164 Chilled foods which result in food rots, off-flavours and off-odours ( Jay 1978, Cousin 1982, Gill 1983). Some of the enzymes produced by Pseudomonas species are extremely heat-resistant and may produce long-term defects(e.g. rancidity or ge-gelation) in thermally processed products with extended shelf-lives Although well adapted to grow at chill temperatures, this group tends to be sensitive to other factors such as the presence of salt or preservatives, lack of oxygen, low(<5.5) pH and a low (<0.98)aw. Should these preservation mechanisms be present in a food, the Gram-negative(oxidase positive)rod- shaped bacteria compete less well and other microbial groups may cause spoilage. Vibrio species are unusual as they tolerate relatively high salt levels ind so may cause spoilage on chilled stored bacon and other cured products Photobacterium phosphorum, a very large marine vibrio is the dominant spoilage microorganism in vacuum packaged cod(Gram and Huss 1996 Overall, this group is not heat-resistant and so is readily removed by mild thermal treatments. Their presence in heat processed foods is usually as a consequence of post-process contamination. 2. Coliform/enteric bacteria This bacterial group also consists of Gram-negative rods, but these may be distinguished from the above group by a negative oxi Traditionally, microbiologists have tended to examine for these groups eparately, as their sources, significance and factors affecting growth differ. This group is frequently used as an indicator of inadequate processing or post-process contamination. Compared with the Gram-negative (oxidase positive)rod-shaped bacteria, the coliform-enteric group is generally less well adapted to growth at temperatures of less than 5-10oC although many may grow at temperatures as low as 0C (Ridell and Korkeala 1997). However, they often dominate the flora at temperatures of 8-15C(Huis int Veld 1996, Cousin 1982). The coliform/enteric group is less sensitive to changes in pH compared with the Gram-negative(oxidase positive)rod-shaped bacteria and so of more significance in mild acid products. They are however, generally sensitive to low aw,preservatives, salt and thermal treatments (Jay 1978) The coliform/enteric group do not necessarily require the presence of oxygen for growth. In addition, they have a fermentative metabolism and so may break down carbohydrates to give acids, which may result in souring of milk(Cousi 1982). In contrast, the metabolism of the Gram-negative (oxidase positive) bacteria is oxidative and fermentation does not occur. Other types of spoilage include the production of pigmented growth, gases, slime, off-odours and off- flavours. Off odours have been described as 'grassy, medicinal, unclean and faecal (Walker and Stringer 1990) Citrobacter. Escherichia. Enterobacter Hafnia, Klebsiella, Proteus and Serratia (Jay 1978, Walker 1988).These microorganisms are widely disseminated in the environment, including in animals. Poor slaughter and dressing practices may contribute to their presence in foodswhich result in food rots, off-flavours and off-odours (Jay 1978, Cousin 1982, Gill 1983). Some of the enzymes produced by Pseudomonas species are extremely heat-resistant and may produce long-term defects (e.g. rancidity or age-gelation) in thermally processed products with extended shelf-lives. Although well adapted to grow at chill temperatures, this group tends to be sensitive to other factors such as the presence of salt or preservatives, lack of oxygen, low (
5.5) pH and a low (
0.98) aw. Should these preservation mechanisms be present in a food, the Gram-negative (oxidase positive) rod￾shaped bacteria compete less well and other microbial groups may cause spoilage. Vibrio species are unusual as they tolerate relatively high salt levels and so may cause spoilage on chilled stored bacon and other cured products. Photobacterium phosphorum, a very large marine vibrio is the dominant spoilage microorganism in vacuum packaged cod (Gram and Huss 1996). Overall, this group is not heat-resistant and so is readily removed by mild thermal treatments. Their presence in heat processed foods is usually as a consequence of post-process contamination. 2. Coliform/enteric bacteria This bacterial group also consists of Gram-negative rods, but these may be distinguished from the above group by a negative oxidase reaction. Traditionally, microbiologists have tended to examine for these groups separately, as their sources, significance and factors affecting growth may differ. This group is frequently used as an indicator of inadequate processing or post-process contamination. Compared with the Gram-negative (oxidase positive) rod-shaped bacteria, the coliform-enteric group is generally less well adapted to growth at temperatures of less than 5–10ºC although many may grow at temperatures as low as 0ºC (Ridell and Korkeala 1997). However, they often dominate the flora at temperatures of 8–15ºC (Huis in’t Veld 1996, Cousin 1982). The coliform/enteric group is less sensitive to changes in pH compared with the Gram-negative (oxidase positive) rod-shaped bacteria and so of more significance in mild acid products. They are however, generally sensitive to low aw, preservatives, salt and thermal treatments (Jay 1978). The coliform/enteric group do not necessarily require the presence of oxygen for growth. In addition, they have a fermentative metabolism and so may break down carbohydrates to give acids, which may result in souring of milk (Cousin 1982). In contrast, the metabolism of the Gram-negative (oxidase positive) bacteria is oxidative and fermentation does not occur. Other types of spoilage include the production of pigmented growth, gases, slime, off-odours and off￾flavours. Off odours have been described as ‘grassy’, medicinal, unclean and faecal (Walker and Stringer 1990). Typical spoilage species include Citrobacter, Escherichia, Enterobacter, Hafnia, Klebsiella, Proteus and Serratia (Jay 1978, Walker 1988). These microorganisms are widely disseminated in the environment, including in animals. Poor slaughter and dressing practices may contribute to their presence in foods. 164 Chilled foods