Thermal inactivation of bacteria-a new predictive model for the combined effect of three environmental factors: temperature, pH and water activity

Abstract An empirical, linear-Arrhenius model for predicting the combined effect of three environmental factors on the thermal inactivation (i.e. death) rate of Escherichia coli , namely inactivation temperature, pH and water activity ( a w ), is synthesised from published, and independent experimental data. For inactivation rates obtained both at isothermic condition, a constant temperature of 58 °C, with a seven level pH range (pH 3 to pH 9), and a three level range of values of a w (0.928 to 0.995); and anisothermic conditions (a range of inactivation temperature 52.05 to 63.10 °C, with a four level pH range of between pH 3 to pH 9, and a three level range of a w from 0.928 to 0.995), the model gave a very high degree of fit. The synthesised model is quadratic in form. For isothermic inactivation the model has three terms (pH, pH 2 , and a w 2 ) and explained 96.7% of the variance accounted for (% V ) in the overall data (i.e. seven levels of pH and three levels of a w ). The model with four terms (l/ T , pH, pH 2 , and a w 2 ) explained 80.7% V in the overall anisothermic data, and between 90.1 and 95.3% V at each level of a w . Comparison with an existing model highlighted the advantages of the newly synthesised model including ease of formulation, and ease of use. The new model form should permit ready integration with equations describing the viscous and thermal properties in design for batch or continuous steriliser design. Findings suggest an additive linear-Arrhenius model is applicable to the combined influence of n environmental factors on the thermal inactivation rate.