162 Chilled foods 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) Ind 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 ersely, many yeasts can grow in the presence or absence of oxygen, ough aerobic growth tends to be more efficient, thereby permitting more rapid growth. 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(pasteuria- 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 produceeffects 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