Chilled foods microbiology 165 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 & C(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 onsiderable periods. The most common form of spoilage is the production of arge 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 actic acid bacteria are also more resistant than the previously discussed spoilage bacteria to slight reductions in the aw and some Pediococcus species are salt- tolerant. Lactic acid bacteria usually predominate on vacuum-packed product 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 enerally 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 <h some chilled foods(e. g. cheese, yoghurts, some salamis)and are essential for 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, many produce antimicrobial compounds in addition to acids (Lucke 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 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, it3. 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 aw 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