaciens’ (for￾mally Altermonas putrefaciens) and the Enterobacteriaceae. Under anaer￾obic conditions LAB have a considerable advantage in growth rate over competing species of facultative anaerobes (Fig. 1.2). The predominant LAB are homofermentative species of Lactobacillus, Carnobacterium and Microbiology of refrigerated meat 9

10 Meat refrigeration Br. thermosphacta 口 Lactobacillus Fig. 1. 2 Effect of temperature on the rates of anaerobic growth of bacteria on meat slices(source: Newton and Gill, 1978) Leuconostoc. Lactococcus spp are much less common. LAB are able to grow at low temperatures and low O2 tensions and tolerate COz. Psy chrophilic species of Clostridium have been recognised as a significant potential problem( Varnam and Sutherland, 1995). Both Br. thermosphacta and Sh putrefaciens are favoured by high pH values. Sh. putrefaciens is unable to grow below pH 6.0 during storage at low temperatures, whereas Br. thermosphacta is unable to grow anaerobi cally below pH 5.8(Gill, 1983). At temperatures below 5C, Enterobacte- riaceae are inhibited in vacuum packs by CO2, low pH and lactic acid. At higher temperatures and pH values, CO2 is markedly less inhibitory and growth is possible, in particular by Serratia liquefaciens and Providencia spp arnam and Sutherland, 1995) The predominant type of spoilage in vacuum-packed chilled meat is souring(Sofos, 1994; Varnam and Sutherland, 1995). This is not normally detectable until bacterial numbers reach &logocfucm- or greater. The exact cause of such spoilage is unknown, but is assumed to result from lactic acid and other end-products of fermentation by dominant LAB High-PH (DFD) vacuum-packed meat spoils rapidly and involves the production of rge quantities of hydrogen sulphide(H2s) by Sh putrefaciens and Enter bacteriaceae (Gill, 1982; Varnam and Sutherland, 1995). Characteristic greening'occurs owing to H2s combining with the muscle pigment to give green sulphmyoglobin; the meat also develops putrid spoilage odours (Gill, 1982) &. Packaging in various gaseous atmospheres has been used as an alterna- ve to vacuum packing. The intention has been to preserve the fresh meat colour and to prevent anaerobic spoilage by using high concentrations of

Leuconostoc. Lactococcus spp. are much less common. LAB are able to grow at low temperatures and low O2 tensions and tolerate CO2. Psy￾chrophilic species of Clostridium have been recognised as a significant potential problem (Varnam and Sutherland, 1995). Both Br. thermosphacta and Sh. putrefaciens are favoured by high pH values. Sh. putrefaciens is unable to grow below pH 6.0 during storage at low temperatures, whereas Br. thermosphacta is unable to grow anaerobi￾cally below pH 5.8 (Gill, 1983). At temperatures below 5 °C, Enterobacte￾riaceae are inhibited in vacuum packs by CO2, low pH and lactic acid. At higher temperatures and pH values, CO2 is markedly less inhibitory and growth is possible, in particular by Serratia liquefaciens and Providencia spp. (Varnam and Sutherland, 1995). The predominant type of spoilage in vacuum-packed chilled meat is souring (Sofos, 1994; Varnam and Sutherland, 1995). This is not normally detectable until bacterial numbers reach 8 log10 cfu cm-2 or greater. The exact cause of such spoilage is unknown, but is assumed to result from lactic acid and other end-products of fermentation by dominant LAB. High-pH (DFD) vacuum-packed meat spoils rapidly and involves the production of large quantities of hydrogen sulphide (H2S) by Sh. putrefaciens and Enter￾obacteriaceae (Gill, 1982; Varnam and Sutherland, 1995). Characteristic ‘greening’ occurs owing to H2S combining with the muscle pigment to give green sulphmyoglobin; the meat also develops putrid spoilage odours (Gill, 1982). Packaging in various gaseous atmospheres has been used as an alterna￾tive to vacuum packing. The intention has been to preserve the fresh meat colour and to prevent anaerobic spoilage by using high concentrations of 10 Meat refrigeration 15 10 5 2 0 10 20 30 40 50 60 Generation time (h) Temperature (°C) Br. thermosphacta Enterobacter Lactobacillus Fig. 1.2 Effect of temperature on the rates of anaerobic growth of bacteria on meat slices (source: Newton and Gill, 1978)

