Change of Thermal Inactivation of Clostridium botulinum Spores during Rice Cooking

Cooked and packed rice (CaPR), a popular rice product in Japan, is distributed with oxygen-absorbing agents and without refrigeration. When the final product was inoculated with spores of several strains of proteolytic Clostridium botulinum at a dose of 10(3) spores per g (2 × 10(5) spores per package) and incubated at 30°C, the bacteria grew and produced neurotoxins in 40 days. To simulate more realistic cases of contamination, the same dose of spores was inoculated before the cooking process. When cooked at 100°C for 30 min, a small number of spores survived and the toxins were detected in some of the samples after incubation for 180 days. However, when cooked at 100°C for 15 min immediately followed by 105°C for 15 min, neither survivors nor the toxins were detected during incubation for 270 days after cooking. Even when inoculated with 105 spores per g of one of the most heat-resistant strains, 213B, viable spores were not detected after cooking. The inactivation by these heating conditions in different media indicated that the spores were inactivated >1,000-fold more in rice suspension than in cooked meat medium or phosphate buffer. It was therefore suggested that rice contains component(s) that facilitates thermal inactivation of C. botulinum.