Purpose: This study investigated the efficacy of repeated treatment with low-dose γ-irradiation to eliminate vegetative and spore form of Bacillus cereus ATCC 12480 from raw rice.
Methods: Rice samples artificially contaminated with spores and vegetative forms of B. cereuswere treated with γ-irradiation at 0, 1.5, 3, 5, 7, 10, 15, 20, 25, and 30 kGy once, twice and thrice successive repeated treatment cycles, respectively, at 1.5, 3 and 5 kGy of γ-irradiation. Microbiological analysis for viable cells and spores count were performed after irradiation treatment; and field scanning electron microscopy was employed to determine the effect of irradiation on spore coat, size and morphology.
Results: Vegetative cells and spores of B. cereus in raw rice tolerated γ-irradiation up to 10 kGy and 20 kGy, respectively, at single treatment and were completely eliminated at 15 kGy and 25 kGy, respectively. Two and three cycles of irradiation treatment at 5 kGy eliminated all vegetative B. cereus. Repeated treatments were significantly more effective than single treatment at high dose (P < 0.05). There was no significant difference between two and three repeated treatment with γ-irradiation at the same dose (P > 0.05). Scanning electron microscopy images revealed that high doses of γ-irradiation slightly alter the morphology of spores by denaturing of exosporium at 20, 25 and 30 kGy. Also at 30 kGy the reduction in spore size and shrinking of spore coats was observed.
Significance : The efficacy of repeated treatment with low-dose γ-irradiation to eliminate B. cereus and the effect of γ-irradiation to viability, spore size and morphology was established. Elimination of foodborne pathogens from food is an ongoing challenge in which this study makes a scientific contribution.