160 Chilled foods Table 7.2 Typical minimum pH and aw values for growth of microorganisms(Anon 1991b, Gould 1989, Mitscherlich and Marth 1984, ACMSF 1995) Microorganism Minimum pH Minimum aw Bacillus cereus Campylobacter jejun 306 0.98 Clostridium botulinum(non-proteolytic) Clostridium botulinum(proteolytic) Clostridium perfringens Escherichia coli Escherichia coli o157: H7 3.84.2 Lactobacillus species Many yeasts and mould 0.8-0.6 Yersinia enterocolitica 4.6 Minimum ph with toxin production 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 nd 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 nd 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 ncreases owing to proteolytic activity of the mould, and this has been related to he ability of Listeria monocytogenes to grow in these products, but not in the unripened cheeses(Terplan et al. 1987) Reduced a The aw is a measure of the amount of water available in a food which may be used for microbial growth. As the aw 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 aw 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 ontent. Thus the intrinsic preservation system (i.e. low aw)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 aw of a product may interact with other preservation factors, including temperature, to maintain the safety of chilled foods(Glass andas 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 aw The aw is a measure of the amount of water available in a food which may be used for microbial growth. As the aw 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 aw 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 aw) 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 aw 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 aw values for growth of microorganisms (Anon. 1991b, Gould 1989, Mitscherlich and Marth 1984, ACMSF 1995) Microorganism Minimum pH Minimum aw 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
2.0 0.8–0.6 Yersinia enterocolitica 4.6 0.95 a Minimum pH with toxin production. 160 Chilled foods