Combined effects of pH and sugar on growth rate of Zygosaccharomyces rouxii, a bakery product spoilage yeast

Abstract

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.

FIG. 1

Linear relationship between specific growth rate (μ) and sugar concentration. Experimental data were obtained at 25°C and pH 3.5 to 5.

FIG. 2

Growth kinetics of Z. rouxii in synthetic medium at 25°C in the presence of high sugar concentrations. The symbols indicate observed values, and the lines are Baranyi fitting curves.

FIG. 3

Effect of pH on the specific growth rate (μ) at 25°C for various sugar concentrations. The symbols indicate observed values, and the lines are nonlinear model curves. (a) Ratkowsky-derived model (equation 4). (b) Rosso-derived model (equation 5). Symbols: ⧫, 300 g of sugar liter⁻¹; □, 400 g of sugar liter⁻¹; ▴, 500 g of sugar liter⁻¹; ■, 600 g of sugar liter⁻¹; ▵, 700 g of sugar liter⁻¹; ●, 800 g of sugar liter⁻¹; ×, 875 g of sugar liter⁻¹; ○, 950 g of sugar liter⁻¹.

FIG. 4

Histograms of residual values. (a) Ratkowsky-derived model (equation 4). (b) Rosso-derived model (equation 5).

FIG. 5

Effects of sugar and pH on the specific growth rate (μ) at 25°C, as determined with the Ratkowsky-derived model (equation 4).