Development of fructophyllic wine yeast for the reduction of residual sugar in wine
Bauer, F F
University of Stellenbosch. Faculty of AgriSciences. Institute for Wine Biotechnology
Muller-Patterton, H E
Berthels, N J
Saccharomyces cerevisiae is a glucophyllic yeast, preferring glucose to any other carbon source that may be present in the growth substrate. Data have confirmed that in bottled wine, fructose is always found in larger amounts than glucose. During fermentation, a strong imbalance in the ratio of fructose and glucose is also suspected to be a leading cause of stuck fermentation.
This project aims at increasing the affinity of S. cerevisiae wine yeast strains for fructose in order to:
– ferment more efficiently to complete dryness
– be less susceptible to stuck fermentation
Following consumer demands and international winemaking practices, a large percentage of quality natural wines are fermented to dryness. However, a small and variable amount of hexoses (glucose and fructose) is always left in the must at the end of fermentation. In many cases, winemakers would like to further decrease this residual sugar because the wine is considered too sweet. However, it is usually not possible to eliminate the remaining sugar, even when inoculating with new yeast cells. Studies in our Institute and elsewhere have shown that the remaining sugar consists mostly of fructose (Fig. 1), which is fermented at a slightly slower rate than glucose by all the wine yeast strains tested thus far. In order to address this problem, the project proposes to
– assess all the most commonly used South African wine yeast strains for their ability to ferment fructose
– study the molecular mechanisms responsible for the preferential use of glucose
– implement three different strategies to address the problem by developing yeast strains which are able to ferment fructose with similar efficiency than glucose.