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 spoilage1.1.1.2 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