Screening of different stress factors and development of growth/no growth models for Zygosaccharomyces rouxii in modified Sabouraud medium, mimicking intermediate moisture foods (IMF).

The microbial stability of intermediate moisture foods (IMF) is linked with the possible growth of osmophilic yeast and xerophilic moulds. As most of these products have a long shelf life the assessment of the microbial stability is often an important hurdle in product innovation. In this study a screening of several Zygosaccharomyces rouxii strains towards individual stress factors was performed and growth/no growth models were developed, incorporating a(w), pH, acetic acid and ethanol concentrations. These stress factors are important for sweet IMF such as chocolate fillings, ganache, marzipan, etc. A comparison was made between a logistic regression model with and without the incorporation of time as an explanatory variable. Next to the model development, a screening of the effect of chemical preservatives (sorbate and benzoate) was performed, in combination with relevant stress factors within the experimental design of the model. The results of the study showed that the influence of the investigated environmental stress factors on the growth/no growth boundary of Z. rouxii is the most significant in the first 30-40 days of incubation. Incorporating time as an explanatory variable in the model had the advantage that the growth/no growth boundary could be predicted at each time between 0 and 60 days of incubation at 22 °C. However, the growth/no growth boundary enlarged significantly leading to a less accurate prediction on the growth probability of Z. rouxii. The developed models can be a useful tool for product developers of sweet IMF. Screening with chemical preservatives revealed that benzoic acid was much less active towards Z. rouxii than sorbic acid or a mixture of both acids.