Modelling the Growth of Bacillus cereus in Cooked Rice Under Variable Temperature Conditions Typical of South West Nigerian Households

Bacillus cereus is a significant cause of food poisoning associated with cooked starchy staples, particularly rice, through emetic and diarrhoeal toxin mechanisms. Understanding its growth dynamics under real-world temperature abuse conditions encountered in South West Nigerian households is critical for evidence-based food safety risk communication. This study modelled the growth kinetics of Bacillus cereus in cooked rice exposed to temperature profiles simulating typical South West Nigerian domestic storage conditions including ambient storage (25 to 32 degrees Celsius), intermittent hot-holding (60 to 75 degrees Celsius), and delayed refrigeration scenarios. Inoculated cooked rice samples were incubated at controlled temperatures mimicking each scenario. Growth data at each temperature were fitted to primary growth models (modified Gompertz and Baranyi models) to determine maximum growth rate, lag phase duration, and maximum population density. Secondary modelling established the temperature-dependency relationship using the square root model. Results showed that maximum specific growth rates increased from 0.12 per hour at 15 degrees Celsius to 0.84 per hour at 37 degrees Celsius. Lag phase duration decreased from over 8 hours at 20 degrees Celsius to less than 2 hours at 37 degrees Celsius. Ambient storage scenarios consistent with typical dry season conditions (30 degrees Celsius) facilitated rapid population growth exceeding infective dose thresholds within 4 hours. The models provide predictive risk data to support quantitative risk assessments for rice-related Bacillus cereus illness in South West Nigeria. Keywords: Bacillus cereus, predictive microbiology, cooked rice, food safety, South West Nigeria.