Part lll Microbiological and non-microbiological hazards
Part III Microbiological and non-microbiological hazards
Chilled foods microbiology S.J. walker and G. Betts, Campden and Chorleywood Food Research association 7.1 Introduction Chilled foods represent a large and rapidly developing market with an extremely wide range of food types. Traditionally these were simple meat, poultry, fish and dairy products but recent trends have moved towards a greater variety and more complex products (Stringer and Dennis 2000). As more innovative products are produced, the variety of ingredients have also increased. Many of these ingredients are sourced around the world and relatively little may be known about their microbiological status. The numbers and types of microorganisms that may be isolated from the full range of chilled foods are very diverse. During the storage of chill products, the microbial flora of the product is not static but affected by many factors, principally the time and temperatures of storage. The spoilage and safety of chilled foods is a complex phenomenon involving physico-chemical, biochemical and biological changes. Often these interact and changes in one affect the rate of change in the others. This review will be concerned only with microbiological issues relation to chilled foods with developments in the manufacture and transport of chilled foods, these ems may now be rapidly disseminated over a wide geographical area,i.e different countries and sometimes continents. Therefore should a microbiol al issue arise it may be similarly widely spread. Consequently, the microbiological status of chilled foods has become more significant. greater surveillance both within and between countries will allow such microbiological ssues to be more rapidly identified, traced and resolved
7.1 Introduction Chilled foods represent a large and rapidly developing market with an extremely wide range of food types. Traditionally these were simple meat, poultry, fish and dairy products but recent trends have moved towards a greater variety and more complex products (Stringer and Dennis 2000). As more innovative products are produced, the variety of ingredients have also increased. Many of these ingredients are sourced around the world and relatively little may be known about their microbiological status. The numbers and types of microorganisms that may be isolated from the full range of chilled foods are very diverse. During the storage of chill products, the microbial flora of the product is not static but affected by many factors, principally the time and temperatures of storage. The spoilage and safety of chilled foods is a complex phenomenon involving physico-chemical, biochemical and biological changes. Often these interact and changes in one affect the rate of change in the others. This review will be concerned only with microbiological issues in relation to chilled foods. With developments in the manufacture and transport of chilled foods, these items may now be rapidly disseminated over a wide geographical area, i.e. different countries and sometimes continents. Therefore should a microbiological issue arise it may be similarly widely spread. Consequently, the microbiological status of chilled foods has become more significant. Greater surveillance both within and between countries will allow such microbiological issues to be more rapidly identified, traced and resolved. 7 Chilled foods microbiology S. J. Walker and G. Betts, Campden and Chorleywood Food Research Association
154 Chilled foods °C 2gEz8 Fig. 7.1 Effect of temperature on the growth of microorganisms 7.2 Why chill? The effect of reducing temperature is to reduce the rate of food deterioration. This applies not only to the chemical and biochemical changes in foods but also to the activities of microorganisms. The effect of temperature on microbial owth is shown in Fig. 7. 1. As the storage temperature decreases, the lag phase before growth(time before an increase in numbers is apparent) extends and the rate of growth decreases. In addition, as the minimum temperature for growth is approached, the maximum population size attainable often decreases. On a cellular basis, the effect of temperature on growth is a complex issue involving he cell membrane structure, substrate uptake, respiration and other enzyme ctivities. These have been discussed by Herbert (1989) The range of temperatures over which microorganisms can grow is extremely wide. Michener and Elliott(1964) reported that a number of microorganisms mainly yeasts, were able to grow below 0C and a pink yeast isolated from oysters was reported to grow at -34oC. Therefore, chilling alone cannot be relied upon to prevent all microbial growth. The use of chill temperatures will, however, reduce the rate and extent of microbial growth 7.3 Classification of growth Microbiologists have attempted to characterise microorganisms based on their abilities to grow at various temperatures. Most only, the cardinal
7.2 Why chill? The effect of reducing temperature is to reduce the rate of food deterioration. This applies not only to the chemical and biochemical changes in foods but also to the activities of microorganisms. The effect of temperature on microbial growth is shown in Fig. 7.1. As the storage temperature decreases, the lag phase before growth (time before an increase in numbers is apparent) extends and the rate of growth decreases. In addition, as the minimum temperature for growth is approached, the maximum population size attainable often decreases. On a cellular basis, the effect of temperature on growth is a complex issue involving the cell membrane structure, substrate uptake, respiration and other enzyme activities. These have been discussed by Herbert (1989). The range of temperatures over which microorganisms can grow is extremely wide. Michener and Elliott (1964) reported that a number of microorganisms, mainly yeasts, were able to grow below 0ºC and a pink yeast isolated from oysters was reported to grow at 34ºC. Therefore, chilling alone cannot be relied upon to prevent all microbial growth. The use of chill temperatures will, however, reduce the rate and extent of microbial growth. 7.3 Classification of growth Microbiologists have attempted to characterise microorganisms based on their abilities to grow at various temperatures. Most commonly, the cardinal Fig. 7.1 Effect of temperature on the growth of microorganisms. 154 Chilled foods
