Improving wine yeast for fructose and nitrogen utilization

[摘要] In the wine industry, the importance of selecting an appropriate yeast strain, generally ofthe species Saccharomyces cerevisiae, to ensure reliable fermentation and to achieve adesired level of quality has been well established. As a consequence, the demand fornew starter cultures with improved or new oenological characteristics is increasing.Appropriately selected starter cultures can reduce the occurrence of stuckfermentations, impart specific aroma profiles and reduce the development of offflavours.Using standard breeding and selection procedures, several wine yeast strains thatwould be less likely than currently existing strains to experience stuck fermentation havepreviously been developed at the Institute for Wine Biotechnology. The target of theseprojects had been to develop strains with improved nitrogen efficiency [defined as theamount of fermented hexoses for a given amount of free amino nitrogen (FAN)],improved fructose utilization and ethanol tolerance. These three parameters are knowncontributors to stuck fermentation. Two of the strains that had been isolated in theseprojects, 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 improveand possibly combine these traits in yeast strains by using hybridization followed byvarious enrichment and directed evolution procedures in a continuous fermentation setup.The strategy was to sequentially subject the population of mass-mated hybrids to anumber of selective environments for a large number of generations. The yeasts weresubjected to a high fructose/glucose ratio for 12 generations, followed by selection in anenvironment with a limited supply of nitrogen for 54 generations and finally to highethanol stress. After each round of enrichment, individual strains were analysed toassess the results.For the hybrid strains selected after enrichment in a medium with a highfructose/glucose ratio, no general improvement could be discerned. However, one ofthe hybrids, hybrid strain 331, fermented fructose better than the parental strains andother hybrid strains. These results may suggest that the selection pressure was notapplied for a sufficient number of generations and may not have been sufficientlystrong. In addition, the parental strain may already performing at a rate that may renderfurther improvement more difficult in this genetic background.The next aim of this study was to enhance fermentation performance of wine yeasthybrid strains in low nitrogen and high sugar conditions. Several hybrid strains 331,RR03 and 05R generated in this study showed improvement in efficiency of nitrogenutilization when compared to the parental strains, indicating a successful selectionstrategy.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 aromaticcompounds and of the sensory profile produced. Such strains would help the wineindustry to control the occurrence of stuck fermentations and to produce quality wines.

[发布机构] Stellenbosch University