Microbiology of refrigerated meat 1 oxygen(50-100%)along with 15-50% carbon dioxide to restrict the growth of Pseudomonas and related species(Nottingham, 1982). The microflora of meat stored in commercially used modified atmosphere packs(MAP) is in general similar to that of vacuum packs( Varnam and Sutherland, 1995).At temperatures below 2C, LAB are dominant, Leuconostoc spp. being the most important. Br. thermosphacta, Pseudomonas spp and Enterobacter- aceae are more prevalent in MAP (modified atmosphere packs)than vacuum packs at storage temperatures ca 5C, rather than 1C. Br. ther- mosphacta is relatively CO2 tolerant and the presence of O2 permits growth of this bacterium at pH values below 5. 8. Prior conditioning in air favours the growth of these bacteria, they are also more prevalent in pork than other meats(Dainty and Mackey, 1992). The spoilage of meat in MAP may involve souring similar to that in vacuum-packed meat. Other characteris- tics include'rancid' and'cheesy odours Chemical rancidity does not appear to be primarily involved and souring is probably caused by the metabolites of LAB or Br. thermosphacta(Varnam and Sutherland, 1995) 1.2.2.3 Spoilage of frozen meat Micro-organisms do not grow below ca. -10.C, thus spoilage is only nor mally relevant to handling before freezing or during thawing. In these contexts, frozen meats behave like their unfrozen counterparts, although growth rates may be faster after thawing, owing to drip Although Salmonella, Staphylococci, and other potential pathogens can survive freezing and frozen storage, the saprophytic flora(spoilage bacte ria)tend to inhibit their growth( Varnam and Sutherland, 1995). During freezing and thawing of food, the temperature favours the growth of psychrophilic organisms, most of which are spoilage organisms. Hence, in nearly all cases, if a frozen product is mishandled, spoilage is apparent before the food becomes a health hazard In the past, carcass meats were imported at temperatures of -5 to 10C. At these temperatures there were problems with the growth of psy- chrotrophic moulds such as strains of Cladosporium, Geotrichum, Mucor Penicillium, rhizopus and Thamnidium, causing whiskers'orspots'of various colours depending on the species. Since little meat is now stored at these temperatures mould spoilage is largely of historic importance subject in great detail at microbiology textbooks continue to discuss this Despite this, many mea 1.1.3 Relative humidity Historically low relative humidities(RH) have been recommended to extend shelf-life. Schmid(1931) recommended a storage temperature for meat of 0C and an RH of 90%. Haines and Smith(1933) later demon- strated that lowering the rh is more effective in controlling bacterial growth on fatty or connective tissue than on lean meat. This was due to a slower rate of diffusion of water to the surface. low rh was mor

oxygen (50–100%) along with 15–50% carbon dioxide to restrict the growth of Pseudomonas and related species (Nottingham, 1982). The microflora of meat stored in commercially used modified atmosphere packs (MAP) is in general similar to that of vacuum packs (Varnam and Sutherland, 1995). At temperatures below 2 °C, LAB are dominant, Leuconostoc spp. being the most important. Br. thermosphacta, Pseudomonas spp. and Enterobacteri￾aceae are more prevalent in MAP (modified atmosphere packs) than vacuum packs at storage temperatures ca. 5 °C, rather than 1 °C. Br. ther￾mosphacta is relatively CO2 tolerant and the presence of O2 permits growth of this bacterium at pH values below 5.8. Prior conditioning in air favours the growth of these bacteria, they are also more prevalent in pork than other meats (Dainty and Mackey, 1992). The spoilage of meat in MAP may involve souring similar to that in vacuum-packed meat. Other characteris￾tics include ‘rancid’ and ‘cheesy’ odours. Chemical rancidity does not appear to be primarily involved and souring is probably caused by the metabolites of LAB or Br. thermosphacta (Varnam and Sutherland, 1995). 1.1.2.2.3 Spoilage of frozen meat Micro-organisms do not grow below ca. -10 °C, thus spoilage is only nor￾mally relevant to handling before freezing or during thawing. In these contexts, frozen meats behave like their unfrozen counterparts, although growth rates may be faster after thawing, owing to drip. Although Salmonella, Staphylococci, and other potential pathogens can survive freezing and frozen storage, the saprophytic flora (spoilage bacte￾ria) tend to inhibit their growth (Varnam and Sutherland, 1995). During freezing and thawing of food, the temperature favours the growth of psychrophilic organisms, most of which are spoilage organisms. Hence, in nearly all cases, if a frozen product is mishandled, spoilage is apparent before the food becomes a health hazard. In the past, carcass meats were imported at temperatures of -5 to -10 °C. At these temperatures there were problems with the growth of psy￾chrotrophic moulds such as strains of Cladosporium, Geotrichum, Mucor, Penicillium, Rhizopus and Thamnidium, causing ‘whiskers’ or ‘spots’ of various colours depending on the species. Since little meat is now stored at these temperatures mould spoilage is largely of historic importance. Despite this, many meat microbiology textbooks continue to discuss this subject in great detail. 1.1.3 Relative humidity Historically low relative humidities (RH) have been recommended to extend shelf-life. Schmid (1931) recommended a storage temperature for meat of 0 °C and an RH of 90%. Haines and Smith (1933) later demon￾strated that lowering the RH is more effective in controlling bacterial growth on fatty or connective tissue than on lean meat. This was due to a slower rate of diffusion of water to the surface. Low RH was more Microbiology of refrigerated meat 11