Improving wine yeast for fructose and nitrogen utilization
Thesis (MSc (Wine Biotechnology))--Stellenbosch University, 2008.
In the wine industry, the importance of selecting an appropriate yeast strain, generally of the species Saccharomyces cerevisiae, to ensure reliable fermentation and to achieve a desired level of quality has been well established. As a consequence, the demand for new starter cultures with improved or new oenological characteristics is increasing. Appropriately selected starter cultures can reduce the occurrence of stuck fermentations, impart specific aroma profiles and reduce the development of offflavours. Using standard breeding and selection procedures, several wine yeast strains that would be less likely than currently existing strains to experience stuck fermentation have previously been developed at the Institute for Wine Biotechnology. The target of these projects had been to develop strains with improved nitrogen efficiency [defined as the amount of fermented hexoses for a given amount of free amino nitrogen (FAN)], improved fructose utilization and ethanol tolerance. These three parameters are known contributors to stuck fermentation. Two of the strains that had been isolated in these projects, strain 116 for nitrogen efficiency and strain 38-1 for efficient fructose utilization, were chosen as parental strains for the current study. The aim was to further improve and possibly combine these traits in yeast strains by using hybridization followed by various enrichment and directed evolution procedures in a continuous fermentation setup. The strategy was to sequentially subject the population of mass-mated hybrids to a number of selective environments for a large number of generations. The yeasts were subjected to a high fructose/glucose ratio for 12 generations, followed by selection in an environment with a limited supply of nitrogen for 54 generations and finally to high ethanol stress. After each round of enrichment, individual strains were analysed to assess the results. For the hybrid strains selected after enrichment in a medium with a high fructose/glucose ratio, no general improvement could be discerned. However, one of the hybrids, hybrid strain 331, fermented fructose better than the parental strains and other hybrid strains. These results may suggest that the selection pressure was not applied for a sufficient number of generations and may not have been sufficiently strong. In addition, the parental strain may already performing at a rate that may render further improvement more difficult in this genetic background. The next aim of this study was to enhance fermentation performance of wine yeast hybrid strains in low nitrogen and high sugar conditions. Several hybrid strains 331, RR03 and 05R generated in this study showed improvement in efficiency of nitrogen utilization when compared to the parental strains, indicating a successful selection strategy. Several strains also showed higher ethanol tolerance, and some strains possessed] combinations of the traits to be improved. Future research will evaluate these hybrids regarding the production of aromatic compounds and of the sensory profile produced. Such strains would help the wine industry to control the occurrence of stuck fermentations and to produce quality wines.