冷冻食品（英文第二版）：34990wp_07

Part III Microbiological and non-microbiological hazards 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 microbiolo- gical 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 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 C034oC. 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 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, or sampling intervals too widely spaced, the resultant value will be erroneous. For example, although an MGT of C00.4oC 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. C2020oC) maximum growth temperature (Eddy, 1960). True psychrophiles are rare in food microbiology and generally limited to some Chilled foods microbiology 155 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 C600–5 C600–5 (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 C6280 a Figures in parentheses are occasionally recorded for microorganisms assigned to a particular classification. 156 Chilled foods understood, e.g. vacuum packed cod (Gram and Huss 1996). In general, a greater understanding is needed of the relationship between specific spoilage microorganisms in particular foods and the deterioration in sensory quality. 7.4.2 Food-borne pathogens With many human pathogens, the greater the number of cells consumed, the greater the chance of microbial invasion, as the larger number of cells may be able to evade/swamp the body’s defence mechanism. Higher numbers may also result in a shorter incubation period before the onset of disease. Consequently, control, and preferably inhibition, of growth in foods is essential. However, with some invasive pathogens (e.g. viruses, Campylobacter), the infectious dose is low and growth in the food may not be necessary. Other pathogenic microorganisms may produce a toxin in the food which results in disease. Preformed toxins are usually produced at high cells densities and so usually growth has occurred. If the toxin is heat stable, it may remain although all microorganisms have been eliminated from the food. Consequently it is important to control growth at all stages of the chill chain. 7.5 Factors affecting the microflora of chilled foods 7.5.1 Initial microflora With healthy animal and plant tissues, microbial contamination is absent or at a low level except for the exterior surfaces. For example, fresh muscle from healthy animals is usually microbiologically sterile, and aseptically drawn milk from healthy cows contains only a few microorganisms (mainly streptococci and micrococci) derived from the teat canal. Similarly, the interior of healthy undamaged vegetables does not contain microorganisms although the exterior may be contaminated with a wide range of microorganisms of soil origin. During slaughter or harvesting, subsequent processing and packaging, these raw materials become contaminated from a wide range of sites. Typically, these sites include water, air, dust, soil, hides/fleece/feathers, animals, people, equipment and other food materials. Consequently, a large range of microorganisms can be isolated from foods. Those which are able to grow may potentially give rise to microbial spoilage or public health issues. The hygienic practices of all food operations, from slaughter/harvesting through retail sale to consumer use, will affect the level of microbial contamination of products. In general, the lower the initial level of contamination, the greater the time until microbial spoilage is evident. 7.5.2 Food type The intrinsic properties (e.g. pH, water activity, acidity, natural antimicrobials) of different foods vary greatly. Such factors affect the ability of microorganisms Chilled foods microbiology 157 to grow and the rate of growth and will be discussed in more detail in subsequent sections of this chapter. With different food types, the nutritional status varies although foods are generally not nutritionally limiting for microorganisms. Foods rich in nutrients (e.g. meat, milk, fish) permit faster growth than those with a lower nutritional status (e.g. vegetables) and so are more prone to spoilage. Slaughter and harvesting practices may affect the intrinsic properties of a food. For example, poor practices in the husbandry and slaughter of pigs may lead to pork being classified as DFD (dark, firm, dry) or PSE (pale, soft, exudative), both of which are more prone to spoilage than ‘normal’ pork. With DFD meat, the pH is higher, so permitting faster growth, whilst nutrient leakage and protein denaturation from PSE meat also allow more rapid microbial proliferation. Even within a single food ingredient or product, variations in the pH, a w and redox potential may occur and so affect the nature and rate of microbial multiplication. The situation may be further complicated in multi-component foods where migration of nutrients and gradients of pH, a w and preservatives may occur. In addition, microorganisms unable to grow on one ingredient may come into contact with a more favourable environment and so permit growth. 7.5.3 Processing Chill storage The time of storage will affect microbial numbers. Generally, microbial numbers increase with time in chilled foods at neutral pH values, low salt concentrations and the absence of preservatives. However, low pH values or high salt concentrations in foods may cause microbial stasis, injury or even death. At chill temperatures however, the rate of death is often reduced and so the microorganism may survive for longer periods compared with higher (e.g. ambient) temperatures. In many cases a combination of processing and preservation factors may be used to achieve a safe, high quality product with an acceptable shelf-life. Such combination treatments have been reviewed by Gould (1996). The ability of individual microorganisms to grow and their rates of growth are affected by temperature. As discussed previously, some microorganisms (mainly psychrotrophs) are better adapted to growth at chill temperatures. Therefore during chill storage not only will the total number of microorganisms change, but also the composition of the microflora will alter. For example, with freshly drawn milk, the microflora is dominated by Gram-positive cocci and rods, which may spoil the product by souring if stored at warm temperatures. At chill temperatures, these microorganisms are largely unable to grow and the microflora rapidly becomes dominated by psychrotrophic Gram-negative rod- shaped bacteria (most commonly Pseudomonas spp.) (Neill, 1974). A similar change in the microflora composition has also been reported for other chill- stored foods (Huis in’t Veld, 1996). 158 Chilled foods Heating As part of their manufacture, many chilled foods undergo a heating process. This will reduce microbial numbers, generally resulting in a pasteurised rather than a sterilised product, otherwise chill storage would be unnecessary. Food pasteurisation treatments have been reviewed by Gaze (1992). The degree of heat applied will affect the types of microorganisms able to survive. In general, the Gram-negative rod-shaped bacteria, which proliferate in chilled foods, are sensitive to heat and are readily eliminated. Although these bacteria may be isolated from, and even spoil, heated foods, their presence is usually attributable to post-heating contamination. Some Gram-positive bacteria are tolerant to mild heat and classified as thermoduric (e.g. some Lactobacillus, Streptococcus and Micrococcus species, Jay 1978). However pasteurisation processes are designed to destroy all vegetative cells. Other bacteria however, produce heat-resistant bodies, called spores, which may survive. The genera of concern are Bacillus and Clostridium species, which include both pathogenic and spoilage strains. Whilst these bacteria are generally out-competed in chilled foods by the Gram-negative rod- shaped bacteria, Bacillus and Clostridium species may grow relatively unhindered in heated foods subsequently stored chilled. Acidification Several types of chilled foods are naturally acidic (e.g. fruit juices) or acidified using either a fermentation process (e.g. yoghurt) or by the direct addition of acids (e.g. coleslaw). As with temperature, microorganisms have pH limits for growth. The pH optima for most pathogenic bacteria is usually in the range 6.8– 7.4 (Jay 1978), which is similar to the human body pH in which they are adapted to grow. Typical minimum pH values for growth are shown in Table 7.2. The minimum pH for the major spoilage bacteria of meat, poultry and dairy products is approximately 5.0 whereas other microbial types, in particular yeasts and moulds, may grow at pH values of 3.0 or less. Consequently, mildly acidified products may be spoiled by acid-tolerant bacteria (lactic acid bacteria and some Enterobacteriaceae) whilst more acid products are spoiled by yeasts and moulds. Both pH and temperature interact, and the minimum pH for growth at optimal temperatures may be significantly less than that at chill temperatures (George et al. 1988). At pH values below the minimum for growth, some microorganisms will die rapidly in the food, whilst others may persist for the life of the product. Of particular concern in acid foods is the pathogen E. coli O157: H7, which is more acid tolerant than other pathogens. It may grow at pH values of 4.0 or below and survive for considerable periods at lower pH values (Conner and Kotrola 1995, Deng et al. 1999). In addition to the pH, the acid type used affects the microbial stability of the foods. The organic acids (lactic, acetic, citric and malic) are more antimicrobial than the inorganic acids (hydrochloric, sulphuric). Care is needed with published literature, as the minimum pH values reported often have used inorganic acids. Therefore the minimum pH for growth in foods is often higher than that quoted, Chilled foods microbiology 159 as organic acids are present. Within the organic acids, the order of decreasing antimicrobial efficiency is usually acetic, lactic, citric then malic acid. With the organic acids, the undissociated form of the acid is effective against microorganisms and the degree of dissociation is dependent on the pH of the food. Organic acids and their use in food systems have been discussed by Kabara and Eklund (1991). The pH and acid composition does not remain constant during the