Growth/no-growth models for heat-treated spores of psychrotrophic Bacillus cereus strains

Refrigerated processed foods of extended durability (REPFED) are a growing group of products. This research aims to minimize the heat treatment, to provide higher quality products, without compromising food safety. One of the microbial risks in REPFEDs are psychrotrophic Bacillus cereus strains due to their high thermal resistance and ability to grow under cold storage. Twenty-five psychrotrophic B. cereus strains, isolated from different REPFEDs were screened for their time to detection (TTD) under cold storage at three temperatures (8, 9, 10°C). The TTD was determined for vegetative cells, spores and heat- treated spores of these strains using Optical Density (OD) measurements. Results show a great inter- and intra-strain variability in TTD. At 8°C only 40.7% of the strains were able to grow, compared to 77.8% at 9 and 10°C. Based on these data two strains, exhibiting spore- growth at 8°C after heat treatment, were selected: a very heat resistant strain (D90°C=90min) and a more heat sensitive strains (D90°C=17min). For each strain a full factorial growth/no- growth model was created with 3 parameters (pH: 5.2–6,4; aw: 0.97–0.99; P90: 0–10 min). Results show that the presence of a heat treatment and the pH are the main factors preventing the growth of B. cereus under cold storage. The minimal pH for growth increases as the heat treatment increases