A cardinal model to describe the effect of water activity on the growth of moulds.

A simple model was proposed to describe the effect of water activity (Aw) on the radial growth rate of moulds. This model is deduced from the cardinal model family proposed by Rosso in 1995, which is only defined from cardinal values of environmental factors (minimum, optimum and maximum values), the growth rate observed at the optimal value of the environmental factors, and n, a shape parameter. For Aw, a simple form of cardinal model is proposed. This form is obtained for n = 2 and Aw(max) = 1.0 (pure water). The final model is so defined from only three parameters: Aw(min), Aw(opt), and optimal radial growth rate (RGR(opt)). This model was successfully fitted on a data set of Aspergillus flavus, Aspergillus nomius, Aspergillus oryzae, Aspergillus parasiticus, Aspergillus candidus, Aspergillus sydowii, Eurotium amstelodami, Eurotium chevalieri, and Xeromyces bisporus. The same quality of fit was obtained for different solutes used to control the Aw (NaCl, glucose/fructose mixture, glycerol), and at different pH values. From this model and using cardinal values extracted from the literature, theoretical evolutions of the RGR of enicillium roqueforti, and Paecilomyces variotii, were proposed and superimposed on data published in the literature. The results showed a good concordance between the predicted and the observed values for these species. The use of this model in Predictive Microbiology is discussed.