Chilled foods microbiology 161 Doyle 1991). In general, yeasts and moulds are more tolerant than bacteria of low aw 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 redominate on chilled fresh meat. The addition of curing sa. c sodin o Gould(1991). As discussed previously, Pseudomonas species tend 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 hanges 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 lowly and so the time to microbial spoilage is increased. The microflora of vacuum-packed meats is usually dominated by lactic acid or Brochothrix thermosphacta(Borch et al. 1996). In some cases, Entero 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. TheDoyle 1991). In general, yeasts and moulds are more tolerant than bacteria of low aw 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