Graduate Thesis Or Dissertation
 

Contamination and growth of Bacillus cereus and Clostridium perfringens in Mexican-style beans

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  • Because Mexican foods have been implicated in a number of outbreaks of foodborne illness in the United States, production procedures used in Mexican restaurants were investigated by interviewing four managers. Two major problems identified through these interviews were failure to cool large quantities of beans rapidly and failure to reheat beans thoroughly before placement on the steam table. Experiments were designed to study the effects of varying temperatures, duration of incubation, and the location in the product as it might affect aeration on growth of B. cereus and C. perfringens, singly and combined, in cooked mashed pinto beans. Growth of both B. cereus and C. perfringens was rapid at 37°C, with numbers of cells associated with illness reached in 4 and 6 hours, respectively. B. cereus may present more of a health hazard, since obvious signs of spoilage did not occur in these beans until 12 hours, whereas C. perfringens caused obvious spoilage of beans within 6 to 8 hours. At 23°C with B. cereus, the numbers associated with illness were found at 12 hours. However, the beans appeared to be spoiled before this level was reached with C. perfringens at 24 hours. Aeration, as indicated by location in the jar, appeared to have more of an effect on B. cereus growth than on C. perfringens, but good growth of the two species occurred in both top and bottom locations. Restaurant samples of bean dip and mashed beans were analyzed for contamination with B. cereus and C. perfringens. Total aerobic and anaerobic counts were determined. Only two of the 42 samples were found to contain B. cereus or C. perfringens and these were present in low numbers. The total aerobic and anaerobic counts varied from less than 100 to 100,000,000 per gram: chiefly lactic acid bacteria which appeared to be related to the seasoning ingredients. However, one batch of bean dip and one of mashed beans were found to contain large numbers of coagulase-positive S. aureus (>100,000/g).
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