Enumeration, isolation and characterization of Bacillus cereus strains from Spanish raw rice

Bacillus cereus was present in 61 samples of raw rice analysed representing unhusked husked and commercial origins. B. cereus in husked and white rice samples did not reach 10(2) cfu g(-1), while in the unhusked rice B. cereus densities exceeded 10(3) cfu g(-1). Processing steps such as drying, husking and polishing reduced the number of B. cereus in the finalproduct. Eight strains with typical morphology of B. cereus on Polymyxin-Mannitol-Egg Yolk-Phenol Red Agar (PMYPA) were isolated According to ISO confirmatory tests, the API System tests and supplementary tests of motility, oxidase activity and enterotoxin production, these isolates were characterized and identified as B. cereus. All strains were motile, oxidase-negative and produced diarrheal enterotoxin in TSB. D and z-values were used to characterize heat resistance of spores obtained from the eight strains of B. cereus characterized A large diversity in heat resistance was observed among the isolates. At 90degreesC D-values ranged from 2.23 to 23.26min, with five groups of D-value means significantly different at the 95% confidence level D-95-and D-100 values calculated for the eight strains ranged from 0.69 to 5.17 min and from 0.43 to 1.09 min, respectively. Statistical analysis revealed that there was significant difference between the D-value means obtained for the strains at each temperature. The z-values for the eight strains of B. cereus tested in this study ranged from 7-42degreesC to 8.20degreesC with an average of 7.7degreesC. (C) 2002 Elsevier Science Ltd. All rights reserved.