life of some foods. Changes in pH will affect the types of microorganisms able to grow and their growth rates. With some foods, fermentation results in a pH decreases during storage whilst in others an increase can be noted. For example, during maturation of mould-ripened cheeses, the pH value of cheese near the surfaces increases owing to proteolytic activity of the mould, and this has been related to the ability of Listeria monocytogenes to grow in these products, but not in the unripened cheeses (Terplan et al. 1987). Reduced a w The a w is a measure of the amount of water available in a food which may be used for microbial growth. As the a w of a food is reduced, the number of microorganisms able to grow and their rate of growth is also reduced (Sperber 1983) (Table 7.2). The a w of a food may be reduced either by the removal of water (i.e. drying) or by the addition of solutes (e.g. salt or sugar). In response to diet and health issues, many jam and sauce products have reduced their sugar content. Thus the intrinsic preservation system (i.e. low a w ) of the product has been compromised and some microorganisms, mainly yeasts, may now grow. These products generally recommend refrigeration after opening to prevent microbial growth. The a w of a product may interact with other preservation factors, including temperature, to maintain the safety of chilled foods (Glass and Table 7.2 Typical minimum pH and a w values for growth of microorganisms (Anon. 1991b, Gould 1989, Mitscherlich and Marth 1984, ACMSF 1995) Microorganism Minimum pH Minimum a w Bacillus cereus 4.9 0.91 Campylobacter jejuni 5.3 0.985 Clostridium botulinum (non-proteolytic) 5.0 0.96 Clostridium botulinum (proteolytic) 4.6 0.93 Clostridium perfringens 5.0 0.93 Escherichia coli 4.4 0.95 Escherichia coli O157:H7 3.8–4.2 0.97 Lactobacillus species 3–3.5 0.95 Pseudomonas species 5.0 0.95 Salmonella species 4.0 0.95 Staphylococcus aureus 4.0 (4.6) a 0.86 Many yeasts and moulds C602.0 0.8–0.6 Yersinia enterocolitica 4.6 0.95 a Minimum pH with toxin production. 160 Chilled foods Doyle 1991). In general, yeasts and moulds are more tolerant than bacteria of low a w values in foods (Jay 1978). As bacterial growth is largely inhibited, yeasts and moulds may then grow and cause the spoilage defects in such products. Preservatives In order to maintain their microbial stability, many chilled products contain natural or added preservatives, e.g. salt, nitrite, benzoate, sorbate. The presence of these compounds affects the type and rate of product spoilage that may occur. Their applications and mechanisms of action have been reviewed in Russell and Gould (1991). As discussed previously, Pseudomonas species tend to predominate on chilled fresh meat. The addition of curing salts (i.e. sodium chloride and potassium nitrite) to pork meat to form bacon, largely inhibits the growth of these microorganisms and spoilage is caused by other microbial groups (e.g. micrococci, staphylococci, lactic acid bacteria) (Gardner 1983, Borch et al. 1996). Similarly, the British sausage is largely a fresh meat product, but is preserved by the addition of sulphite. This prevents the growth of the Pseudomonas species, and microbial spoilage will be caused by sulphite- resistant Brochothrix thermosphacta or yeasts (Gardner 1983). The number and type of microorganisms able to grow in preservative- containing chilled foods depend on the food type, preservative type, pH of the food, preservative concentration, time of storage and other preservation mechanisms in the food. Overall, yeasts and moulds tend to be more resistant to preservatives compared with bacteria and so may dominate the final spoilage microflora. Recent trends in food processing have tended to reduce or eliminate the use of preservatives. Care is needed with such an approach, as even small changes may compromise the product safety and microbiological stability. Storage atmosphere The use of modified atmospheres, including vacuum packaging, for the storage of chilled foods is increasing. Often these are chosen to maintain sensory characteristics of a product, but many will also inhibit or retard the development of the ‘normal’ spoilage microflora. Pseudomonas species, the major spoilage group in chilled proteinaceous foods, require the presence of oxygen to grow. Therefore, the use of vacuum packaging or modified atmospheres excluding oxygen will prevent the growth of this microbial group. Whilst other microorganisms can grow in the absence of oxygen, they generally grow more slowly and so the time to microbial spoilage is increased. The spoilage microflora of vacuum-packed meats is usually dominated by lactic acid bacteria or Brochothrix thermosphacta (Borch et al. 1996). In some cases, Enterobacter- iaceae or coliforms may cause the spoilage of vacuum-packed and modified- atmosphere-packed (MAP) foods (Gill and Molin 1991). Most commercial MAP gas mixtures for chilled food usually contain a combination of carbon dioxide, nitrogen and oxygen. The inhibition of bacteria becomes more pronounced as the amount of carbon dioxide increases. The Chilled foods microbiology 161 effects of carbon dioxide on microbial growth have been discussed by Gill and Molin (1991). More recently, the use of other gases (including the noble gases) and high levels of oxygen have been used to extend the shelf-life of chilled foods (Day 2000). Good temperature control is essential to obtain the maximum potential benefits of modified-atmosphere and vacuum packing. Should temperature abuse occur, the rate of spoilage will be similar to that without the atmosphere. It has been suggested that modified atmospheres will inhibit the ‘normal’ spoilage microflora of a food, but the growth of some anaerobic or facultatively anaerobic pathogens (e.g. Clostridium species, Listeria monocytogenes, Yersinia enterocolitica, Salmonella species and Aeromonas hydrophila) will be largely unaffected. Consequently the food may appear satisfactory but contain food- poisoning microorganisms. Of particular concern is the potential for growth of psychrotrophic Clostridium botulinum, which has been addressed by Betts (1996). In general, moulds require oxygen for growth and so are unlikely to create problems in vacuum-packaged or MAP (excluding oxygen) foods. Conversely, many yeasts can grow in the presence or absence of oxygen, although aerobic growth tends to be more efficient, thereby permitting more rapid growth. Combinations Many chill products do not depend on a single preservation system for their microbial stability, but a combination of the factors described above. These can be effective in controlling microbial growth (Gould 1996). With such foods, care is needed during their manufacture, distribution and sale because inadequate control of one factor may permit rapid growth. Furthermore, the use of two or more systems in combination may select for a particular microbial type (Gould and Jones 1989). For example, ‘sous-vide’ processing involves the vacuum packaging of foods, followed by a relatively mild heat treatment (pasteurisa- tion). The heat treatment will eliminate vegetative microorganisms but not spore-forming bacteria. During subsequent chill storage (up to 30 days) in vacuum packaging, anaerobic spore-forming bacteria, including Cl. botulinum, may grow in the absence of other microorganisms. In order to prevent this happening, the product should be stored below the minimum temperature for growth of Cl. botulinum, the formulation of the product adjusted to prevent growth, or the heat treatment applied increased (Betts, 1992). 7.6 Spoilage microorganisms Microbiological spoilage of chilled foods may take diverse forms, but all are generally as a consequence of growth which manifests itself in a change in the sensory characteristics. In the simplest form, this may be due to growth per se and often the production of visible growth, and this is common in moulds which produce large often pigmented colonies. Bacteria and yeasts may also produce 162 Chilled foods visible (sometimes pigmented) colonies on foods. Other forms of spoilage including the production of gases, slime (extracellular polysaccharide material), diffusible pigments and enzymes which may produce softening, rotting, off- odours and off-flavours from the breakdown of food components. The taints produced by microbial spoilage have been reviewed by Dainty (1996) and Whitfield (1998). Spoilage is usually most rapid in proteinaceous chilled foods such as red meats, poultry, fish, shellfish, milk and some dairy products. These products allow good microbial growth as they are highly nutritious, have a high moisture content and relatively neutral pH value. In an attempt to reduce the spoilage rates of these foods, they are often modified as discussed previously. For chilled products, these modifications may not entirely prevent microbial growth and spoilage, but do limit the rate and nature of spoilage. In general the microorganisms responsible for spoilage of a food are those which are best able to grow in the presence of the preservation mechanisms that are operating within that food. Care is needed to distinguish between those microorganisms present in spoiled food and those responsible for the spoilage defect (often called specific spoilage organisms or SSO) which may be only a fraction of the microflora (Gram and Huss 1996). Consequently, the relationship between sensory spoilage and microbial numbers is often only poorly correlated. Traditional microbiology is often of limited value for control of spoilage microorganisms as the time taken to get results represents a significant proportion of the shelf-life. Recently more rapid and molecular techniques have become available for the detection of general or specific spoilage organisms (Venkitanarayanen et al. 1997, Gutie′rrez et al. 1997). For discussion in this chapter, spoilage microorganisms have been arbitrarily divided into six categories: Gram-negative (oxidase positive) rod-shape bacteria; coliform enterics; Gram-positive spore-forming bacteria; lactic acid bacteria; other bacteria; yeasts and moulds. 