Contamination flows of Bacillus cereus and spore-forming aerobic bacteria in a cooked, pasteurized and chilled zucchini puree processing line

A food processing plant producing pasteurized purees and its zucchini puree processing line were examined for contamination with aerobic and facultative anaerobic bacterial spores during a day's operation. Multiplication of spores was also monitored in the product stored under different conditions. High concentrations of Bacillus cereus spores were found in the soil in which the zucchinis were grown (4.6+/-0.3 log CFU/g), with a background spore population of 6.1+/-0.2 log CFU/g. In the processing plant, no B. cereus or psychrotrophic bacterial spores were detected on equipment. B. cereus and psychrotrophic bacterial spores were detected after enrichment in all samples of raw zucchinis, washed zucchinis, of two ingredients (starch and milk proteins) and in processed puree at each processing step. Steam cooking of raw zucchinis and pasteurization of puree in the final package significantly reduced spore numbers to 0.5+/-0.3 log CFU/g in the processed food. During storage, numbers of spore-forming bacteria increased up to 7.8+/-0.1 log CFU/g in puree after 5 days at 20-25degreesC, 7.5+/-0.3 log CFU/g after 21 days at 10degreesC and 3.8+/-1.1 log CFU/g after 21 days at 4degreesC. B. cereus counts reached 6.4+/-0.5 log CFU/g at 20-25degreesC, 4.6+/-1.9 log CFU/g at 10degreesC, and remained below the detection threshold (1.7 log CFU/g) at 4degreesC. Our findings indicate that raw vegetables and texturing agents such as milk proteins and starch, in spite of their low levels of contamination with bacterial spores and the heat treatments they undergo, may significantly contribute to the final contamination of cooked chilled foods. This contamination resulted in growth of B. cereus and psychrotrophic bacterial spores during storage of vegetable puree. Ways to eliminate such contamination in the processing line are discussed. (C) 2002 Elsevier Science B.V. All rights reserved.