Metabolic Analysis of Saccharomyces cerevisiae during Alcoholic Fermentation

Sugar utilization activity of yeast cells decreases in the presence of alcohol, followed eventually by a loss in cell viability during alcoholic fermentation. Commercial yeast strains, given an identical initial nutrient concentration and composition, will produce different concentrations of biomass. Theoretically, strains that use nutrients more efficiently to create more biomass should outperform strains with lower biomass. Consequently, these strains should be able to produce and tolerate higher levels of ethanol. This study improved the understanding of the relationship between the nutritional environment and growth rate in Saccharomyces cerevisiae, in order to develop a rational approach to strain modification for modifying ethanol tolerance. The extracellular concentration of key nutrients and products of yeast was assessed throughout batch fermentations. Thirty-four commercial yeast strains were compiled with a wide range of growth and ethanol tolerance attributes. Nutrient utilization and fermentation kinetic parameters were studied for each strain. Maximum optical density for these strains ranged from 2.68 to 6.17, and ethanol production ranged from 4.6 to 15.1 %v/v. The results shown in this study demonstrate that growth rate is yeast strain dependent when supplied with the same nutritional environment. PLSR modeling showed high correlation between biomass production and ethanol tolerance. Extracellular analysis showed that strains with high biomass produced the lowest amount of byproducts in the form of succinic acid, acetic acid and glycerol, and the highest amounts of ethanol. These findings are important because understanding ethanol tolerance will enable the modulation of yeast strains with other desirable characteristics.;Yeast metabolism has been widely studied over the past decades but drivers of cell growth and ethanol tolerance are still not well understood at a fundamental level. Metabolomic approaches promise to deliver tools to better understand and study cellular behavior in yeast. Global metabolic profiling has become a powerful tool in understanding how different cells respond to the same nutritional environment. This study assessed differences in the extracellular and intracellular metabolic profiles of four commercial wine yeast strains with different biomass production under the same nutritional environment aiming to better understand metabolic driving forces for cell growth. When supplied with the same nutritional and cultivation conditions, yeast strains completed the fermentation and had similar residual sugar concentrations left in the fermentation broth, between 3.7 to 11.9 g/L. The total biomass production between the strains varied with maximum optical densities ranging from 5.36 to 8.10. However, the range of total ethanol production for all the strains was from 12.33 to 12.78 %v/v. When examining extracellular metabolites in the media, strains secreted different concentrations of glycerol, acetic acid and succinic acid. Similarly, different nutrient consumption rates were observed when malic acid, tartaric acid, and citric acid were quantified. PLSR analysis showed a positive correlation between long-chain fatty acids and biomass; and a negative correlation between Pentose Phosphate Pathway intermediates and intracellular glycerol with biomass. These differences can likely be attributed to differences in nutrient utilization and activity of metabolic pathways between the strains during the exponential and stationary phase.