1. Gram-negative (oxidase positive) rod-shaped bacteria Overall, this group comprises the most common spoilage microorganisms of fresh chilled products. The minimum growth temperatures are often 0–3oC and they grow relatively rapidly at 5–10oC. Although they may represent only a small proportion of the initial microflora, they rapidly dominate the microflora of fresh proteinaceous chilled stored foods (Huis in’t Veld 1996, Cousin 1982, Gill 1983). Within this general group, the genus Pseudomonas is most common although other genera include Acinetobacter, Aeromonas, Alcaligenes, Alter- omonas, Flavobacterium, Moraxella, Shewenella and Vibrio species (Walker and Stringer 1990). These microorganisms are common in the environment, particularly in water, and so many easily contaminate foods. Often they may proliferate on inadequately cleaned surfaces of food processing plant or equipment and so contaminate foods. The Gram-negative (oxidase positive) rods may spoil products by the production of diffusible pigments, slime material on surfaces and enzymes Chilled foods microbiology 163 which result in food rots, off-flavours and off-odours (Jay 1978, Cousin 1982, Gill 1983). Some of the enzymes produced by Pseudomonas species are extremely heat-resistant and may produce long-term defects (e.g. rancidity or age-gelation) in thermally processed products with extended shelf-lives. Although well adapted to grow at chill temperatures, this group tends to be sensitive to other factors such as the presence of salt or preservatives, lack of oxygen, low (C605.5) pH and a low (C600.98) a w . Should these preservation mechanisms be present in a food, the Gram-negative (oxidase positive) rod- shaped bacteria compete less well and other microbial groups may cause spoilage. Vibrio species are unusual as they tolerate relatively high salt levels and so may cause spoilage on chilled stored bacon and other cured products. Photobacterium phosphorum, a very large marine vibrio is the dominant spoilage microorganism in vacuum packaged cod (Gram and Huss 1996). Overall, this group is not heat-resistant and so is readily removed by mild thermal treatments. Their presence in heat processed foods is usually as a consequence of post-process contamination. 2. Coliform/enteric bacteria This bacterial group also consists of Gram-negative rods, but these may be distinguished from the above group by a negative oxidase reaction. Traditionally, microbiologists have tended to examine for these groups separately, as their sources, significance and factors affecting growth may differ. This group is frequently used as an indicator of inadequate processing or post-process contamination. Compared with the Gram-negative (oxidase positive) rod-shaped bacteria, the coliform-enteric group is generally less well adapted to growth at temperatures of less than 5–10oC although many may grow at temperatures as low as 0oC (Ridell and Korkeala 1997). However, they often dominate the flora at temperatures of 8–15oC (Huis in’t Veld 1996, Cousin 1982). The coliform/enteric group is less sensitive to changes in pH compared with the Gram-negative (oxidase positive) rod-shaped bacteria and so of more significance in mild acid products. They are however, generally sensitive to low a w , preservatives, salt and thermal treatments (Jay 1978). The coliform/enteric group do not necessarily require the presence of oxygen for growth. In addition, they have a fermentative metabolism and so may break down carbohydrates to give acids, which may result in souring of milk (Cousin 1982). In contrast, the metabolism of the Gram-negative (oxidase positive) bacteria is oxidative and fermentation does not occur. Other types of spoilage include the production of pigmented growth, gases, slime, off-odours and off- flavours. Off odours have been described as ‘grassy’, medicinal, unclean and faecal (Walker and Stringer 1990). Typical spoilage species include Citrobacter, Escherichia, Enterobacter, Hafnia, Klebsiella, Proteus and Serratia (Jay 1978, Walker 1988). These microorganisms are widely disseminated in the environment, including in animals. Poor slaughter and dressing practices may contribute to their presence in foods. 164 Chilled foods 3. Gram-positive spore-forming bacteria This group, of particular significance, can produce heat-resistant bodies (spores) which can survive many thermal processes. Such heating may destroy all vegetative cells, leaving the relatively slow growing spore-formers to dominate the microflora. The minimum growth temperatures are often 0–5oC, although growth is often slow below 8oC (Huis in’t Veld 1996, Cousin 1982, Coghill and Juffs 1979). The genera of concern in this group are Bacillus and Clostridium species. Again, these are common in the environment and spores may survive for considerable periods. The most common form of spoilage is the production of large quantities of gas which may result in pack or product blowing (Cousin 1982, Walker 1988). The heat resistance of psychrotrophic strains is considered to be lower than that of mesophilic strains (Reinheimer and Bargagna 1989), but the former group is of concern in chilled pasteurised foods. 4. Lactic acid bacteria At chill temperatures, lactic acid-producing bacteria grow slowly if at all. Consequently, if they are to cause spoilage, growth of most other bacterial species must be inhibited. This group is more tolerant of low pH than other spoilage bacteria and may multiply at pH values as low as 3.6 (Jay 1978). The lactic acid bacteria are also more resistant than the previously discussed spoilage bacteria to slight reductions in the a w and some Pediococcus species are salt- tolerant. Lactic acid bacteria usually predominate on vacuum-packed products and in some modified-atmosphere-stored foods, and may even grow in atmospheres containing 100% carbon dioxide (Gill and Molin 1991). This bacterial group comprises both rod- and coccus-shaped Gram-positive bacteria and typical genera include Carnobacterium, Lactobacillus, Leuconostoc, Pediococcus and Streptococcus species (Borch et al. 1996). Spoilage is generally by the production of acid which results in souring with or without concomitant gas production (Walker and Stringer 1990). Lactic acid-producing bacteria are deliberately added during the manufacture of some chilled foods (e.g. cheese, yoghurts, some salamis) and are essential for the development of the desired product characteristics. In addition, there is much interest in the potential use of lactic acid bacteria as a novel preservation system, as many produce antimicrobial compounds in addition to acids (Lu¨cke and Earnshaw 1991). 5. Other bacteria Depending on the food type and preservation system operating, other microorganisms may also cause problems in chilled foods. For example, Brochothrix thermosphacta is a Gram-positive rod-shaped bacterium which is occasionally present on raw meats but does not normally create a spoilage problem. Products preserved with sulphite (e.g. fresh British sausage) may encourage the development of this bacterium (Gardner 1981). Furthermore, it Chilled foods microbiology 165 can grow in atmospheres with a low oxygen level and/or high carbon dioxide concentration and so may cause problems in vacuum-packed or modified- atmosphere-packed meat products. In vacuum-packed sliced meats, this microorganism produces an objectionable pungent ‘cheesy’ odour. Micrococcus species are Gram-positive cocci which can grow in the presence of high salt concentrations. They tend not to grow well at chill temperatures but can cause souring and slime production on cured meats and in curing brines should temperature abuse occur (Gardner 1983). Other microorganisms that may cause spoilage problems in cured meats and/or vacuum-packed meat products are Corynebacterium, Kurthia and Arthobacter species (Gardner 1983, Gould and Russell 1991). 6. Yeasts and moulds Compared with bacteria, both yeasts and moulds grow more slowly in foods permitting good growth and so are generally out-competed. Therefore this group is seldom responsible for the spoilage of fresh proteinaceous foods. If, however, the conditions in the food are altered to limit bacterial growth, the role of yeasts and moulds may become more significant. Many yeasts can grow at temperatures less than 0oC (Michener and Elliott 1964). Furthermore, yeasts and moulds are generally more resistant than bacteria to low pH, reduced a w values and the presence of preservatives (Jay 1978). Moulds tend to require oxygen for growth whereas many yeasts can grow in the presence or absence of oxygen. Most yeasts and moulds are not heat-resistant and are readily destroyed by a thermal process. The mould genus Byssochlamys however, may produce relatively heat-resistant ascospores (Bayne and Michener 1979). Freshly collected meat, poultry, fish and dairy products rarely contain yeasts or moulds but they rapidly become contaminated from the environment. In particular, air movements may be an important vector of transmission, especially with mould ascospores. Typical spoilage yeasts include Candida, Debaryo- myces, Hansenula, Kluveromyces, Rhodotorula, Saccharomyces, Torula and Zygosaccharomyces species (Walker and Stringer 1990, Pitt and Hocking 1985). Moulds that may be isolated from spoiled chilled foods include Aspergillus, Cladosporium, Geotrichum, Mucor, Penicillium, Rhizopus and Thamnidium species (Pitt and Hocking 1985, Filtenborg et al. 1996). Fungal spoilage may be characterised by the production of highly visible, often pigmented, growth, slime, fermentation of sugars to form acid, gas or alcohol, and the development of off-odours and off-flavours. Odours and flavours have been described as yeasty, fruity, musty, rancid and ammoniacal. As with the lactic acid bacteria, yeasts and moulds are sometimes deliberately added to food products. For example, the development of Penicillium camembertii on the surfaces of Brie and Camembert cheeses is essential for the desired flavour, odour and texture characteristics. This mould growing on other types of cheeses would be described as a spoilage defect. 