The breeding of yeast strains for novel oenological outcomes
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.