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A mathematical model efficiently gives an indication of growth, and propionate and acetate production by a Pectinatus species during beer spoilage. This allows for the estimation of the risk of contamination of beer in a brewery process. The optimal conditions for the growth rate as estimated from the model are a temperature of 35 degrees Celsius, a pH of 6.5 and 0% ethanol concentration. The optimal conditions for cell density and cell yield are 32 degree Celsius, a pH of 6.0 and 1% ethanol. Ethanol concentrations above 8% and pH values below 4.0 are unfavourable for growth.

The effect of water activity (a(sub w)) on the heat resistance of Bacillus cereus spores (decimal reduction time or D) was studied. A linear relationship between log D and 1 - a(sub w) was observed. The combined influences of temperature, pH and water activity were then investigated and a four parameter model employed to fit the data. Interactions between the parameters were seen but they accounted for less than 2.4% of the total variation and were not considered in the model.

The generalized linear-Bigelow model extends the concept of standard decimal reduction time Dt* into a D-value which is also connected with a reference pH. This research proposed a generalized linear-Bigelow model to describe the effects of temperature and pH on spore heat resistance. The model was tested by applying three published sets of data. This model does not take account of interactions between temperature and pH, but its goodness of fit is proof of its robustness.

A new primary model, based on two mixed Weibull distributions of cell resistance is proposed to describe the survival curves and the change in the pattern with the modifications of resistance of two assumed subpopulations of bacteria. The parameters of the model were stable and showed consistent evolution according to the initial physiological state of the bacterial population.