|dc.description.abstract||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
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.||en