Modelling of the growth/no growth interface of Wallemia sebi and Eurotium herbariorum as a function of pH, aw and ethanol concentration.

High sugar products (sugar content > 50%) are generally considered to be stable against all forms of microbial spoilage during a prolonged shelf life of several months. However, one specific subgroup of micro-organisms, the xerophilic moulds, can develop quite fast on the surface of food products with a reduced water activity (< 0.85). The chance whether these xerophilic moulds are able to grow on the food product depends on the combination of intrinsic factors (e.g., water activity and pH) and the storage conditions (e.g., temperature). This study examines the development of growth/no growth models for the xerophilic moulds Wallemia sebi and Eurotium herbariorum in a sugar rich broth. Growth/no growth models are predictive models that are designed to give a prediction about the probability of growth of a spoilage micro-organism under a specific set of environmental conditions. In this research, a water activity between 0.75 and 0.90, a pH between 5.0 and 6.2, an ethanol concentration between 0% and 5% (g EtOH/g H2O) and their interactions were tested. The inoculated media were stored at 22 °C (± 1 °C) during a prolonged test period (up to 120 days). The obtained models were also validated in a chocolate-based food product (ganache). The resulting growth/no growth models show that the growth of W. sebi and E. herbariorum can be inhibited for a prolonged time (> 3 months) if an ethanol concentration of 5% on the water phase is present in the food product, irrespective of water activity values between 0.89 and 0.755. The necessary amount of ethanol for shorter shelf lives can be calculated with the models that were built. Although the models have not been validated thoroughly in actual food products, the preliminary results that were obtained by testing the model on a ganache indicated that the models are capable of delivering safe predictions.