Physiological response of Saccharomyces cerevisiae to citral combined with thermal treatment

Yeasts are responsible for spoilage of fruit juices and beverages because of their low pH and high sugar content. Their control is generally guaranteed by thermal treatments that however can affect organoleptic and nutritional properties. A strategy to reduce thermal damage is the combination of mild treatments with natural antimicrobials. This work studied the effect of citral on thermal inactivation of a Saccharomyces cerevisiae strain isolated from spoiled beverages, also in relation to medium pH, through plate counting and flow cytometry (FCM), to assess cell viability and membrane integrity. The results of treatments at 60 °C for 30 min showed that sublethal concentrations of citral increased death kinetics, especially at lower pH. Conversely, FCM analysis performed on cells treated at 60 °C for 10 min evidenced higher mortality at pH 6.0, while at pH 4.0 the population was mainly injured. This different behavior was likely due to higher membrane permeability observed at pH 6.0. No recovery of injured cells to a viable status was observed after 3 h of incubation. Since the fate of these cells in foods is a great concern, these results are helpful to increase knowledge about the physiological response of S. cerevisiae and to optimize strategies for food stability.