The high thermal resistance of Bacillus cereus spores inoculated in rice flour

Abstract

Numerous studies highlight the increased thermal resistance of bacteria in low-moisture food, mainly due to Salmonella outbreaks reported in products such as milk powder and peanut butter. However, most research focuses on bacterial vegetative forms. This study assessed the thermal resistance of Bacillus cereus spores isolated from U.S. pre-launch spacecraft, known to withstand multiple cleaning protocols. B. cereus spores were inoculated in rice flour with an initial water activity (a_w) of 0.48 and treated at temperatures of 90°, 95°, and 100°C. The a_w change was evaluated at specific temperature intervals using an Aqualab Vapor Sorption Analyzer and relative humidity sensors. The D-values at the selected temperatures were determined using aluminum thermal death test cells. The a_w value increased (P < 0.05) from 0.48 to 0.85 ± 0.02, 0.86 ± 0.01, and 0.89 ± 0.02 at 90°, 95°, and 100°C. The D-values from the spores ranged from 20.33 to 27.8 h and 30.8–40.7 h for the selected B. cereus strains. A high thermal resistance was observed in the spores, primarily due to the harsh environments where these bacteria samples were collected. This work demonstrates the spores' ability to survive thermal treatment and the necessity to develop alternative methods for inactivating these bacteria in low-moisture foods.