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 andEnterobacteriaceae 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