Control of microbial hazards storage temperature

Refrigeration delays spoilage of products and growth of spoilage organisms. This has been clearly shown on cooked pasteurised and chilled purées of vegetables. On a courgette purée, the time to growth of 105cfug-1 increased from 5 days to 15 days and the time to the appearance of noticeable off-odours (the first detected sign of spoilage) increased from 12 days to 36 days when the temperature decreased from 10°C to 4°C (Carlin et al., 2000b). The growth of most pathogenic bacteria is inhibited at low temperature. The minimal growth temperature of proteolytic (Group I) C. botulinum is 10°C, that of E. coli (including entero-haemorrhagic E. coli) and Salmonella is 7-8°C and of psychrotrophic strains of B. cereus is 4°C. Only non-proteolytic (Group II) C. botulinum and L. monocy-togenes are able to grow at lower temperatures (3°C and 0°C, respectively) (ICMSF, 1996).

When growing in vegetable substrate or in foods, significant delays or absence in growth are generally observed even above the minimum growth temperature. Psychrotrophic isolates of B. cereus were unable to grow in a courgette broth at 7°C, while growth was observed in a nutrient broth (Choma et al., 2000). Many strains of C. botulinum failed to grow in vegetable substrate at temperatures (15°C for Group I proteolytic C. botulinum and 10°C for Group II non-proteolytic C. botulinum) substantially higher than the minimum growth temperature, while growth was detected in nutrient broth after a few days of incubation (Carlin and Peck, 1996; Braconnier, 2001). These differences were not simply due to pH, as vegetables with similar pHs showed marked differences in growth potential of the bacterium. However, time to toxin production of C. botulinum in vegetable-based foods is in the range reported for other food groups (e.g. meat, fish and poultry).

Slight differences in temperatures can induce high differences in the populations of bacteria. For instance, B. cereus was never detected in cooked pasteurised and chilled purées of vegetables stored for 46 days at 4°C, whereas 17 out of 50 samples were positive after a storage of 20-32 days at 10°C, with some counts being higher than 105 B. cereus cfug-1 (Choma et al., 2000).

Some outbreaks have been caused by products exposed for a prolonged time at ambient temperature (20-30°C), when they should have been kept refrigerated. Challenge tests with such incubation temperatures have therefore been performed and show growth to critical levels and/or toxin production of L. monocytogenes, B. cereus and C. botulinum within 1 to 5 days. At the same time spoilage was not observed in many instances and the product was still acceptable to the consumer (Notermans et al., 1981; Lund et al., 1988).

In conclusion, the growth potential of pathogenic bacteria in cooked chilled foods containing vegetables depends strongly on storage temperature and on the nature of the vegetable substrate, interactions between both factors being more significant at low temperatures.

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