Contamination and Growth of Bacillus cereus and Clostridium perfringens in Mexican-Style Beans

Abstract

Two major problems in production procedures used in Mexican restaurants identified through interviews with managers 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, incubation time, and location in the product on growth of Bacillus cereus and Clostridium 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 h, respectively. B. cereus may present more of a health hazard, since obvious signs of spoilage did not occur in these beans until 12 h, whereas C. perfringens caused obvious spoilage of beans within 6 to 8 h. Numbers of B. cereus usually associated with illness were found at 12 h at 23°C. The beans appeared to be spoiled before this level was reached with C. perfringens at 24 h. Good growth of both species occurred in both top and bottom locations. Of 42 restaurant samples of bean dip and mashed beans analyzed for contamination with B. cereus and C. perfringens, only two samples were found to contain either organism and these were present in low numbers. Two samples were, however, found to contain large numbers of coagulase-positive S. aureus (> 100,000/g).