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 reducing 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 bacterial growth. If this is the case then mould growth on frozen meats is indicative 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 affecting 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). Listeria 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 refrigerated meats since refrigeration can not be relied on to prevent growth (Doyle, 1987). On the other hand, psychrotrophic pathogens are not particularly 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