Microorganisms that present a safety hazard in cooked chilled foods containing vegetables have the following characteristics:
• they are natural contaminants of raw vegetables
• they have been implicated in outbreaks of food poisoning following consumption of vegetable-based foods
• they are able to survive, at least to some extent, the mild heat treatment received by the products during processing
• they are able to grow at temperatures of refrigeration.
The list of the bacteria complying with these conditions is quite large. However Listeria monocytogenes, Clostridium botulinum and Bacillus cereus, according to most experts, are the major concern because of natural contamination, the ability to grow at low temperatures and high resistance to heat (ACMSF, 1992; Peck, 1997; Carlin et al., 2000a). These bacteria are widely distributed in the environment and may be isolated from vegetables. Surveys for the presence of L. monocytogenes show that 0-85% of fresh vegetable samples are positive, with contamination levels lower than 100 L. monocytogenes cfug-1 (Beuchat, 1996; Nguyen-the and Carlin, 2000). This level of contamination is lower than that observed on meat products for instance. Surveys for the presence of C. botulinum show 0-100% of fresh vegetable samples are positive, with a maximal contamination level likely to be lower than one C. botulinum spore/g (Notermans, 1993; Lund and Peck, 2000). The three species of bacteria have been implicated in outbreaks of food poisoning following the consumption of vegetable-based foods, in both fresh, minimally processed vegetables and heat-processed vegetables. Among non-spore forming bacteria, L. monocytogenes is considered to be a relatively heat-resistant organism, when compared to other pathogenic bacteria, and is the most psychrotrophic bacterium among the known pathogens with a lower growth limit at about 0°C. Other pathogens such as Escherichia coli and Salmonella have a similar heat resistance and a lesser ability to grow at low temperatures. Endospores produced by C. botulinum and B. cereus confer a high resistance to the heat process applied to cooked chilled foods. Some strains of C. botulinum (Group II or non-proteolytic strains) or of B. cereus are able to grow at temperatures as low as 3°C and 4°C, respectively (ICMSF, 1996; Lund and
Peck, 2000). Despite being a spore former C. perfringens is generally not considered to be a safety concern for cooked chilled foods containing vegetables, because of its poor ability to grow at temperatures lower than 15°C in a vegetable substrate (Labbe, 2000).
L. monocytogenes, C. botulinum and B. cereus have a strong ability to grow on a vegetable substrate, even at low temperature (Table 11.2). Possible growth of L. monocytogenes was shown on a range of raw and cooked vegetable substrates (Farber and Peterkin, 2000). Growth of B. cereus and C. botulinum was shown in a range of cooked vegetable substrates, at temperatures close to minimal growth temperature. More generally most cooked vegetables at a pH above 4.6 (5.0) appear to be able to support growth and toxin production by proteolytic (non-proteolytic) C. botulinum, whatever the supposed nutrient value (Carlin and Peck, 1995, 1996).
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