158 Chilled foods 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, so exudative), both of which are more prone to spoilage thannormal 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, aw 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, aw and preservatives ay 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 hange, but also the composition of the microflora will alter. For example, with freshly drawn milk, the microflora is dominated by Gram-positive cocci and ods, which may spoil the product by souring if stored at warm temperatures. At hill 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- (Huis in't Veld, 1996)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, aw 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, aw 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