166 Chilled foods 7.7 Pathogenic microorganisms Foods may be considered to be microbiologically unsafe owing to the presence of microorganisms which may invade the body (e.g. Salmonella, Listeria monocytogenes, E. coli O157:H7 and Campylobacter) or those which produce a toxin ingested with a food (e.g. Clostridium botulinum, Staphylococcus aureus and Bacillus cereus). The growth of pathogenic microorganisms in foods may not necessarily result in spoilage, and so the absence of deleterious sensory changes cannot be relied upon as an indicator of microbial safety. Furthermore, some toxins are resistant to heating and so may remain in a food after viable microorganisms have been removed. It is therefore essential that an effective programme is used to ensure the safety of foods from production, through processing, storage and distribution to consumption. Within the UK, the recent trends in food poisoning and the issues contributing to this have been extensively reviewed by Border and Norton (1997). As discussed previously, storage at chill temperatures cannot prevent all microbial growth, but can prevent the growth of some types and retard the rate of growth in others. As far back as 1936, Prescott and Geer recommended that foods permitting growth of microorganisms should be stored at less than 10oC (50oF) and preferably ca. 4oC (39oF) to prevent the growth of pathogens or toxin production. That was sound advice in terms of the food-borne pathogens recognised at that time. The risk of growth by food-borne pathogens is a combination of the minimum growth temperatures, the growth rate at chill temperatures and the time and temperature(s) of storage. The minimum growth temperatures of pathogenic bacteria have been discussed by Walker and Stringer (1990). Whilst the majority of food-borne disease is caused by relatively few bacterial types – mainly Salmonella and Campylobacter (Border and Norton 1997), the number of bacteria recognised as food-borne pathogens, however, has steadily increased. Whilst this may, in part, reflect a true underlying increase in the incidence, it may also be due to a greater awareness of these microorganisms and improvements in methodologies. For discussion in this chapter, the pathogenic bacteria of concern for chilled foods can be arbitrarily divided as follows. Microorganisms capable of growth at temperatures below 5oC This group is potentially of greatest concern as they continue to multiply even with ‘good’ refrigeration temperatures. Although growth may continue, temperature control is critical and the growth rate becomes increasingly slow as the temperature is reduced (see Fig. 7.1). In addition, temperature control can interact effectively with other factors to prevent or greatly limit growth. Listeria monocytogenes The bacterium now identified as L. monocytogenes was first recognised as a human pathogen in 1926 (Murray et al.), but its role in food-borne disease was Chilled foods microbiology 167 not apparent until the late 1970s. Reported cases in the UK increased dramatically during the 1980s, and decreased during subsequent years. The symptoms of disease are protean and range from a mild flu-like illness to meningitis, septicaemia, stillbirths and abortions (Ralovich 1987). In general, the major symptoms of disease are restricted to the pregnant mother, foetus, elderly and immunocompromised. With the latter three groups, the mortality level can be high (McLauchlin 1987). The epidemiology of L. monocytogenes has been discussed by Schuchat et al. (1991). A very wide range of foods including meat, poultry, dairy products, seafoods and vegetables have been reported to be contaminated with L. monocytogenes and have been reviewed by Bell and Kyriakides (1998b). Whilst the total absence of L. monocytogenes from raw meats, poultry and vegetables is difficult to ensure, the bacterium has been isolated from products which have undergone a listericidal thermal process (Lund 1990). Such isolations are of concern as many of these chilled foods may be consumed without further heating. The presence of L. monocytogenes on cooked foods suggests that post-process contamination may have occurred. Several studies have shown this bacterium has been isolated from a wide range of sites in several types of factory (Cox et al. 1989) and may be spread by some cleaning procedures (Holah et al. 1993). Sites of particular concern include those where water is present. Environmental control of Listeria, particularly in key areas of production (e.g. after cooking) is crucial to the prevention of product contamination. Whilst the number of cases of reported listeriosis in England and Wales peaked dramatically between 1986 and 1988 which was associated with contaminated imported pa?te′. Following public warnings about this, the number of cases declined to the annual rate prior to this (100–150 cases per year) (Border and Norton 1997). The major concern with L. monocytogenes is its ability to grow at low temperatures, and a minimum growth temperature of C00.4oC has been reported (Walker et al. 1990). Temperature control will however, retard the rate of growth (Fig. 7.2). Conversely, temperature abuse during storage of a food can exacerbate problems. Listeria monocytogenes is more resistant than many other vegetative bacteria to some, but not all, of the preservation mechanisms used in food manufacture (e.g. chilling, reduced water activity) and these have been reviewed by Walker (1990). Whilst resistance may be noted to these preservation systems when examined individually, foods are complex and interactions may occur which effectively prevent growth. The use of predictive models (see Section 7.9) for microbiology is an efficient method to identify such interactions. L. monocytogenes is not considered to be a classically heat-resistant bacterium. It is generally accepted that conventional HTST milk pasteurisation (71.7oC/15 seconds) will eliminate this microorganism when freely suspended in milk (Bradshaw et al. 1991). In other foods, decimal reduction times of 8–16 seconds have been reported at 70oC (Gaze et al. 1989). It has been recommended (Anon. 1989) that foods subject to a cook-chill process be heated to a minimum of 70oC for 2 minutes (or the thermal equivalent) to ensure the effective 168 Chilled foods elimination of this bacterium. Overall, the control of L. monocytogenes in foods and food environments (Holah 1999) is of concern to food processors. Yersinia enterocolitica Like L. monocytogenes, Y. enterocolitica was first described over 50 years ago (Schliefstein and Coleman 1939) but largely ignored as an agent of food-borne disease until the 1970s. Outbreaks of disease have implicated chilled foods such as pasteurised milk (Tacket et al. 1984), tofu (Tacket et al. 1985) and chocolate milk (Black et al. 1978). Whilst the reported incidence of Y. enterocolitica in gastrointestinal samples is generally low, it has increased, but as before, this may be due not only to a true underlying increase but also to a greater awareness of this bacterium, recognition of symptoms and improved methodologies. In some countries (e.g. Belgium and the Netherlands) disease by Y. enterocolitica has surpassed that of Shigella and even rivals that of Salmonella (Doyle 1990). The symptoms of human yersiniosis are protean (Schiemann 1989). Overall, acute gastroenteritis is the most common symptom, particularly with children, and is characterised by diarrhoea, abdominal pain, fever and less commonly, vomiting. With adolescents, abdominal pain may be localised in the right iliac fossa area of the body and misdiagnosed as appendicitis. In the outbreak involving chocolate milk, 17/257 (6.6%) of the cases had their appendix removed (Black et al. 1978). The mortality rate from human yersiniosis is low Fig. 7.2 Effect of temperature on the generation time of L. monocytogenes and Y. enterocolitca Chilled foods microbiology 169 and, other than cases involving appendectomies, the symptoms are generally self-limiting and rarely require treatment (Schiemann 1989). With adults, secondary symptoms may occur several weeks after the typical gastrointestinal symptoms disappear. Most commonly these are post-infectious polyarthritis and erythema nodosum (Schiemann, 1989). A wide variety of foods have been reported to be contaminated with Y. enterocolitica including many chilled products, i.e. raw and cooked meats, poultry, seafoods, milk, dairy products and vegetables (Greenwood and Hooper 1989). Care is needed as the isolates responsible for disease generally belong to a few specific bio-serotypes, whilst those from foods and the environment belong to a wide range of bio-serotypes (Gilmour and Walker 1988, Logue et al. 1996). Therefore the pathogenic significance of food isolates should be ascertained before the food is condemned as a health risk. The bio-serotypes responsible for human disease are frequently isolated from pigs and occasionally pork products (Schiemann 1989). The minimum reported growth temperature for Y. enterocolitica is C01.3oC and the bacterium grows relatively well at chill temperatures (Walker et al. 1990b). As with L. monocytogenes, reducing the storage temperature has an increasingly dramatic effect on the growth of Y. enterocolitica (Fig. 7.2). Furthermore, storage at refrigeration temperatures will interact with other preservation factors present in foods to prevent growth of this bacterium. Factors affecting the growth of Y. enterocolitica have been discussed by Walker and Stringer (1990). Y. enterocolitica is a heat-sensitive bacterium and will be readily eliminated from foods by heating (Lovett et al. 1982). It has, however, been reported from cooked meats, seafoods and pasteurised dairy products, which indicates that post-process contamination had occurred. Greater attention is required for the environmental control of Y. enterocolitica in food manufacturing establishments. As yet, relatively little is published on this aspect. Aeromonas hydrophila The role of A. hydrophila as an agent of food-borne disease is still a matter of controversy as no fully documented outbreaks have been reported. This bacterium however, does possess many of the characteristics of other pathogenic bacteria (Cahill 1990). As with Y. enterocolitica, the number of reported cases of A. hydrophila gastroenteritis in England and Wales has risen during the 1980s (Anon. 1991a). The reasons for this are as described previously. Incidents of food-borne disease implicating A. hydrophila have included oysters and prawns (Todd et al. 1989) – both chilled foods. Within the genus Aeromonas, some of the other major