Thesis (PhD (Viticulture and Oenology. Wine Biotechnology))--Stellenbosch University, 2008. / The occurrence of sluggish and stuck fermentations continues to be a serious problem
in the global wine industry, leading to loss of product, low quality wines, cellar
management problems and consequently to significant financial losses. Comprehensive
research has shown that many different factors can act either in isolation, or more
commonly synergistically, to negatively affect fermentative activity of wine yeast strains
of the species Saccharomyces cerevisiae. The individual factors most commonly
referred to in the literature are various nutrient and oxygen limitations. However, other
factors have been shown to contribute to the problem. Because of the mostly synergistic
nature of the impacts, no single factor can usually be identified as the primary cause of
stuck fermentation.
In this study, several strategies to evolutionarily engineer wine yeast strains that are
expected to reduce the occurrence of stuck and sluggish fermentations are investigated.
In particular, the investigations focus on improving the ability of wine yeast to better
respond to two of the factors that commonly contribute to the occurrence of such
fermentations, nitrogen limitation and the development of an unfavorable ratio of
glucose and fructose during fermentation.
The evolutionary engineering strategies relied on mass-mating or mutagenesis of
successful commercial wine yeast strains to generate yeast populations of diverse
genetic backgrounds. These culture populations were then exposed to enrichment
procedures either in continuous or sequential batch cultivation conditions while applying
specific evolutionary selection pressures.
In one of the stragegies, yeast populations were subjected to continuous cultivation
under hexose, and especially fructose, limitation. The data show that the strains
selected after this procedure were usually able to out-compete the parental strains in
these selective conditions. However, the improved phenotype was not detectable when
strains were evaluated in laboratory scale wine fermentations.
In contrast, the selection procedure in continuous cultivation in nitrogen limiting
conditions proved to be highly efficient for the generation of yeast strains with higher
total fermentative capacity in low nitrogen musts.
Furthermore, yeast strains selected after mutagenesis and sequential batch cultivation
in synthetic musts with a very low glucose on fructose ratio showed a fructose specific
improvement in fermentative capacity. This phenotype, which corresponds to the
desired outcome, was also present in laboratory scale wine fermentations, where the
discrepancy between glucose and fructose utilization of the selected strains was
significantly reduced when compared to the parents.
Finally, a novel strategy for the rectification of stuck fermentations was adjusted to
industrial conditions. The strategy is based on the use of a natural isolate of the yeast
species Zygosaccharomyces bailii, which is known for its preference of fructose. This
process was successfully established and implemented in the wine industry.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/1344 |
| Date | 12 1900 |
| Creators | Horsch, Heidi K. |
| Contributors | Bauer, F. F., Gafner, J., Stellenbosch University. Faculty of AgriSciences. Dept. of Viticulture and Oenology. Institute for Wine Biotechnology. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Rights | Stellenbosch University |
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