Thermal Sterilisation - a New Predictive Model for the Combined Effect of Three Environmental Factors: Temperature, pH and Water Activity

An empirical, linear-Arrhenius model for predicting the combined effect of three environmental factors on the thermal inactivation (ie death) rate of Escherichia coli namely, sterilisation temperature, pH and water activity (a{w}), is synthesised from published and independent experimental data. The model gave a very high degree of fit to 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} from 0.928 to 0.995), and an-isothermic conditions (a range of sterilisation temperature from 52.05 C to 63.10 C with four level pH range between pH 3 to pH 9 and a three level range of aw from 0.928 to 0.995). The synthesised model is quadratic in form. For isothermic sterilisation the model has three terms (pH/ pH^/ and a ) and explained 96.7% of the variance accounted for (%V) in the overall data (ie seven levels of pH and three levels of aw). The model with four terms (1/T; pH; ph{2}; and a{w}{2} ) explained 80.7 %V in the overall an-isothermic data, and; between 90.1 %V 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.