Chilled foods microbiology 157 understood, e.g. vacuum packed cod( Gram and Huss 1996). In general, a greater understanding is needed of the relationship between specific spoilage microorganisms in particular foods and the deterioration in sensory quality 7.4.2 Food-borne pathogens ith many human pathogens, the greater the number of cells consumed, the greater the chance of microbial invasion, as the larger number of cells may be able to evade/swamp the bodys defence mechanism. Higher numbers may also result in a shorter incubation period before the onset of disease. Consequently ontrol, and preferably inhibition, of growth in foods is essential. However, with ome invasive pathogens(e.g. viruses, Campylobacter), the infectious dose is ow and growth in the food may not be necessary. Other pathogenic microorganisms may produce a toxin in the food which results in disease Preformed toxins are usually produced at high cells densities and so usuall growth has occurred. If the toxin is heat stable, it may remain although all microorganisms have been eliminated from the food. Consequently it is important to control growth at all stages of the chill chain 7.5 Factors affecting the microfora of chilled foods 7.5.1 Initial microflora with healthy animal and plant tissues, microbial contamination is absent or at a low level except for the exterior surfaces. For example, fresh muscle from healthy animals is usually microbiologically sterile, and aseptically drawn milk from healthy cows contains only a few microorganisms(mainly streptococci and micrococci)derived from the teat canal. Similarly, the interior of healthy damaged vegetables does not contain microorganisms although the exterior ay be contaminated with a wide range of microorganisms of soil origin During slaughter or harvesting, subsequent processing and packaging, these raw materials become contaminated from a wide range of sites. Typically, these sites nclude water, air, dust, soil, hides/fleece/feathers, animals, people, equipment and other food materials. Consequently, a large range of microorganisms can be isolated from foods. Those which are able to grow may potentially give rise to microbial spoilage or public health issues. The hygienic practices of all food operations, from slaughter/harvesting through retail sale to consumer use, will affect the level of microbial contamination of products. In general, the lower the initial level of contamination, the greater the time until microbial spoilage is evident 7.5.2 Food type The intrinsic properties(e.g. pH, water activity, acidity, natural antimicrobials) of different foods vary greatly. Such factors affect the ability of microorganismsunderstood, e.g. vacuum packed cod (Gram and Huss 1996). In general, a greater understanding is needed of the relationship between specific spoilage microorganisms in particular foods and the deterioration in sensory quality. 7.4.2 Food-borne pathogens With many human pathogens, the greater the number of cells consumed, the greater the chance of microbial invasion, as the larger number of cells may be able to evade/swamp the body’s defence mechanism. Higher numbers may also result in a shorter incubation period before the onset of disease. Consequently, control, and preferably inhibition, of growth in foods is essential. However, with some invasive pathogens (e.g. viruses, Campylobacter), the infectious dose is low and growth in the food may not be necessary. Other pathogenic microorganisms may produce a toxin in the food which results in disease. Preformed toxins are usually produced at high cells densities and so usually growth has occurred. If the toxin is heat stable, it may remain although all microorganisms have been eliminated from the food. Consequently it is important to control growth at all stages of the chill chain. 7.5 Factors affecting the microflora of chilled foods 7.5.1 Initial microflora With healthy animal and plant tissues, microbial contamination is absent or at a low level except for the exterior surfaces. For example, fresh muscle from healthy animals is usually microbiologically sterile, and aseptically drawn milk from healthy cows contains only a few microorganisms (mainly streptococci and micrococci) derived from the teat canal. Similarly, the interior of healthy undamaged vegetables does not contain microorganisms although the exterior may be contaminated with a wide range of microorganisms of soil origin. During slaughter or harvesting, subsequent processing and packaging, these raw materials become contaminated from a wide range of sites. Typically, these sites include water, air, dust, soil, hides/fleece/feathers, animals, people, equipment and other food materials. Consequently, a large range of microorganisms can be isolated from foods. Those which are able to grow may potentially give rise to microbial spoilage or public health issues. The hygienic practices of all food operations, from slaughter/harvesting through retail sale to consumer use, will affect the level of microbial contamination of products. In general, the lower the initial level of contamination, the greater the time until microbial spoilage is evident. 7.5.2 Food type The intrinsic properties (e.g. pH, water activity, acidity, natural antimicrobials) of different foods vary greatly. Such factors affect the ability of microorganisms Chilled foods microbiology 157