Calculating the efficacy of heat sterilization processes

Abstract

The isothermal semi-logarithmic survival curves of certain bacterial spores, C. botulinum and B. sporothermodurans among them, are non-linear. Hence, the methods to calculate the efficacy of processes to destroy them need to be revised. These spores’ survival curves could be described by a power law model, which is based on the assumption that the spores’ heat resistances have a Weibull type distribution, with a practically temperature independent shape factor. The temperature dependence of the ‘rate parameter’ of the power law model (related to the reciprocal of the distribution’s scale factor) could be described by a log logistic or a discontinuous linear model. The survival characteristics of the two spores are described in terms of the power law model’s exponents and the log logistic and discontinuous linear models’ two parameters; the temperature level where the inactivation accelerates and the rate at which it rises with temperature at the lethal range. These three parameters, together with the temperature profiles were used to simulate the outcome of different heat processes by solving, numerically, a rate based survival model. The resulting survival curves could then be compared and the processes’ lethality assessed in terms of the final survival ratio that they had produced. The method to calculate the survival curves is applicable to thermal processes having either a continuous or a discontinuous temperature profile. The same survival model could also be used to estimate the spores’ survival parameters directly from the non-isothermal survival curves in simulated inactivation data to which a scatter had been added.