Modelling the effect of temperature and water activity on the growth of two ochratoxigenic strains of Aspergillus carbonarius from Greek wine grapes

Aims: To develop descriptive models for the combined effect of temperature (10-40 degrees C) and water activity (0.850-0.980) on the growth of two ochratoxin A producing strains of Aspergillus carbonarius from Greek wine grapes on a synthetic grape juice medium. Methods and Results: Fungal growth was measured as changes in colony diameter on a daily basis. The maximum specific colony growth rates (mu(max)) were determined by fitting the primary model of Baranyi describing the change in colony diameter (mm) with respect to time (days). Secondary models, relating mu(max) with temperature and a(w) were developed and comparatively evaluated based on polynomial, Parra, Miles, Davey and Rosso equations. No growth was observed at 0.850 a(w) (water activity) regardless of temperature, as well as at marginal temperature levels assayed (10 and 40 degrees C) regardless of water activity. The data set was fitted successfully in all models as indicated by the values of regression coefficients and root mean square error. Models with biological interpretable parameters were highly rated compared with the polynomial model, providing realistic cardinal values for temperature and a(w). The optimum values for growth were found in the range 0.960-0.970 a(w) and 34-35 degrees C respectively for both strains. The developed models were validated on independently derived data from the literature and presented reasonably good predictions as inferred by graphical plots and statistical indices (bias and accuracy factors). Conclusions: The effect of temperature and a(w) on the growth of A. carbonarius strains could be satisfactorily predicted under the current experimental conditions, and the proposed models could serve as a tool for this purpose. Significance and Impact of the Study: The results could be successfully employed as an empirical approach in the development and prediction of risk models of contamination of grapes and grape products by A. carbonarius.