Phytosterols and ergosterol role during wine alcoholic fermentation for 27 Saccharomyces cerevisiae strains

Abstract

Sterols are a class of the eukaryotic lipidome that is essential for the maintenance of the cell membrane integrity and their good functionality (Daum et al., 1998). During alcoholic fermentation, they ensure yeast growth, metabolism and viability, as well as resistance to osmotic stress and ethanol inhibition (Mannazzu et al., 2008). Musts clarified in excess lead to the loss of solid particles rich in sterols, resulting in sluggish and stuck fermentations (Casalta et al., 2013). Two sterol sources can support yeasts to adapt to fermentation stress conditions: ergosterol, produced by yeast in aerobic conditions, and phytosterols, plant sterols found in grape musts imported by yeasts in the absence of oxygen (Nes, 1987). Little is known about the physiological impact of the assimilation of phytosterols in comparison to ergosterol and the influence of sterol nature on fermentation kinetics parameters. Moreover, studies done until today analyzed a limited number of yeasts strains. For this reason, the aim of this work is to compare the fermentation performances of 27 Saccharomyces cerevisiae strains with phytosterols and ergosterol on two conditions: sterol stress (sterol starvation) and osmotic stress (the most common stress during fermentation due to high concentrations of sugars).Experiments were performed in 300 mL fermenters without oxygen. Fermentation kinetics were monitored continuously through CO2 production in order to obtain parameters, such as the maximum fermentation rate (Vmax) or total CO2 production. Cell count and cell viability were measured around 80% of fermentation progress. Central carbon metabolism (CCM) metabolites (acetate, glycerol, succinate and residual sugars) were quantified at the end of fermentation.Principal Component Analysis with biological, kinetic and CCM variables revealed the huge phenotype diversity of strains in this study. Analysis of variance (ANOVA) indicated that both the strain and the nature of sterol explained the differences on yeast performances in fermentation. It should be noted that cellular viability is a key parameter in both sterol and osmotic stress. Indeed, strains with a high viability at the end of the fermentation finished fermenting earlier. Finally, ergosterol allowed a better completion of fermentation in both stress conditions tested.These results highlighted the role of sterols in wine alcoholic fermentation to ensure yeast growth and avoid sluggish or stuck fermentations. Interestingly, sterol nature affected yeast viability, biomass, kinetics parameters and biosynthesis of CCM metabolites