Elimination of Bacillus cereus contamination in raw rice by electron beam irradiation

Abstract

Microbial loads of unhusked and husked rice grains were examined to evaluate the efficacy of decontaminating rice with electron beam irradiation. The effect of irradiation with electron beams on Bacillus cereus counts in inoculated husked rice as well as the sensitivity of eight strains of B. cereus to electron beam irradiation in distilled water were also investigated. Unhusked and husked rice samples were irradiated with doses ranging from 0 to 7.5 kGy. The microbial counts performed were total mesophilic aerobes, coliforms, Escherichia coli, B. cereus, sulfite-reducing clostridia, and fungi (molds and yeast). The microbiological quality of unhusked rice was very poor as compared to husked rice. The dose of electron beams necessary to destroy coliforms, E. coli, B. cereus, sulfite-reducing clostridia and fungi in unhusked and husked rice were 7.5 and 1.1 kGy, respectively. The 7.5 kGy dose was enough for decontamination of husked rice as well as reduced aerobic plate counts to average levels of 2.66 log cfu g −1 in unhusked rice. It also reduced the B. cereus load to levels lower than 10 cfu g −1 in husked rice inoculated with 10 4 spores g −1 of the strain EPSO-41WR. Irradiation at 1.1 kGy reduced aerobic plate counts to average levels of 4.03 log cfu g −1 in husked rice. Diversity in radiation resistance was observed among eight strains of B. cereus isolated from Spanish raw rice, with average D 10 values ranging from 2.07 to 2.68 kGy. Strain EPSO-46HR* was the most radiation-resistant, on the contrary strain EPSO-47HR was the least radiation-resistant.