Effect of storage temperature on the lag time of Geobacillus stearothermophilus individual spores

Abstract

The lag times (lambda) of Geobacillus stearothermophilus single spores were studied at different storage temperatures ranging from 45 to 59 degrees C using the Bioscreen C method. A significant variability of lambda was observed among individual spores at all temperatures tested. The storage temperature affected both the position and the spread of the lambda distributions. The minimum mean value of lambda (i.e. 10.87 h) was observed at 55 degrees C, while moving away from this temperature resulted in an increase for both the mean and standard deviation of lambda, A Cardinal Model with Inflection (CMI) was fitted to the reverse mean lambda, and the estimated values for the cardinal parameters T-min, T-max, T-opt and the optimum mean lambda of G. stearothermophilus were found to be 38.1, 64.2, 53.6 degrees C and 10.3 h, respectively. To interpret the observations, a probabilistic growth model for G. stearothermophilus individual spores, taking into account lambda variability, was developed. The model describes the growth of a population, initially consisting of No spores, over time as the sum of cells in each of the N-0 imminent subpopulations originating from a single spore. Growth simulations for different initial contamination levels showed that for low N-0 the number of cells in the population at any time is highly variable. An increase in N-0 to levels exceeding 100 spores results in a significant decrease of the above variability and a shorter lambda of the population. Considering that the number of G. stearothermophilus surviving spores in the final product is usually very low, the data provided in this work can be used to evaluate the probability distribution of the time-to-spoilage and enable decision-making based on the "acceptable level of risk".