motile species (i.e. A. hydrophila, A. sobria and A. caviae) may be considered to be pathogenic (Stelma 1989). All three of these species have been isolated from a variety of chilled foods (Abeyta and Wekell 1988, Fricker and Tompsett 1989). The minimum reported growth temperature for A. hydrophila is C00.1 to 1.2oC (four strains tested) and so growth will occur at chill temperatures (Walker and 170 Chilled foods Stringer 1987). As with the previous psychrotrophic pathogens, temperature control is important and temperature abuse will greatly increase the rate of growth. Relatively little is published about the heat resistance of A. hydrophila but the bacterium is considered to be heat-sensitive and so may be readily eliminated from foods (Palumbo and Buchanan 1988). The effects of other factors (e.g. pH, salt, preservatives etc.) on the growth of A. hydrophila have been reviewed by Palumbo and Buchanan (1988). Little has been published on the presence of A. hydrophila in the processing environment, but it is likely that it will be isolated particularly from wet areas. Bacillus cereus The role of B. cereus as a spoilage bacterium of chilled foods is well recognised (Griffiths and Phillips 1990). Many such strains may grow at temperatures as low as 1oC (Coghill and Juffs 1979). This bacterium may also cause food-borne disease but the number of reported cases is generally low (Border and Norton 1997). The minimum reported growth temperatures of these strains is usually 10–15oC (Goepfert et al. 1972, Johnson 1984) although some isolates from outbreaks which involved vegetable pie, pasteurised milk and cod were able to grow and produce toxins at 4oC (van Netten et al. 1990, Jaquette and Beuchat 1998). In addition, psychrotrophic, presumptively enterotoxigenic strains were frequently isolated from pasteurised milks and some cook-chill meats (van Netten et al. 1990). If temperature abuse of the product occurred, the time until the toxin was detected was reduced by 50% when the temperature was raised from 4 to 7oC. Bacillus cereus may be of particular significance in foods which have been heated or pasteurised, as the heat treatment may have eliminated other competitor microorganisms. During subsequent chilled storage, spores which may survive the heat treatment may germinate and grow. Although little published information is available, the heat resistance of psychrotrophic B. cereus (and other related species) is generally lower than that of the mesophilic strains (Reinheimer and Bargagna 1989). Other Bacillus species (i.e. B. subtilis and B. licheniformis) may also cause human disease (Kramer and Gilbert 1989). Although psychrotrophic strains of these have been isolated from milk, their association with human disease is at present unclear. Clostridium botulinum Human botulism is caused by the ingestion of a neurotoxin, and, based on the antigenic analysis of this, seven types can be distinguished (named A–G) (Hauschild 1989). Traditionally, food-borne disease was caused by types A and B. It is now well recognised that types E and F may also cause disease following the ingestion of preformed toxin. The strains responsible for disease can be divided into two main groups. Firstly, types A and some strains of B and F are proteolytic and so often cause putrefaction of foods if substantial growth occurs (Hauschild 1989). Secondly, types E and others of B and F are non-proteolytic and so the consequences of growth in foods will be less pronounced (Hauschild, Chilled foods microbiology 171 1989). The minimum growth temperature of the mesophilic proteolytic strains is considered to be 10oC and so these are of limited significance with chilled foods. In 1961, Schmidt et al. reported that type E Cl. botulinum was able to grow and produce toxin in a beef stew after incubation at 3.3oC for 32 days. It is now recognised that non-proteolytic strains of types B and F are also capable of growth and toxin production at 5oC or less (Ecklund et al. 1967, Simunovic et al. 1985). Therefore, these non-proteolytic strain may grow in chilled foods. The growth of non-proteolytic Cl. botulinum is of particular concern in ‘sous- vide’ processing. This consists of packing foods under vacuum in air- impermeable sealed bags which are then heat processed and stored chilled for extended periods. Whilst the time and temperature of cooking is specific to the food type, it will destroy vegetative microbial cells but may not be sufficient to destroy bacterial spores. These may subsequently germinate and grow in the absence of air during refrigerated storage (Betts 1992). It should be noted that the heat resistance of the psychrotrophic non- proteolytic strains is considerably lower than that of the mesophilic proteolytic strains (Table 7.3). The risk of botulism from ‘sous-vide’ products can be minimised by the use of an appropriate time–temperature profile during heating, adequately controlled chilled storage and/or alterations in the product formulation to prevent growth (Betts 1992, Betts 1996). The minimum pH and a w values for growth also differ between the proteolytic and non-proteolytic strains (Table 7.3). Overall, the non-proteolytic strains are less resistant to low pH and a w values (Hauschild 1989). Microorganisms capable of initiating growth at temperatures of 5–10oC There are a number of other pathogenic bacteria which, although unable to grow at temperatures below 5oC, may grow if temperature abuse occurs. These include Salmonella species, Escherichia coli and Staphylococcus aureus,with generally accepted minimum growth temperatures of 5.1, 7.1 and 7.7oC respectively (Alcock 1987, Angelotti et al. 1961) At temperatures up to 10oC, the growth rate of these bacteria is generally slow (Matches and Liston 1968). These bacteria do, however, cause food-borne disease, frequently implicating Table 7.3 Comparison of proteolytic and non-proteolytic strains of Clostridium botulinum (Betts 1992, Hauschild 1989) Cl. botulinum Proteolytic Non-proteolytic Minimum temperature 10–12oC 3.3–5.0oC Minimum pH 4.6 5.0 Maximum salt 10% 5–6.5% Minimum a w 0.93 0.95–0.97 D value at 100oC for spores 25 min. C600.1 min 172 Chilled foods chilled foods. Psychrotrophic strains of salmonellae have very occasionally been reported and this may be of more concern with regard to the public health issues of chilled foods (d’Aoust 1991). Several types of E. coli are well recognised as agents of food-borne disease. At present the type of most concern is E. coli 0157:H7 and other verocytotoxigenic E. coli (VTEC) which may produce severe haemorrhagic colitis (Kaper and O’Brien 1998). Limited growth of some strains may occur at 5–10oC (Alcock 1987, Kauppi et al. 1998). This organism has been reviewed by Bell and Kyriakides (1998a). Whilst Staph. aureus may grow at temperatures as low at 7.7oC, disease is caused by the ingestion of a preformed toxin. The minimum temperature for toxin production is greater than for growth and has been reported to be 14.3oC (Alcock 1987). Overall, the bacterial species above do not grow at temperatures below 5oC, but may survive at these temperatures. Often, pathogens and spoilage bacteria will survive adverse conditions (e.g. low pH or high salt) better at refrigeration temperatures compared with higher temperatures (Faith et al. 1998). Therefore, if the infectious dose of the bacterium is low and/or growth of the pathogen has already occurred (e.g. during slow cooling), growth during chilled storage may not be a prerequisite for disease. Microorganisms capable of initiating growth at temperatures greater than 10oC These species include mesophilic Cl. botulinum, mesophilic B. cereus and other Bacillus species, Cl. perfringens and Campylobacter species. In general, these will not grow below 10oC and growth is limited at temperatures between 10 and 15oC (Walker and Stringer 1990). Of particular concern in this bacterial group are the Campylobacter species which comprise the most commonly reported cause of gastrointestinal disease in the UK (Border and Norton 1997). Although many of the reported cases are sporadic, outbreaks have frequently implicated the consumption of raw milk and undercooked chicken (Skirrow 1990). This bacterial group is unusual as the minimum temperature for growth is 25–30oC and so it will not grow on most foods. The infectious dose of the microorganism is very low and so growth may not be necessary for disease to occur (Butzler and Oosterom 1991). Whilst disease caused by the mesophilic spore-forming bacteria has implicated chilled foods, this is usually as a consequence of poor temperature control during cooling after cooking (Gould and Russell 1991, Shaw 1998). These bacteria may grow extremely rapidly during a long slow cooling regime after cooking and then persist during chilled storage. 7.8 Temperature control With chilled foods, good temperature control is essential, not only to maintain the microbiological safety and quality of foods, but also to minimise changes in Chilled foods microbiology 173 the biochemical and physical properties of the food. The temperatures of storage of chilled foods may vary greatly during manufacture, distribution, retail sale and in the home. Consequently, during the life of a chilled food, considerable opportunities exist for temperature abuse to occur. The greater the abuse of temperature, then the greater the potential for microbial growth to occur. This may result in a product becoming unsafe and/or a loss in product quality. Temperature control is the key issue with regard to chilled foods and an integral part of the preservation system. In many of the stages in the food chain after primary chilling, the refrigeration equipment is designed to maintain the product temperature. It may not be able rapidly to reduce the temperature of foods that have been abused at higher temperature. 