Validation of a predictive model for the growth of Botrytis cinerea and Penicillium expansum on grape berries

The objective of this study was to develop and to validate a model for predicting the combined effect of temperature and aw on the radial growth rate, μ, of Botrytis cinerea and Penicillium expansum on grape berries. The proposed strategy was based on the gamma-concept developed previously [Zwietering, M.H., Wijtzes, T., de Wit, J.C., van't Riet, K. 1992. A decision support system for prediction of the microbial spoilage in foods. Journal of Food Protection. 12, 973-979]: μ = μopt.γ(T).γ(aw), where the gamma functions were cardinal models with inflection (CMI), μopt the radial growth rate on grape berries. Firstly, the cardinal temperatures and aw's were estimated independently from experiments carried out on Potato Dextrose Agar. Secondly, the gamma concept was validated i/ on a synthetic grape juice medium (SGJ) and ii/ on a grape juice agar (GJA). Accuracy and bias factors were closer to 1 with the latter analogue, thus suggesting that GJA should be preferred to SGJ. Thirdly, an experimental protocol taken into account the isotropic nature of fungal growth was developed for estimating μopt on grape berries. This study demonstrated that CMI's can be validated on agri-food products over a wide range of temperature and aw using the described methodology.