Variability in the numbers of bacteria remaining in saline solution and whole milk following mild heat treatment has been studied with Listeria innocua, Enterococcus faecalis, Salmonella enterica serovar Enteritidis, and Pseudomonas fluorescens. As expected, the most heat-resistant bacterium was E. faecalis, while P. fluorescens was the least heat resistant, and all bacteria showed greater thermal resistance in whole milk than in saline solution. Despite the differences in the inactivation kinetics of these bacteria in different media, the variability in the final number of bacteria was affected neither by the species nor by the heating substrate, but it did depend on the intensity of the heat treatment. The more severe the heat treatment was, the lower the average number of surviving bacteria but the greater the variability. Our results indicated that the inactivation times for the cells within a population are not identically distributed random variables and that, therefore, the population includes subpopulations of cells with different distributions for the heat resistance parameters. A linear relationship between the variability of the log of the final bacterial concentration and the logarithmic reduction in the size of the bacterial population was found.The safety and quality of many foods, such as minimally processed foods and some ready-to-eat products, are affected by a combination of multiple sublethal stresses due to processing and storage conditions (17, 32). The use of mild preservative processes instead of harsher ones affects the microbiological risk. It is therefore important to identify potential hazards and to perform accurate quantitative microbial risk assessments including probabilistic modeling approaches (17).It is unlikely that food products are contaminated by one unique bacterial strain or species. In fact, even cells from a pure culture derived from one cell are not necessarily identical (25). Nauta (16) differentiated between uncertainty, which is due to the lack of perfect knowledge about a parameter and which is reducible by further measurements, and variability, which reflects the true variability of a population and is therefore irreducible. He showed the importance of both parameters in quantitative microbial risk assessment models. The estimation of variability usually refers to the variation in the responses of the individual cells within the population, which may be homogeneous or heterogeneous. In a homogeneous population, the heat resistance parameters of the individual cells are identically distributed random variables. Heterogeneous populations include subpopulations of cells with different distributions for the heat resistance parameters. The heterogeneity of the individual cells or spores within a population is the most accepted explanation for the tails of inactivation curves (1, 30) and unexpected increases in the variability of the number of survivors after heat treatments (7,8,30).The microbial concentration in a food product at a given time is affected by the initial...

Summary When conventional preservative treatments are applied, such as heat or acid, the maximum specific growth rate (μmax) of survivors is the same as that of untreated cells. However, when new nonthermal technology is applied, the effects of it on the kinetics of the microorganism can be unpredictable. In this sense, Cabeza et al. (2010) reported longer doubling times after irradiating with accelerated electron beam. The aim of this work was to study the effect of electron beam irradiation on the μmax of Bacillus cereus and compare it with a conventional inactivation treatment (heat). To prove this, μmax was estimated in ham at 12 °C and in TSB at 22 °C after 0, 2, 3 or 4 log reduction by irradiation; likewise, μmax was estimated in whole milk at 12 °C and in TSB at 22 °C after the same log reduction using heat treatments. Our findings show that irradiation affected the μmax of survivor cells. Irradiation intensity was inversely proportional to μmax, such that greater intensity was associated with lower μmax. At the same time, growth temperature had an effect on the decrease in μmax: the radiation‐induced reductions in μmax were greater at 12 °C than at 22 °C. In summary, E‐beam irradiation decreases the μmax of B. cereus, while heat treatment does not. This suggests that the shelf life of irradiated foods must be longer than that of heat‐preserved foods after the application of a similar inactivation treatment.