7.9 Predictive microbiology As discussed previously, chill temperatures will not prevent microbial growth completely and additional preservative factors, such as reduced pH and water activity, may be required to extend the time period before significant microbial growth occurs. Traditionally, the effect of combinations of preservation systems on target organisms would have been tested using laboratory studies (often called challenge tests). Whilst challenge tests have an important role, they tend to be expensive, time consuming and results obtained are limited to the specific conditions tested. Should any of these change, the test needs to be repeated. However, the chilled foods market is very dynamic and there is a great demand for continual development of new products (Stringer and Dennis 2000). These need to be developed and marketed rapidly. Predictive microbiology is a tool which can provide rapid reliable answers concerning the likely growth of specific organisms under defined conditions, including conditions not previously examined. Models can be used to predict the probability of growth, the time until growth occurs or the growth rate of microorganisms. The use of predictive models to describe the microbial kinetics is not new and reference to these techniques can be found in publications dating from the 1920s (Esty and Meyer 1922). Microbiological modelling has been reviewed by Gould (1989a) and McMeekin et al. (1993). The development of a microbiological model generally uses the following stages: ? careful selection and appropriate preparation of the target microorganism ? inoculation of the target microorganism into a growth medium (micro- biological media or food) with defined characteristics ? storage of the medium under controlled conditions ? sampling of the medium for the target microorganism at relevant intervals ? construction of a model to describe the target microorganism’s response ? validation of the model’s predictions – preferably in food to ensure the predictions are meaningful ? refinement or further enhancement of the model. 174 Chilled foods The types of models which have been used vary greatly and include the Arrhenius equation, non-linear Arrhenius (Schoolfield) models, Be?lehra′dek-type (Ratkowsky or square root) models, polynomial models, mechanistic models (all reviewed by McMeekin et al. 1993) and a dynamic modelling approach (Baranyi and Roberts 1994). 7.9.1 Food pathogens Over the past decade, there has been considerable work done on predictive modelling of a wide range of pathogenic bacteria, e.g. kinetic growth models have been published for Salmonella (Gibson et al. 1988), L. monocytogenes (Farber et al. 1996), Cl. botulinum (Graham et al. 1996). In order to make such models accessible to food manufacturers, there is a requirement for them to be packaged as user-friendly software. There are two main systems currently available for predicting the growth of food pathogens. In the UK, the Food MicroModel system is the largest and most comprehensive system. It was developed from a Ministry of Agriculture Fisheries and Food sponsored research programme and the software is available for purchase from Leatherhead Food Research Association (Leatherhead, UK). There is an extensive range of pathogen models in the system including those shown in Table 7.4. The models in the Food MicroModel system were produced from data obtained in laboratory growth media and validated by comparing predictions from the model with data obtained from the literature or obtained from inoculated food studies. Another comprehensive modelling programme has been produced in the USA by the United States Department of Agriculture (USDA). It is called the Pathogen Modeling Program and was designed by Dr Robert L. Buchanan and Dr Richard Whiting. The models in this system include those shown in Table 7.5. This programme is available free of charge and can be obtained from the internet (http://www.arserrc.gov). The models in this programme have been produced from extensive growth data in laboratory media, but have not been validated in foods. Table 7.4 Some pathogen models Growth models Thermal death models Aeromonas hydrophila Cl. botulinum Bacillus cereus E. coli O157:H7 Clostridium botulinum L. monocytogenes Clostridium perfringens Salmonella Escherichia coli O157:H7 Y. enterocolitica Listeria monocytogenes Salmonella Staphylococcus aureus Yersinia enterocolitica Chilled foods microbiology 175 7.9.2 Food spoilage With regard to the modelling of food spoilage organisms, there are few systems available although many individual models have been published. Work in Tasmania has developed Pseudomonas predictor models applicable to milk and raw meats (McMeekin and Ross 1996). Campden and Chorleywood Food Research Association (CCFRA) has developed a collection of models which can be used to assess spoilage rates or likely stability of foods, including chilled foods. This collection of models is called Forecast and is available to potential users via an enquiry service (+44 (0) 1386 842000) which runs the model on behalf of clients after a detailed consultation with respect to their needs. The consultancy aspect of this approach also allows subsequent expert interpretation and consideration of model validation status. Table 7.6 shows the range of models currently available within Forecast. All models within the Forecast system have been produced from data obtained in laboratory media and have been validated in relevant foods using literature data or inoculated challenge test studies. Limited models on spoilage organisms are available in the Food MicroModel programme previously mentioned and these include: Brochothrix thermosphac- ta, Saccharomyces cerevisiae, Lactobacillus plantarum, Zygosaccharomyces bailii. In addition to bacteria and yeasts, models have also been developed for mould growth (Val?′k et al. 1999). Furthermore, Membre′ and Kubaczka (1998) have applied similar models to product degradation (i.e. pectin breakdown) rather than just microbial growth. Table 7.5 Some models in the Pathogen Modeling Program (USDA) Growth models Survival models A. hydrophila E. coli O157:H7 B. cereus L. monocytogenes E. coli O157:H7 Salmonella Salmonella spp. S. aureus Shigella flexneri S. aureus Y. enterocolitica Table 7.6 Current options for CCFRA Forecast models Model pH Salt (% w/v) Temperature oC Bacillus spp. 4.0–7.0 0.5–10.0 5–25 Pseudomonas spp. 5.5–7.0 0.0–4.0 0–15 Enterobacteriaceae 4.0–7.0 0.5–10 0–30 Yeasts (chilled) 2.5–6.3 0.5–10 1–22 Lactic acid bacteria 2.9–5.8 0.5–10 2–30 176 Chilled foods 7.9.3 Practical application of models Figure 7.3 shows how a model has been put to practical use by comparing predicted values of numbers against predetermined standards for termination of shelf-life. Many other potential applications exist, for example: ? What level of microorganisms will be present under different temperatures of storage? ? How much salt is needed to restrict microbial numbers to a pre-set level after one week storage at 8oC? ? What will be the effect of increasing the product pH from 5.0 to 5.4? Several authors have reported deficiencies or inaccuracies in model predictions (Dalgaard and J?rgensen 1998, Hygtia¨ et al. 1999) in that they predict faster growth than that observed in foods. However, many of the models, particularly those for pathogens, are designed to be ‘fail safe’ and foods may contain additional antimicrobial factors not present in the model, which may inhibit or prevent the predicted growth. Consequently, it is important to determine that any models used contain the important preservation factors relevant to the study and that the model has been validated in appropriate foods. Most of the models developed have been based on single organisms or groups of organisms in pure cultures and may not therefore take into account any effects of Fig. 7.3. A graphical representation of predictions made using CCFRA forecast conditions: pH 6.0, salt 3% w/v, temperature of storage 6oC. The user’s tolerance for enterobactericeae, Pseudomonas spp. and Bacillus spp. are clearly shown in relation to the predicted shelf-life. Chilled foods microbiology 177 microbial interaction and competition likely to be seen in foods. Pin and Baranyi (1998) have used modelling techniques to examine the interactions between spoilage bacteria. In the wrong hands, the information from predictive models may be misused and may have serious consequences. It is important that the right questions be asked in order to obtain useful information. There are many advantages to the use of predictive models in the development and manufacture of chilled foods. They can help to focus resources during product development to assess the microbiological safety and stability of hundreds of different ingredient combinations before stepping into the development kitchen. Predictive models can be used as decision-making tools to allow productive focusing of effort in process and product development and risk and hazard assessment. They can be of great value in complex HACCP studies if used correctly. They should be followed up with targeted practical trials and challenge tests. Used in this way, predictive models can be powerful tools for industrial food microbiologist. Recently, several workers have proposed the development of predictive models with computational neural networks (Hajmeer et al. 1997) and their incorporation in decision support systems for microbiological quality and safety (Wijtzes et al. 1998). Predictive models also have a role to play in education and training, in that they allow demonstration of microbial behaviour and risk without the need for expensive laboratory exercises. It should be stressed that microbiological models will never completely remove the requirements for microbiological expertise or to conduct microbiological challenge tests and shelf-life studies, but can be very useful for an indication of the safety and stability of chilled products and ingredients. 7.10 Conclusions Chilled foods comprise a diverse and complex group of commodities which contain a large number of ingredients. The composition and number of microorganisms present is affected by the indigenous microflora, microorgan- isms contaminating before and after processing, the growth rates and abilities of the microorganisms, the spoilage abilities of the microorganisms, the intrinsic properties of the food, the effects of processing and packaging, and the time and temperatures of storage. Consequently, the microbial safety and spoilage of chilled foods is very complex, but certain general principles may be applied. 