Chilled foods micro 163 visible(sometimes pigmented) colonies on foods. Other forms of spoilage ding the production of gases, slime(extracellular polysaccharide material) diffusible pigments and enzymes which may produce softening, rotting, off- dours and off-flavours from the breakdown of food components. The taints produced by microbial spoilage have been reviewed by Dainty(1996)and Whitfield (1998) Spoilage is usually most rapid in proteinaceous chilled foods such as red allow good microbial growth as they are highly nutritious, have a high moisture content and relatively neutral pH value. In an attempt to reduce the spoilage rates of these foods, they are often modified as discussed usly. For chilled products, these modifications may not entirely prevent microbial growth and poilage, but do limit the rate and nature of spoilage microorganisms responsible for spoilage of a food are those which are best able to grow in the presence of the preservation mechanisms that are operating within that food. Care is needed to distinguish between those microorganisms oresent in spoiled food and those responsible for the spoilage defect (often called specific spoilage organisms or sso) which may be only a fraction of the microflora( Gram and Huss 1996). Consequently, the relationship between sensory spoilage and microbial numbers is often only poorly correlated Traditional microbiology is often of limited value for control of spoilage microorganisms as the time taken to get results represents a significant proportion of the shelf-life. Recently more rapid and molecular techniques have become available for the detection of general or specific spoilage organisms (Venkitanarayanen et al. 1997, Gutierrez et al. 1997). For discussion in this chapter, spoilage microorganisms have been arbitrarily divided into six categories: Gram-negative(oxidase positive) rod-shape bacteria; coliform enterics; Gram-positive spore-forming bacteria; lactic acid bacteria; other bacteria; yeasts and mould 1. Gram-negative (oxidase positive) rod-shaped bacteria Overall, this group comprises the most common spoilage microorganisms of fresh chilled products. The minimum growth temperatures are often 0-3oC and they grow relatively rapidly at 5-10C. Although they may represent only mall proportion of the initial microflora, they rapidly dominate the microflora of fresh proteinaceous chilled stored foods(Huis in't Veld 1996, Cousin 1982 Gill 1983). Within this general group, the genus Pseudomonas is most common although other genera include Acinetobacter, Aeromonas, Alcaligenes, Alter monas, Flavobacterium, Moraxella, Shewenella and Vibrio species(Walker and Stringer 1990). These microorganisms are common in the environment, particularly in water, and so many easily contaminate foods. Often they may proliferate on inadequately cleaned surfaces of food processing plant or equipment and so contaminate foods The Gram-negative (oxidase positive) rods may spoil products by the production of diffusible pigments, slime material on surfaces and enzymesvisible (sometimes pigmented) colonies on foods. Other forms of spoilage including the production of gases, slime (extracellular polysaccharide material), diffusible pigments and enzymes which may produce softening, rotting, off￾odours and off-flavours from the breakdown of food components. The taints produced by microbial spoilage have been reviewed by Dainty (1996) and Whitfield (1998). Spoilage is usually most rapid in proteinaceous chilled foods such as red meats, poultry, fish, shellfish, milk and some dairy products. These products allow good microbial growth as they are highly nutritious, have a high moisture content and relatively neutral pH value. In an attempt to reduce the spoilage rates of these foods, they are often modified as discussed previously. For chilled products, these modifications may not entirely prevent microbial growth and spoilage, but do limit the rate and nature of spoilage. In general the microorganisms responsible for spoilage of a food are those which are best able to grow in the presence of the preservation mechanisms that are operating within that food. Care is needed to distinguish between those microorganisms present in spoiled food and those responsible for the spoilage defect (often called specific spoilage organisms or SSO) which may be only a fraction of the microflora (Gram and Huss 1996). Consequently, the relationship between sensory spoilage and microbial numbers is often only poorly correlated. Traditional microbiology is often of limited value for control of spoilage microorganisms as the time taken to get results represents a significant proportion of the shelf-life. Recently more rapid and molecular techniques have become available for the detection of general or specific spoilage organisms (Venkitanarayanen et al. 1997, Gutie´rrez et al. 1997). For discussion in this chapter, spoilage microorganisms have been arbitrarily divided into six categories: Gram-negative (oxidase positive) rod-shape bacteria; coliform enterics; Gram-positive spore-forming bacteria; lactic acid bacteria; other bacteria; yeasts and moulds. 1. Gram-negative (oxidase positive) rod-shaped bacteria Overall, this group comprises the most common spoilage microorganisms of fresh chilled products. The minimum growth temperatures are often 0–3ºC and they grow relatively rapidly at 5–10ºC. Although they may represent only a small proportion of the initial microflora, they rapidly dominate the microflora of fresh proteinaceous chilled stored foods (Huis in’t Veld 1996, Cousin 1982, Gill 1983). Within this general group, the genus Pseudomonas is most common although other genera include Acinetobacter, Aeromonas, Alcaligenes, Alter￾omonas, Flavobacterium, Moraxella, Shewenella and Vibrio species (Walker and Stringer 1990). These microorganisms are common in the environment, particularly in water, and so many easily contaminate foods. Often they may proliferate on inadequately cleaned surfaces of food processing plant or equipment and so contaminate foods. The Gram-negative (oxidase positive) rods may spoil products by the production of diffusible pigments, slime material on surfaces and enzymes Chilled foods microbiology 163