Analysis of the variability in the number of viable bacteria after mild heat treatment of food

Abstract

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.

FIG. 1.

Distributions of the logarithm values for the surviving bacterial concentrations in saline solution and whole milk after several heat treatments. Variances labeled with the same letter are not significantly different.

FIG. 2.

Relationship between the logarithm of the final bacterial concentration and its standard deviation after several heat treatments. Results are shown for E. faecalis (circles), L. innocua (squares), P. fluorescens (triangles), and S. Enteritidis (diamonds) in saline solution (filled symbols) and whole milk (empty symbols). The dashed line shows the relationship expected by assuming that the inactivation times of the cells within a population are identically distributed and independent of one another.

FIG. 3.

Relationship between the standard deviation of the log of the final bacterial concentration and the decrease in the log of the bacterial concentration after several heat treatments. The symbols are the same as those used in Fig. 2. The continuous lines show the relationships found in our experiments, while the dashed lines show the expected relationships for the log₁₀ of the initial bacterial concentrations (X₀) used in our experiments when the inactivation times for the cells within a population are assumed to be identically distributed and independent of one another.