Chilled foods microbiology 155 temperatures for growth(minimum, optimum and maximum growth tempera- tures) are used. with chilled foods, the factor of most concern is the minimum growth temperature(Mgt), which represents the lowest temperature at which growth of a particular microorganism can occur. If the MGT of a microorganism is greater than 10oC, then this microorganism will not grow during chill storage Whilst MGT values for microorganisms have been published, care is needed. If the time period for the investigation reporting this value was too short sampling intervals too widely spaced, the resultant value will be erroneous. For example, although an MGT of -04C has been reported for Listeria monocytogenes, the lag phase before growth was in excess of 15 days(Walker et al, 1990a). Had the study terminated before this time, the reported MGT would have been higher. The MGT is affected by other factors including the pH salt, preservatives and previous heat treatments. a true estimate of the mgt can be determined only when other factors are optimal for growth If a microorganism is stored below its MGT, gradual death may occur, but often the microorganism will survive and growth will resume should the temperature subsequently be raised. It was noted by Alcock(1984) that the survival of salmonellae was worse at temperatures just below the MGt compared with lower temperatures. Storage at temperatures below the minimum for growth should not be considered to be a lethal process for microorganisms as in many cases, growth will resume if the temperature is subsequently raised The optimum growth temperature represents the temperature at which the biochemical processes governing growth of a particular microorganism are overall operating most efficiently. At this temperature, the lag phase before growth is minimised and the growth rate maximised. As the temperature rises above the optimum, the rate of growth decreases until the maximum growth mperature is reached. In general, the maximum growth temperature is only a few degrees (Celsius) higher than the optimum. With some specialised microorganisms, isolated from hot springs, the maximum growth temperature may exceed 90oC (Jay, 1978). At temperatures just above the maximum for growth, cell injury starts to occur. If the temperature is subsequently reduced, then growth may resume, although a period of time may be required to permit cell repair. At higher temperatures, the inactivation of one or more critical enzymes in the microorganism becomes irreversible and cell damage occurs, ading to cell death. Such microorganisms will not be able to repair and resume growth if temperatures are reduced. The concepts of cell injury and death have been discussed by Gould (1989b) Based on the relative positions of the cardinal temperatures, microorganisms can be divided into four main groups, viz., psychrophile, psychrotroph, mesophile and thermophile (table 7. 1). with chilled foods, the groups of most concern are the psychrophiles and psychrotrophs. In the past, these terms have been used synonymously, which has led to much confusion. It is now accepted that the term psychrophile' should only be used for microorganisms which have a low (i.e. <20C) maximum growth temperature(Eddy, 1960). True psychrophiles are rare in food microbiology and generally limited to sometemperatures for growth (minimum, optimum and maximum growth tempera￾tures) are used. With chilled foods, the factor of most concern is the minimum growth temperature (MGT), which represents the lowest temperature at which growth of a particular microorganism can occur. If the MGT of a microorganism is greater than 10ºC, then this microorganism will not grow during chill storage. Whilst MGT values for microorganisms have been published, care is needed. If the time period for the investigation reporting this value was too short, or sampling intervals too widely spaced, the resultant value will be erroneous. For example, although an MGT of
0.4ºC has been reported for Listeria monocytogenes, the lag phase before growth was in excess of 15 days (Walker et al., 1990a). Had the study terminated before this time, the reported MGT would have been higher. The MGT is affected by other factors including the pH, salt, preservatives and previous heat treatments. A true estimate of the MGT can be determined only when other factors are optimal for growth. If a microorganism is stored below its MGT, gradual death may occur, but often the microorganism will survive and growth will resume should the temperature subsequently be raised. It was noted by Alcock (1984) that the survival of salmonellae was worse at temperatures just below the MGT compared with lower temperatures. Storage at temperatures below the minimum for growth should not be considered to be a lethal process for microorganisms as in many cases, growth will resume if the temperature is subsequently raised. The optimum growth temperature represents the temperature at which the biochemical processes governing growth of a particular microorganism are overall operating most efficiently. At this temperature, the lag phase before growth is minimised and the growth rate maximised. As the temperature rises above the optimum, the rate of growth decreases until the maximum growth temperature is reached. In general, the maximum growth temperature is only a few degrees (Celsius) higher than the optimum. With some specialised microorganisms, isolated from hot springs, the maximum growth temperature may exceed 90ºC (Jay, 1978). At temperatures just above the maximum for growth, cell injury starts to occur. If the temperature is subsequently reduced, then growth may resume, although a period of time may be required to permit cell repair. At higher temperatures, the inactivation of one or more critical enzymes in the microorganism becomes irreversible and cell damage occurs, leading to cell death. Such microorganisms will not be able to repair and resume growth if temperatures are reduced. The concepts of cell injury and death have been discussed by Gould (1989b). Based on the relative positions of the cardinal temperatures, microorganisms can be divided into four main groups, viz., psychrophile, psychrotroph, mesophile and thermophile (Table 7.1). With chilled foods, the groups of most concern are the psychrophiles and psychrotrophs. In the past, these terms have been used synonymously, which has led to much confusion. It is now accepted that the term ‘psychrophile’ should only be used for microorganisms which have a low (i.e. 20ºC) maximum growth temperature (Eddy, 1960). True psychrophiles are rare in food microbiology and generally limited to some Chilled foods microbiology 155