Chilled foods microbiology 167 7.7 Pathogenic microorganisms Foods may be considered to be microbiologically unsafe owing to the presence of microorganisms which may invade the body(e.g. Salmonella, Listeria monocytogenes, Ecoli o157: H7 and Campylobacter)or those which produce a toxin ingested with a food(e.g. Clostridium botulinum, Staphylococcus aureus and Bacillus cereus). The growth of pathogenic microorganisms in foods may not necessarily result in spoilage, and so the absence of deleterious sensory hanges cannot be relied upon as an indicator of microbial safety. Furthermore, some toxins are resistant to heating and so may remain in a food after viable microorganisms have been removed. It is therefore essential that an effective programme is used to ensure the safety of foods from production, through processing, storage and distribution to consumption. Within the Uk, the recent trends in food poisoning and the issues contributing to this have been extensively reviewed by border and Norton(1997) As discussed previously, storage at chill temperatures cannot prevent all microbial growth, but can prevent the growth of some types and retard the rate of growth in others. As far back as 1936, Prescott and Geer recommended that foods permitting growth of microorganisms should be stored at less than 10C (50F) and preferably ca. 4C(39F)to prevent the growth of pathogens or toxin production. That was sound advice in terms of the food-borne pathogens recognised at that time. The risk of growth by food-borne pathogens is a combination of the minimum growth temperatures, the growth rate at chil temperatures and the time and temperature(s)of storage. The minimum growth temperatures of pathogenic bacteria have been discussed by Walker and Stringer Whilst the majority of food-borne disease is caused by relatively few bacterial types- mainly Salmonella and Campylobacter(Border and Norton 1997), the number of bacteria recognised as food-borne pathogens, however, has steadily increased. Whilst this may, in part, reflect a true underlying increase in the incidence, it may also be due to a greater awareness of these microorganisms and improvements in methodologies. For discussion in this chapter, the pathogen bacteria of concern for chilled foods can be arbitrarily divided as follows Microorganisms capable of growth at temperatures below 50 This group is potentially of greatest concern as they continue to multiply even with 'good refrigeration temperatures. Although growth may continue, temperature control is critical and the growth rate becomes increasingly slow as the temperature is reduced(see Fig. 7. 1). In addition, temperature control can interact effectively with other factors to prevent or greatly limit growth Listeria monocytogenes The bacterium now identified as L. monocytogenes was first recognised as a human pathogen in 1926(Murray et al.), but its role in food-borne disease was7.7 Pathogenic microorganisms Foods may be considered to be microbiologically unsafe owing to the presence of microorganisms which may invade the body (e.g. Salmonella, Listeria monocytogenes, E. coli O157:H7 and Campylobacter) or those which produce a toxin ingested with a food (e.g. Clostridium botulinum, Staphylococcus aureus and Bacillus cereus). The growth of pathogenic microorganisms in foods may not necessarily result in spoilage, and so the absence of deleterious sensory changes cannot be relied upon as an indicator of microbial safety. Furthermore, some toxins are resistant to heating and so may remain in a food after viable microorganisms have been removed. It is therefore essential that an effective programme is used to ensure the safety of foods from production, through processing, storage and distribution to consumption. Within the UK, the recent trends in food poisoning and the issues contributing to this have been extensively reviewed by Border and Norton (1997). As discussed previously, storage at chill temperatures cannot prevent all microbial growth, but can prevent the growth of some types and retard the rate of growth in others. As far back as 1936, Prescott and Geer recommended that foods permitting growth of microorganisms should be stored at less than 10ºC (50ºF) and preferably ca. 4ºC (39ºF) to prevent the growth of pathogens or toxin production. That was sound advice in terms of the food-borne pathogens recognised at that time. The risk of growth by food-borne pathogens is a combination of the minimum growth temperatures, the growth rate at chill temperatures and the time and temperature(s) of storage. The minimum growth temperatures of pathogenic bacteria have been discussed by Walker and Stringer (1990). Whilst the majority of food-borne disease is caused by relatively few bacterial types – mainly Salmonella and Campylobacter (Border and Norton 1997), the number of bacteria recognised as food-borne pathogens, however, has steadily increased. Whilst this may, in part, reflect a true underlying increase in the incidence, it may also be due to a greater awareness of these microorganisms and improvements in methodologies. For discussion in this chapter, the pathogenic bacteria of concern for chilled foods can be arbitrarily divided as follows. Microorganisms capable of growth at temperatures below 5ºC This group is potentially of greatest concern as they continue to multiply even with ‘good’ refrigeration temperatures. Although growth may continue, temperature control is critical and the growth rate becomes increasingly slow as the temperature is reduced (see Fig. 7.1). In addition, temperature control can interact effectively with other factors to prevent or greatly limit growth. Listeria monocytogenes The bacterium now identified as L. monocytogenes was first recognised as a human pathogen in 1926 (Murray et al.), but its role in food-borne disease was Chilled foods microbiology 167