| Summary: | Thesis (MSc (Wine Biotechnology))--University of Stellenbosch, 2005. === The quality of wine is influenced by a variety of factors, most noticeably the quality of
the grapes, winemaking practices and the yeast strains used for alcoholic fermentation.
Although several yeast strains are present in the must at the beginning of fermentation,
strains of S. cerevisiae quickly dominate and survive alcoholic fermentations. This
dominance of S. cerevisiae prompted research that led to the development of a
multitude of industrial yeast starter cultures. Starter cultures are usually capable of quick
and complete fermentations, with minimal production of deleterious substances such as
volatile acidity, H2S, SO2 and ethyl carbamate. Yeast strains should be able to survive
the stressful environment created during alcoholic fermentation, whilst possibly offering
novel oenological benefits such as pectinolytic activity, killer activity and malic acid
degradation. The increased production of volatile esters and higher alcohols may also
be desirable, as this will allow the production of wines that are more aromatic.
In this study, VIN13 was crossed with S. paradoxus strain RO88 and WE14 by using a
micomanipulator. VIN13 was chosen for its fast and complete fermentation ability and
moderate aroma production potential. Other factors such as the presence of killer
activity and low production of volatile sulphur compounds also favoured the selection of
VIN13. S. paradoxus strain RO88 was selected for its ability to degrade malic acid and
the favourable impact on aroma production during fermentation. Hybrids between these
yeasts may have the potential to produce more aromatic wines, with the added bonus of
pectinolytic activity and a strong fermentation capacity. The first crossing yielded 5
hybrids between VIN13 and S. paradoxus strain RO88. Two of these hybrids stood out
in the sense that they were able to degrade more malic acid than VIN13 and they also
possessed killer and pectinolytic activity. Cinsaut wine was made and the 2 hybrids
were shown to have higher aroma compound capacity than the parental yeasts. This
was also confirmed during sensory evaluation. The second crossing between VIN13
and WE14 yielded 10 hybrids with low H2S production potential and killer activity. WE14
was selected for its ability to produce very aromatic wines and also the slower
fermentation capacity. Hybrids between these yeast may have the potential to produce
wines with an increased aromatic content and the fermentation rate might be slower,
thereby improving the aroma profile of the wine. After microvinification, 5 hybrids were
selected on the basis of fermentation rate differing from that of the parental yeasts and
favourable oenological traits, such as fast and complete fermentation, high production of
glycerol and low production of volatile acidity. Pinotage wine was made and it was
shown that some of the hybrids produced more esters and higher alcohols than the
parental yeasts. Sensory evaluation also showed the aroma production potential of the
hybrids, as some of the hybrids were shown to score higher for banana, cherry and
tobacco characteristics.
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