Modelling the growth/no growth boundary of Zygosaccharomyces bailii in acidic conditions: A contribution to the alternative method to preserve foods without using chemical preservatives

The aim of the study was to develop mathematical models describing growth/no growth (G/NG) boundaries of the highly resistant food spoilage yeast—Zygosaccharomyces bailii—in different environmental conditions, taking acidified sauces as the target product. By applying these models, the stability of products with characteristics within the investigated pH, aw and acetic acid ranges can be evaluated. Besides, the well-defined no growth regions can be used in the development of guidelines regarding formulation of new shelf-stable foods without using chemical preservatives, which would facilitate the innovation of additive-free products. Experiments were performed at different temperatures and periods (22 °C for 45 and 60 days, 30 °C for 45 days) in 150 modified Sabouraud media characterized by high amount of sugars (glucose and fructose, 15% (w/v)), acetic acid (0.0–2.5% (v/v), 6 levels), pH (3.0–5.0, 5 levels) and aw (0.93–0.97, 5 levels). These time and temperature combinations were chosen as they are commonly applied for shelf-stable foods. The media were inoculated with ca. 4.5 log CFU/ml and yeast growth was monitored daily using optical density measurements. Every condition was examined in 20 replicates in order to yield accurate growth probabilities. Three separate ordinary logistic regression models were developed for different tested temperatures and incubation time. The total acetic acid concentration was considered as variable for all models. In general, when one intrinsic inhibitory factor became more stringent, the G/NG boundary shifted to less stressful conditions of the other two factors, resulting in enlarged no growth zones. Abrupt changes of growth probability often occurred around the transition zones (between growth and no growth regions), which indicates that minor variations in environmental conditions near the G/NG boundaries can cause a significant impact on the growth probability. When comparing growth after 45 days between the two tested temperatures, an unexpected phenomenon was observed: the no growth region at 30 °C was larger than the one at 22 °C, though it is known that 30 °C is the optimal growth temperature for Z. bailii. These results show that lowering temperature does not always lead to a reduced growth of the yeast (i.e. more stable foods) and storing shelf-stable products at the higher temperature (30 °C) is not always the worst case. In addition, at 22 °C, there was no significant difference in no growth zones between the two incubation periods (45 and 60 days), implying that the no growth zones remain unchanged if the experimental time is sufficiently long.