Effects of pH on Toxin Production in Hermetically Packaged Boiled Rice by Clostridium botulinum Types A and B and Heat Resistance of Those Spores

Abstract

The effects of pH on botulinal toxin production in boiled rice and the heat resistance of the spores were investigated. Toxin was detected in boiled rice (pH above 5.0) packed in a plastic film pouch or semi-rigid plastic tray with deoxidant and inoculated with mixed spores (ca. 10⁶ per container) of Clostridium botulinum 62A, 90A, 213B and B1G4 strains following incubation for one to six months at 30°C. Lowered pH (5.0) and cooking, for 19 min at 99°C, failed to prevent toxigenesis. The heat resistance of C. botulinum spores in rice gruel (pH 5.3) was assessed. D (decimal reduction time at 100°C) and z (a slope index of the thermal death time curve) were 9 min and 10°C for 62A, and 7 min and 9°C for 213B, respectively. Raw rice was also examined for contamination with clostridial spores. No clostridial spores could be found in milled rice grains. The number of spores per 100 g was 3 to 73 CFU for brown rice and more than 100 CFU for bran.