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 produce
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) 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 foodpoisoning 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 (pasteurisation). 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
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 enzymes
visible (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, offodours 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, Alteromonas, 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