Low Ethanol Yielding Yeast

Project Number
IWBT-Y 08-01

Project title
Low ethanol yielding yeast

Project leader
Bauer, F F

University of Stellenbosch. Faculty of AgriSciences. Institute for Wine Biotechnology

Team members
Bauer, F F
Bosch, S
Tai, S L
Madlanga, N H
Wepener, I

Project description
The average ethanol content of South African and international wines have increased significantly over the past decade. This appears to be mainly due to a shift in viticultural practices, with grapes being left to ripen for longer periods of time resulting in higher levels of fermentable sugars in the must. Higher initial sugar concentrations in the must logically lead to a higher ethanol level in the wine. In many cases, such high ethanol levels have a negative impact on wine quality and consumer perceptions. An additional problem associated with high initial sugar concentrations in must is the increased risk of experiencing problem fermentations, either sluggish or stuck, and ending up with wines with high residual sugar concentrations (residual sugar > 5 g/l). Discussions with wine makers indicate that these problems are some of the most serious issues affecting the future competitiveness of the South African wine industry.

Several mechanical-physical methods for the reduction of ethanol have been developed, including methods based on reverse osmosis and on centrifugal forces in a spinning cone. However, these methods of ethanol removal are labour-intensive, expensive and may in some cases have a significant impact on the final quality of wine, resulting in altered taste, aroma or texture .

Wine yeast strains have been evaluated for their intrinsic ethanol yields (the amount of ethanol that is produced per unit of sugar consumed). These studies show that the ethanol yields of individual strains are dependent on environmental conditions (including availability of oxygen, temperature and others) and that different strains have different ethanol yields. However, all variations fall within a narrow band of values, and the differences are too small to have a significant impact on final ethanol concentrations. For this reason, approaching the problem from a biotechnological perspective appears the most promising option to provide an easy to implement and cost-effective solution.

The aims of the project are to identify biotechnological targets for strain improvement (ethanol yield and fermentation efficiency) and to apply these insights to develop strains that have high fermentation efficiency combined with significantly lower ethanol yields.

Bosch, S and Bauer, F F. 2008. Understanding carbon flux in Saccharomyces cerevisiae: The case of fermentations with high sugar content. Paper presented at the Yeast Genetics and Molecular Biology Meeting. 22-27 July, Toronto, Canada.

Rossouw, D and Bauer, F F. 2010. Comparative omics of wine yeast strains. Paper presented at the 7th International Symposium on Innovations in Enology and Intervitis Interfructa. 9-11 May, Stuttgart, Germany.

Vivier, M A, Du Toit, M and Bauer, F F. 2011. Wine metabolomics: How yeast and bacteria transform the grape metabolome. Paper presented at the WAC 2011: 2nd Wine Active Compounds Conference. 24-26 March, Beaune, France.

Bauer, F F. 2011. Yeast – the wine builder. Paper presented at the 34th World Congress of Vine and Wine and the 9th General Assembly of the OIV. 20-27 June 2011, Pronto, Portugal.

Rossouw, D, Olivares-Hernandes, R, Nielsen, J, Bauer, F F. 2009. A comparative ‘omics’ approach to investigate differences in wine yeast physiology and metabolism during fermentation, Applied and Environmental Microbiology, Mnth Oct v. 75 (20) (p. 6600-6012)

Rossouw, D, Van den Dool, A H, Jacobson, D, Bauer, F F. 2010. Comparative transcriptomic and proteomic profiling of industrial wine yeast strains, Applied and Environmental Microbiology, Mnth Jun v. 76 (p. 3911-3923)

Jain, V K, Divol, B, Prior, B A, Bauer, F F. 2011. Effect of alternative NAD+ regenerating pathways on the formation of primary and secondary aroma compounds in a Saccharomyces cerevisiae glycerol defective mutant, Applied and Environmental Biotechnology, v. 93 (p. 131-141)

Rossouw, D, Jacobson, D, Bauer, F F. 2012. Transcriptional regulation and the diversification of metabolism in wine yeast strains, Genetics, v. 190 (1) (p. 251-261)

Franken, J, Brandt, B A, Tai, S L, Bauer, F F. 2013. Biosynthesis of Levan, a bacterial extracellular polysaccharide, in the yeast Saccharomyces cerevisiae, PLos One, v. 8 (10) (p. e77499)

Rossouw, D, Heyns, E H, Setati, M E, Bosch, S, Bauer, F F. 2013. Adjusting trehalose metabolism to modify ethanol yields in wine yeast strains, Applied and Environmental Microbiology, Mnth Jun v. 17 (12) (p. 3911–3923)


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