Metabolic Basis of Differing Ethanol Yields in Saccharomyces

The goal of this study is the metabolomic analysis of ten commercial wine yeast strains that show differences in fermentation behavior and in final ethanol yield. Five of these strains tend to give lower and five higher ethanol yields during grape juice fermentation. Since non-sterile cold-settled juices were to be used to mimic wine production conditions, genetically marked strains of each commercial strain were generated. These strains were marked with a drug resistance gene that encodes resistance to kanamycin or G418. Initial experiments compared the fermentation performance of these strains to their parental strains and there were no differences noted. The kanamycin resistance gene was inserted into the HO gene which is required for yeast mating and not expressed in diploid cells thus it presents an attractive location at which to insert foreign marker DNA. The strains were grown in six replicate fermentations for the metabolomic analysis. Ethanol yields and dominance of the fermentation by the strains were monitored. One strain was not able to complete the fermentation of this juice but it was still taken for metabolomic analysis as this may reveal why it eventually arrested fermentation. Comparisons of the metabolomics data for high and low ethanol-producing strains suggests that low ethanol producers maintain active growth longer than high ethanol producers thus directing more sugar carbon towards biosynthetic intermediates and cellular components than high producers. High ethanol producing strains also showed evidence of the accumulation of degradation products of plant cell wall polysaccharides. This has not been observed before and offers the possibility that some of these components may be catabolized as well at this time which would boost yields of ethanol even further. Analysis of the metabolite pool levels of strains with intermediate ethanol production is currently underway.