Dimensionless analysis of the microbial growth rate dependence on sub-optimal temperatures

The influence of sub-optimal temperatures (T) on the microbial growth rate (mu) has been assessed by means of dimensionless variables: T(dim) = [T-T(min)]/[T(opt)-T(min)] and mu(dim) = mu/mu(opt). T(min) represents the temperature at which there is no growth, T(opt) the optimum temperature at which the growth rate, mu(opt), is maximum. Data sets, growth rate vs temperature, have been taken from the literature for 12 organisms (psychrotrophs, mesophiles and thermophiles). In order to compare these organisms, the power law function has been used: [mu(dim)] = [T(dim)](alpha) The parameters mu(opt) and T(opt) are determined from direct readings whereas T(min) and alpha are estimated by means of a non-linear regression. An accurate estimation of T(min) is obtained providing low growth rate data are available. A wide range of optimal temperatures where the growth rate almost equals mu(opt) prevents one from obtaining a narrow confidence interval for cu. On the basis of the analysis hereafter developed, thermophiles are characterized by values of the power cu less than mesophiles and psychrotrophs. Almost all of these values are significantly different from two, previously determined for Staphylococcus xylosus and widely used for predicting the microbial growth in foods.