1. The microbiological status of all raw materials should be known and only materials of good quality used. 2. All stages of processing should be defined, monitored and controlled to ensure their correct operation. This is of particular significance in foods which rely on a combination of factors to ensure microbial stability. 3. The temperatures and times of chill storage should be controlled during all stages, from raw materials through retail sale and preferably to the home. 178 Chilled foods The lower the temperature throughout the process, the slower the rate of growth. 4. Attention must be given to the hygiene of the entire process to ensure that microbial contamination is minimised. These objectives may be best achieved through the application of a quality system including Hazard Analysis Critical Control Points (HACCP) (Leaper 1997) which may be powerfully integrated with other systems, including risk analysis (Jouve et al. 1998). The use of appropriate and validated models may greatly help in the decision-making processes of HACCP and risk analysis. Finally, greater education of all involved in food manufacture, distribution and retail sale and better education of the consumer in areas of hygiene and temperature control will be of great benefit. 7.11 References ABEYTA C and WEKELL M M, (1988) Potential sources of Aeromonas hydrophila. J. Food Safety 9 11–22. ADVISORY COMMITTEE ON THE MICROBIOLOGICAL SAFETY OF FOOD (ACMSF), (1995) Report on Verocytotoxin-producing Escherichia coli. HMSO, London. ALCOCK S J, (1984) Growth characteristics of food-poisoning organisms at suboptimal temperatures. II Salmonellae, Campden Food Preservation Research Association Technical Memorandum No 364. ALCOCK S J, (1987) Growth characteristics of food-poisoning organisms at suboptimal temperatures, Campden Food Preservation Research Associa- tion Technical Memorandum No. 440. ANGELOTTI R, FOTER M J and LEWIS K H, (1961) Time–temperature effects of salmonellae and staphylococci in foods, Am. J. Pub. Health, 36 559–63. ANON, (1989) Guidelines for Cook-Chill and Cook-Freeze Catering Systems, HMSO, London. ANON, (1991a) The Microbiological Safety of Food, Part II HMSO, London. ANON, (1991b) Principles and Practices for the Safe Processing of Foods, Butterworth-Heinemann, Oxford. BARANYI J and ROBERTS T A, (1994) A dynamic approach to predicting bacterial growth in food, International Journal of Food Microbiology, 23 277–94. BAYNE H G and MICHENER H D, (1979) Heat resistance of Byssochlamys ascospores Appl. Environ. Microbiol., 37 449–53. BELL C and KYRIAKIDES A, (1998a) E. coli: a practical approach to the organism and its control in foods, Blackie Academic and Professional, London. BELL C and KYRIAKIDES A, (1998b) Listeria: a practical approach to the organism and its control in foods, Blackie Academic and Professional, London. BETTS G D, (1992) The microbiological safety of sous-vide processing, Campden and Chorleywood Food Research Association Technical Manual No. 39. Chilled foods microbiology 179 BETTS G D, (1996) A code of practice for the manufacture of vacuum and modified atmosphere packaging chilled foods, Campden and Chorleywood Food Research Association, CCFRA Guideline No. 11. BLACK R E, JACKSON R L, TSAI T, MEDVESKY M, SHAYEGANI M, FEELEY J C, MACLEOD K I E, and WAKELEE A W, (1978) Epidemic Yersinia enterocolitica infection due to contaminated chocolate milk, New Engl. J. Med., 298 7679. BORCH E, KANT-MUERMANS M-L, and BLIXT, Y. (1996) Bacterial spoilage of meat and cured meat products. International Journal of Food Microbiology 33 103–20. BORDER P and NORTON M, (1997) Safer eating: microbiological food poisoning and its prevention. The Parliamentary Office of Science and Technology, London. BRADSHAW J G, PEELER J T and TWEDT R M, (1991) Thermal resistance of Listeria spp. in milk, J. Food Prot., 54 12–4. BUTZLER J P and OOSTEROM J, (1991) Campylobacter: pathogenicity and significance in foods, Int. J. Food Microbiol., 12 1–8. CAHILL M M (1990) Virulence factors in motile Aeromonas species: a review, J. Appl. Bacteriol., 69 1–16. COGHILL D and JUFFS H S, (1979) Incidence of psychrotrophic spore-forming bacteria in pasteurised milk and cream products and effect of temperature on their growth, Australian J. Dairy Technol., 3 150–3. CONNER D E and KOTROLA J S, (1995) Growth and survival of Escherichia coli O157:H7 under acidic conditions, Applied and Environmental Microbiol- ogy, 61 382–5. COUSIN M A, (1982) Presence and activity of psychrotrophic microorganisms in milk and dairy products: a review, J. Food Prot., 45, 172–207. COX L J, KLEISS T, CORDIER J L, CORDELLANA C, KONKEL P, PEDRAZZINI C, BEUMER R and SIEBENGA A, (1989) Listeria spp. in food processing, non-food processing and domestic environments, Food Microbiol, 6 49–61. DAINTY R H, (1996) Chemical/biochemical detection of spoilage, International Journal of Food Microbiology, 33 19–34. DALGAARD P and J?RGENSEN L V, (1998) Predicted and observed growth of Listeria monocytogenes in seafood challenge tests and naturally contami- nated cold smoked salmon, International Journal of Food Microbiology, 40 105–15. DAY B P F, (2000) Chilled food packaging. In: Stringer, M. F. and Dennis, C. (ed.), Chilled Foods: a comprehensive guide, 2nd edn. Woodhead Publishing Ltd., Cambridge, pp. 137–50. D’AOUST J Y, (1991) Psychrotrophy and foodborne Salmonella, Int. J. Food Microbiol., 13 207–16. DENG Y, RYU J H and BEUCHAT L R, (2000) Tolerance of acid adopted and non- adopted Escherichia coli O157:H7 cells to reduced pH as affected by type of acidulant. Journal of Applied Microbiology, 86 203–10. DOYLE M P, (1990) Pathogenic Escherichia coli, Yersinia enterocolitica and Vibrio parahaemolyticus, The Lancet, 336 1111–15. 180 Chilled foods ECKLUND M W, WIELER D L and POYSKY F T, (1967) Outbreak and toxin production of non-proteolytic type B Clostridium botulinum at 3.3 to 5.6oC, J. Bacteriol, 93 1461–2. EDDY B P, (1960) The use and meaning of the term ‘psychrophilic’, J. Appl. Bacteriol., 23 189–90. ESTY J R and MEYER K F, (1992) The heat resistance of spores of Cl. botulinum and allied anaerobes, J. Infect. Dis., 31 650–63. FAITH N G, WIERZBA R K, IHNOT A M, ROERING A M, LORANG T D, KASPER, C W and LUCHANSKY J B (1998) Survival of Escherichia coli O157 in full and reduced fat pepperoni after manufacture of sticks, storage of slices at 4o or 21oC under air vacuum and baking of slices on frozen pizza at 135, 191 and 246oC, Journal of Food Protection, 61 383–9. FARBER J M, CAI Y and ROSS W H, (1996) Predictive modelling of the growth of Listeria monocytogenes in CO 2 environments, International Journal of Food Microbiology, 32 133–44. FILTENBORG O, FRISVAD J C and THRANE U, (1996) Moulds in food spoilage. International Journal of Food Microbiology, 33 85–102. FRICKER C R and TOMPSETT S, (1989) Aeromonas spp. in foods: a significant cause of food poisoning, Int. J. Food Microbiol., 9 17–23. GARDNER G A, (1981) Brochothrix thermosphacta (Microbacterium thermo- sphactum) in the spoilage of meats: a review. In: Roberts, T. A. et al. (eds) Psychrotrophic Microorganisms in Spoilage and Pathogenicity, Academic Press, London, pp. 139–73. GARDNER G A, (1983) Microbial spoilage of cured meats. In: Roberts, T. A. and Skinner, F. A. (eds) Food Microbiology: Advances and Prospects, Academic Press, London pp. 179–202. GAZE J E, (1992) Food pasteurisation treatments, Campden Food & Drink Research Association Technical Manual No. 27. GAZE J E, BROWN G D, GASKELL D E and BANKS J G, (1989) Heat resistance of Listeria monocytogenes in homogenates of chicken, beef steak and carrot, Food Microbiol., 6 251–59. GEORGE S M, LUND B M and BROCKLEHURST T F, (1988) The effect of pH and temperature on initiation of growth of Listeria monocytogenes, Letters in Appl. Microbiol., 6 153–6. GIBSON A M, BRATCHELL N and ROBERTS T A, (1988) Predicting microbial growth: growth responses of salmonellae in a laboratory medium as affected by pH, sodium chloride and storage temperature, Int. J. Food Microbiol., 6 155–78. GILL C D and MOLIN G, (1991) Modified atmospheres and vacuum packaging. In: Russell, N. J. and Gould, G. W. (eds), Food Preservatives, Blackie and Son Ltd., Glasgow, pp. 172–99. GILL C O, (1983) Meat spoilage and evaluation of the potential storage life of fresh meat, J. Food Prot., 46, 444–52. GILMOUR A and WALKER S J, (1988) Isolation and identification of Yersinia enterocolitica and Yersinia enterocolitica-like bacteria J. Appl. Bacteriol. Suppl., 65 213S–236S. Chilled foods microbiology 181 GLASS K A and DOYLE M P, (1991) Relationship between water activity of fresh pasta and toxin production by proteolytic Clostridium botulinum., J. Food Prot., 54 162–5. GOEPFERT J M, SPIRA W M and KIM H U, (1972) Bacillus cereus food poisoning: a review. J. Milk Food Technol. 35, 213–27. GOULD G, (1989a). Predictive modelling of microbial growth and survival in foods, Food Sci. Technol. Today, 3 89–92. GOULD G W, (1989b) Heat-induced injury and inactivation. In: Gould, G. W. (ed.), Mechanisms of Action of Food Preservation Procedures, Elsevier Appl. Sci. London, pp. 11–42. GOULD G W, (1996) Methods for preservation and extension of shelf-life, International Journal of Food Microbiology, 33 51–64. GOULD G W and JONES M V, (1989) Combination and synergistic effects. In: Gould, G W (ed.), Mechanisms of Action of Food Preservation Procedures, Elsevier Appl. Sci., London, pp. 400–21. GOULD G W and RUSSELL N J, (1991) Major food-poisoning and food-spoilage microorganisms. In: Russell, N. J. and Gould, G. W. (eds), Food Preservatives, Blackie and Son Ltd., Glasgow, pp. 1–21. GRAHAM A F, MASON D R and PECK M W, (1996) Predictive model of the effect of temperature, pH and sodium chloride on growth from spores of non- proteolytic Clostridium botulinum, International Journal of Food Micro- biology, 31 69–85. GRAM L and HUSS H H, (1996) Microbiological spoilage of fish and fish products, International Journal of Food Microbiology, 53 121–38. GREENWOOD M H and HOOPER W L, (1989) Improved methods for the isolation of Yersinia species from milk and foods Food Microbiol., 6 99–104. GRIFFITHS M W and PHILLIPS J D, (1990) Incidence, source and some properties of psychrotrophic Bacillus spp. found in raw and pasteurized milk, J. Soc. Dairy Technol, 43 62–6. GUTIE ′ RREZ R, GARE ′ IA T, GONZALEZ I, SANZ B, HERNA ′ NDEZ P E and MARTIN R, (1997) A quantitative