A quantitative approach for studying the effect of heat treatment conditions on resistance and recovery of Bacillus cereus spores.

When a bacterial population undergoes an unfavourable transient increase in temperature, a death phase followed by a lag and growth phase are observed for the surviving and cultivable population. The lag phase is of great interest in regard to food safety, but for bacterial spores, very few studies have been carried out on the evolution of lag time versus heat treatment duration. The experiments monitored on spores of two strains of Bacillus cereus showed a biphasic evolution of the lag time for germination of stressed spores with increase in heat treatment duration, at 90 degrees C, 95 degrees C and 100 degrees C and for different recovery conditions in laboratory medium. Initially, the lag time increased with the duration of the thermal stress up to an optimum. Subsequently, the lag time decreased, with increasing stress duration, to a threshold. The evolution of the lag time to germination of surviving spores, under the range of temperature tested, was similar to the evolution of the lag time to growth after mild-temperature treatments observed with vegetative cells of Escherichia coli and Listeria monocytogenes by Breand et al. (1997) [Breand, S., Fardel, G., Flandrois, J.-P., Rosso, L., Tomassone, R., 1997. A model describing the relationship between lag time and mild temperature increase duration. Int. J. Food Microbiol. 38, 157-167]. The mathematical model established from these investigations could also be applied in the case of heat-stressed spores. Moreover, it was observed that there was a significant linear relationship between the D-value and the treatment duration that gave the maximum lag time.