Combined Effects of pH and Sugar on Growth Rate of Zygosaccharomyces rouxii, a Bakery Product Spoilage Yeast

The effects of citric acid-modified pH (pH 2.5, 2.75, 3, 3.5, 4, 4.5, 5, and 5.5) and a 30% glucose–70% sucrose mixture (300, 400, 500, 600, 700, 800, 875, and 900 g/liter) on an osmophilic yeast, Zygosaccharomyces rouxii, were determined by using synthetic medium. One hundred experiments were carried out; 50-ml culture flasks were inoculated with 10(3) CFU ml(−1) by using a collection strain and a wild-type strain cocktail. The biomass was measured by counting cell colonies, and growth curves were fitted by using a Baranyi equation. The growth rate decreased linearly with sugar concentration, while the effect of pH was nonlinear. Indeed, the optimal pH range was found to be pH 3.5 to 5, and pH 2.5 resulted in a 30% reduction in the growth rate. Finally, we evaluated the performance of two nonlinear predictive models developed previously to describe bacterial contamination. Equations derived from the Rosso and Ratkowsky models gave similar results; however, the model that included dimensionless terms based on the Ratkowsky equation was preferred because it contained fewer estimated parameters and also because biological interpretation of the results was easier.

本研究采用合成培养基，探究柠檬酸调节的pH值（设置为2.5、2.75、3、3.5、4、4.5、5及5.5）与30%葡萄糖-70%蔗糖混合体系（浓度梯度为300、400、500、600、700、800、875及900 g/L）对嗜渗酵母鲁氏接合酵母（Zygosaccharomyces rouxii）的影响。本次实验共开展100组测试：采用保藏菌株与野生型菌株混合接种液，以10³ 菌落形成单位（CFU）·mL⁻¹的接种量接种至50 mL培养瓶中。通过菌落计数法测定菌体生物量，并采用Baranyi方程拟合生长曲线。菌体生长速率随糖浓度升高呈线性下降趋势，而pH对生长的影响则呈非线性特征。研究确定该菌株的最优pH范围为3.5~5，当pH为2.5时，其生长速率降低30%。最后，本研究评估了此前开发的两款用于描述细菌污染的非线性预测模型的性能。基于Rosso模型与Ratkowsky模型推导得到的方程预测结果相近；但基于Ratkowsky方程构建的无量纲项模型所需估计参数更少，且结果的生物学解释更为简便，因此更受青睐。