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 some
temperatures for growth (minimum, optimum and maximum growth temperatures) 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
156 Chilled foods Table 7.1 Classification of microbial growth (Jay 1978, Walker and Stringer 1990, lorita 1973) C) Psychrophile Psychrotroph Minimum 5to10 30to)240 20-30(35) Maximum 35(40-42) (70to2>80 Figures in parentheses are occasionally recorded for microorganisms assigned to a particular microorganisms from deep-sea fish. The major spoilage microorganisms of hilled foods are psychrotrophic in nature 7. 4 The impact of microbial growth Under suitable conditions, most microorganisms will grow or multiply. Bacteria each cell divides to form twe daughter cells. Consequently, the bacterial population undergoes an exponential increase in numbers. Under ideal conditions some bacteria may grow and divide every 20 minutes and so one bacterial cell may increase to 16 million cells in 8 hours. Under adverse conditions, e.g. chilled storage, the generation time (doubling time) will be increased. For example with an increased time of two hours, the population obtained after 8 hours would be only 16 cells. Even under ideal conditions, growth does not continue unchecked and is limited by a range of factors including the depletion of nutrients, build-up of toxic by-products changes to the environmental conditions or a lack of space 7.4.1 Food spoilage ring growth in foods, bacteria will consume nutrients from the food and produce metabolic by-products such as gases or acids. In addition, they may produce a number of enzymes which results in the breakdown of the cell structure or of components (e.g. lipases and proteases). When only a few spoilage microorganisms are present, the consequences of growth may not be apparent. If however, the microorganisms have multiplied then the production of gases, acid, off-odours, off-flavours or deterioration in structure in the food may become unacceptable. In addition, the number of microorganisms may be apparent as a visible colony, production of slime or an increase in the turbidity of liquids. Some of the enzymes produced by spoilage bacteria may remain active, even when a thermal process has destroyed the causative microorganisms The relationship between microbial numbers and food spoilage is complex and depends on the number, type and activity of the microorganisms present, the type of food and the intrinsic and extrinsic conditions. In some cases this is well
microorganisms from deep-sea fish. The major spoilage microorganisms of chilled foods are psychrotrophic in nature. 7.4 The impact of microbial growth Under suitable conditions, most microorganisms will grow or multiply. Bacteria multiply by the process of binary fission, i.e. each cell divides to form two daughter cells. Consequently, the bacterial population undergoes an exponential increase in numbers. Under ideal conditions some bacteria may grow and divide every 20 minutes and so one bacterial cell may increase to 16 million cells in 8 hours. Under adverse conditions, e.g. chilled storage, the generation time (doubling time) will be increased. For example with an increased time of two hours, the population obtained after 8 hours would be only 16 cells. Even under ideal conditions, growth does not continue unchecked and is limited by a range of factors including the depletion of nutrients, build-up of toxic by-products, changes to the environmental conditions or a lack of space. 7.4.1 Food spoilage During growth in foods, bacteria will consume nutrients from the food and produce metabolic by-products such as gases or acids. In addition, they may produce a number of enzymes which results in the breakdown of the cell structure or of components (e.g. lipases and proteases). When only a few spoilage microorganisms are present, the consequences of growth may not be apparent. If however, the microorganisms have multiplied then the production of gases, acid, off-odours, off-flavours or deterioration in structure in the food may become unacceptable. In addition, the number of microorganisms may be apparent as a visible colony, production of slime or an increase in the turbidity of liquids. Some of the enzymes produced by spoilage bacteria may remain active, even when a thermal process has destroyed the causative microorganisms in the food. The relationship between microbial numbers and food spoilage is complex and depends on the number, type and activity of the microorganisms present, the type of food and the intrinsic and extrinsic conditions. In some cases this is well Table 7.1 Classification of microbial growth (Jay 1978, Walker and Stringer 1990, Morita 1973) Temperature (˚C) Psychrophile Psychrotroph Mesophile Thermophile Minimum 0–5 0–5 (5 to)a 10 (30 to)a 40 Optimum 12–18 20–30 (35)a 30–40 55–65 Maximum 20 35 (40–42)a 45 (70 to)a 80 a Figures in parentheses are occasionally recorded for microorganisms assigned to a particular classification. 156 Chilled foods