PCR-ELISA for the rapid enumeration of bacteria in refrigerated raw milk, Journal of Applied Microbiology, 83 518–23. HAJMEER M N, BASHEER I A and NAJJAR Y M, (1997) Computational neural networks for predictive microbiology II. Application to microbial growth, International Journal of Food Microbiology, 34 51–66. HAUSCHILD A H W, (1989) Clostridium botulinum. In: Doyle, M. P. (ed.), Foodborne Bacterial Pathogens, Marcel Dekker, New York, pp. 111–89. HERBERT R A, (1989) Microbial growth at low temperatures. In: Gould, G. W. (ed.), Mechanisms of Action of Food Preservation Procedures, Elsevier Appl. Sci., London, pp. 71–96. HOLAH J T, (1999) Effective microbiological sampling of food processing environments, Campden and Chorleywood Food Research Association, Guideline No. 20. HOLAH J T, TAYLOR J and HOLDER J S, (1993) The spread of Listeria by cleaning systems, Campden Food & Drink Research Association Technical 182 Chilled foods Memorandum No. 673. HUIS IN’T VELD J H T, (1996) Microbial and biochemical spoilage of foods: an overview, International Journal of Food Microbiology 33,1–18. HYGTIA ¨ E, HIELM S, MOKKILA M, KINNUNEN A and KORKEALA H, (1999) Predicted and observed growth and toxigenesis by Clostridium botulinum type E in vacuum-packaged fishery product challenge tests, International Journal of Food Microbiology, 47 161–9. JAQUETTE C B and BEUCHAT L R, (1998) Combined effects of pH, nisin and temperature on growth and survival of psychrotrophic Bacillus cereus, Journal of Food Protection, 61 563–70. JAY J M, (1978) Modern Food Microbiology, 2nd edn. D. van Nostrand Co., New York. JOHNSON K M, (1984) Bacillus cereus foodborne illness – an update, J. Food Prot., 47 145–53. JOUVE J L, STRINGER M F and BAIRD-PARKER A C, (1998) Food Safety Management Tools, ILSI – Europe, Brussels. KABARA J J and EKLUND T, (1991) Organic acids and their esters. In: Russell N J and Gould G W (eds), Food Preservatives, Blackie and Son Ltd., Glasgow, pp. 44–71. KAPER B and O’BRIEN A D, (1998) Escherichia coli O157:H7 and other Shiga toxin-producing E. coli strains, ASM Press, Washington. KAUPPI K L, O’SULLIVAN D J and TATINI S R, (1998) Influence of nitrogen source on low temperature growth of verocytotoxigenic Escherichia coli, Food Microbiology, 15 355–64. KRAMER J M and GILBERT R J, (1989) Bacillus cereus and other Bacillus species. In: Doyle, M P (ed.), Foodborne Bacterial Pathogens, Marcel Dekker, New York, pp. 21–69. LEAPER S, (1997) HACCP: a practical guide (2nd edn), Campden and Chorleywood Food Research Association Technical Manual, No. 38. LOGUE C M, SHERIDAN G, WAUTERS G, MCDOWELL D A and BLAIR I S, (1996) Yersinia spp and numbers, with particular reference to Y.enterocolitica occurring on Irish meat and meat products, and the influence of alkali treatment on their isolation, International Journal of Food Microbiology, 33 257–74. LOVETT L, BRADSHAW J G and PEELER J T, (1982) Thermal inactivation of Yersinia enterocolitica in milk Appl. Environ. Microbiol., 44 517–19. LU ¨ CKEFKand EARNSHAW R G, (1991) Starter cultures. In: Russell, N J and Gould, G W (eds), Food Preservatives, Blackie and Son Ltd., Glasgow, pp. 215–34. LUND B M, (1990) The prevention of foodborne listeriosis, Br. Food J., 92 13–22. MCLAUCHLIN J A, (1987) A review: Listeria monocytogenes, recent advances in the taxonomy and epidemiology of listeriosis in humans, J. Appl. Bacteriol., 63,1–2. MCMEEKIN T A and ROSS T, (1996) Shelf-life prediction: status and future possibilities, International Journal of Food Microbiology, 31 65–84. Chilled foods microbiology 183 MCMEEKIN T A, OLLEY J N, ROSS T and RATKOWSKY D A, (1993) Predictive microbiology: theory and application, Research Studies Press, Somerset. MATCHES J R and LISTON J, (1968) Low temperature growth of Salmonella, J. Food Sci., 33 641–5. MEMBRE ′ JMand KUBACZKA M, (1998) Degradation of pectin compounds during pasteurized vegetable juice spoilage by Chryseomonas luteola: a predictive microbiology approach, International of Food Microbiology, 42 159–66. MICHENER H D and ELLIOTT R P, (1964) Minimum growth temperatures for food poisoning, fecal indicator and psychrophilic microorganisms, Adv. in Food Res., 13 349–96. MITSCHERLICH E and MARTH E H, (1984) Microbial Survival in the Environment, Springer-Verlag, Berlin. MORITA R Y, (1973) Psychrophilic bacteria, Bacteriol. Rev., 39 144–67. MURRAY E G D, WEBB R A and SWAN M B R, (1926) A disease of rabbits characterised by a large mononuclear leucocytosis caused by a hitherto undescribed bacillus Bacterium monocytogenes (n. sp), J. Pathol. Bacteriol, 29 407–39. NEILL S D, (1974) A study of the microflora of raw milk stored at low temperature, PhD Thesis, The Queen’s University of Belfast, Northern Ireland. PALUMBO S A and BUCHANAN R L, (1988) Factors affecting growth or survival of Aeromonas hydrophila in foods, J. Food Safety, 9 37–51. PIN C and BARANYI J, (1998) Predictive models as means to quantify the interactions of spoilage organisms, International Journal of Food Micro- biology, 41 59–72. PITT J I and HOCKING A D (1985) Spoilage of fresh and perishable foods. In: Fungi and Food Spoilage, Academic Press, Sydney, pp. 365–82. PRESCOTT S C and GEER L P, (1936) Observations on food poisoning organisms under refrigeration conditions, Refrigeration Engineering, 32 211–2, 282–3. RALOVICH B S, (1987) Epidemiology and significance of listeriosis in the European countries. In: Schonberg, A. (ed.), Listeriosis: Joint WHO/ROI Consultation on Prevention and Control, Vet. Med. Hefte, Berlin, pp. 51–5. REINHEIMER J A and BARGAGNA M L, (1989). Response of psychrotrophic strains of Bacillus to different heat treatments, Microbiol-Aliments-Nutr., 5 117– 22. RIDELL J and KORKEALA H, (1997) Minimum growth temperatures of Hafnia alvei and other Enterobacteriaceae isolated from refrigerated meat determined with a temperature gradient incubator. International Journal of Food Microbiology 35, 287–92. RUSSELL N J and GOULD G W, (1991) Food preservatives, Blackie, Glasgow. SCHIEMANN D A, (1989) Yersinia enterocolitica and Yersinia pseudotuberculosis. In: Doyle, M. P. (ed.), Foodborne Bacterial Pathogens, Marcel Dekker, New York, pp. 601–72. SCHLIEFSTEIN J I and COLEMAN M B, (1939) An unidentified microorganism resembling B. lignieri and Past. pseudotuberculosis, and pathogenic for 184 Chilled foods man, New York State J. Med., 39 1749–53. SCHMIDT C F, LECHOWICH R V and FOLINAZZO J F, (1961) Growth and toxin production by Type E Clostridium botulinum below 40oF, J. Food Sci., 26 626–34. SCHUCHAT A, SWAMINATHAN B and BROOME C V, (1991) Epidemiology of human listeriosis, Clin. Microbiol. Rev., 4 169–83. SHAW R, (1998) Identification and prevention of hazards associated with slow cooling of hams and other large cooked meats and meat products, Campden and Chorleywood Food Research Association Review No. 8. SIMUNOVIC J, OBLINGER J L and ADAMS J P, (1985) Potential for growth of non- proteolytic types of Clostridium botulinum in pasteurized and restructured meat products: a review, J. Food Prot., 48 265–76. SKIRROW M B, (1990) Campylobacter, The Lancet, 336 921–3. SPERBER W H, (1983) Influence of water activity on foodborne bacteria – a review, J. Food Prot., 46 142–50. STELMA G N, (1989) Aeromonas hydrophila, In: Doyle, M. P. (ed.) Foodborne Bacterial Pathogens, Marcel Dekker, New York, pp. 1–19. STRINGER M F and DENNIS C, (2000) The market for chilled foods, In Chilled Foods: a comprehensive guide, 2nd edn. Woodhead Publishing, Cambridge. TACKET C O, NAVAIN J P, SATTIN R, LOFGREN J R, KONIGSBERG C, RENDTORFF R C, RAUSA A, DAVIS B R and COHEN M L, (1984) A multistate outbreak of infections caused by Yersinia enterocolitica transmitted by pasteurised milk, J. American Med. Assoc., 51 483–6. TACKET C O, BALLARD L, HARRIS N, ALLARD L, NOLAN C, QUAN T and COHEN M L, (1985) An outbreak of Yersinia enterocolitica infections caused by contaminated tofu, American J. Epidemiol., 121 705–11. TERPLAN G, SCHOEN R, SPRINGMEYER W, DEGLE I and BECKER H, (1987) Investigations on incidence, origin and behaviour of Listeria in cheese. In: Scho¨nberg, A. (ed.), Listeriosis – Joint WHO/ROI Consultation on Prevention and Control., Vet. Med. Hefte, Berlin, pp. 98–105. TODD L S, HARDY J C, STRINGER M F and BARTHOLOMEW B A, (1989) Toxin production by strains of Aeromonas hydrophila grown in laboratory media and prawn pure′e, Int. J. Food Microbiol., 9 145–56. VALI ′ K L, BARANYI J and GO ¨ RNER F, (1999) Predicting fungal growth: the effect of water activity on Penecillium roquefortii, International Journal of Food Microbiology, 47 141–46. VAN NETTEN R, VAN DE MOOSDIJK A, VAN HOENSEL P and MOSSEL D A A, (1990) Psychrotrophic strains of Bacillus cereus producing enterotoxin. J. Appl. Bacteriol., 69 73–9. VENKITANARAYANEN K S, FAUSTMAN C, CRIVELLO J F, KHAN M I, HOAGLAND T A and BERRY B W, (1997) Rapid estimation of spoilage bacterial load in aerobically stored meat by a quantitative polymerase chain reaction, Journal of Applied Microbiology, 82 359–64. WALKER S J, (1988) Major spoilage microorganisms in milk and dairy products, J. Soc. Dairy Technol., 41 91–2. Chilled foods microbiology 185 WALKER S J and STRINGER M F, (1987) Growth of Listeria monocytogenes and Aeromonas hydrophila at chill temperatures, Campden Food and Drink Research Association Technical Memorandum No 462. WALKER S J and STRINGER M F, (1990) Microbiology of chilled foods. In: Gormley, T. R (ed.), Chilled Foods – The State of the Art. Elsevier Appl. Sci., Barking, pp. 269–304. WALKER S J, (1990) Listeria monocytogenes: an emerging pathogen. In: Turner, A. (ed.), Food Technology International Europe, Sterling Publications, London, pp. 237–40. WALKER S J, ARCHER P and BANKS J G, (1990a) Growth of Listeria monocytogenes at refrigeration temperatures, J. Appl. Bacteriol., 68 157–62. WALKER S J, ARCHER P and BANKS J G, (1990b) Growth of Yersinia enterocolitica at chill temperatures in milk and other media Milchwissenschaft, 45 503–6. WHITFIELD F B, (1998) Microbiology of food taints. International Journal of Food Science and Technology, 33 31–51. WIJTZES T, VAN’T RIET K, HUIS IN’T VELD J H J and ZWIETERING M H, (1998) A decision support system for the prediction of microbial food safety and quality, International Journal of Food Microbiology, 42 79–90. 186 Chilled foods