Effect of temperature and water activity on growth and ochratoxin A production boundaries of two Aspergillus carbonarius isolates on a simulated grape juice medium.

AIMS: To develop and validate a logistic regression model to predict the growth and ochratoxin A (OTA) production boundaries of two Aspergillus carbonarius isolates on a synthetic grape juice medium as a function of temperature and water activity (a(w)). METHODS AND RESULTS: A full factorial design was followed between the factors considered. The a(w) levels assayed were 0.850, 0.880, 0.900, 0.920, 0.940, 0.960, 0.980 and the incubation temperatures were 10, 15, 20, 25, 30, 35 and 40 degrees C. Growth and OTA production responses were evaluated for a period of 25 days. Regarding growth boundaries, the degree of agreement between predictions and observations was >99% concordant for both isolates. The erroneously predicted growth cases were 3.4-4.1% false-positives and 0.7-1.4% false-negatives. No growth was observed at 10 degrees C and 40 degrees C for all a(w) levels assayed, with the exception of 0.980 a(w)/40 degrees C, where weak growth was observed. Similarly, OTA production was correctly predicted with a concordance rate >98% for the two isolates with 0.7-1.4% accounting for false-positives and 2.0-2.7% false-negatives. No OTA production was detected at 10 degrees C or 40 degrees C regardless of a(w), and at 0.850 a(w) at all incubation temperatures. With respect to time, the OTA production boundary shifted to lower temperatures (15-20 degrees C) as opposed to the growth boundary that shifted to higher temperature levels (25-30 degrees C). Using two literature datasets for growth and OTA production of A. carbonarius on the same growth medium, the logistic model gave one false-positive and three false-negative predictions out of 68 growth cases and 13 false-positive predictions out of 45 OTA production cases. CONCLUSIONS: The results of this study suggest that the logistic regression model can be successfully used to predict growth and OTA production interfaces for A. carbonarius in relation to temperature and a(w). SIGNIFICANCE AND IMPACT OF THE STUDY: The proposed modelling approach helps the understanding of fungal-food ecosystem relations and it could be employed in risk analysis implementation plans to predict the risk of contamination of grapes and grape products by A. carbonarius.

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.