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A high concentration of acetic acid has been suggested to be a cause of incomplete fermentation. Acetic acid can reduce the growth and fermentation activity of Saccharomyces cerevisiae, and incomplete ferments can contain a high concentration of acetic acid, although the origin is often not clear. Here we examined the concentration of acetic acid which could hinder or prevent the restarting of an incomplete Cabernet Sauvignon fermentation by three acclimatized wine yeast cultures. By taking advantage of new technology which combines reverse osmosis and ion exchange, the concentration of acetic acid in an incomplete commercial ferment was adjusted to between 0.5 and 4.0 g/L. At a concentration up to 4 g/L, acetic acid did not prevent the restarting of the incomplete ferment. No lag period was observed in any of the fermentations. The amount of sugar fermented after 20–21 days was dependent on the strain of Sacch. cerevisiae and was correlated with the initial concentration of acetic acid at the time of restarting, between 0.5 and 2.0 g/L, with the concentration of residual sugar increasing with the concentration of acetic acid. In ferments which contained 4 g acetic acid/L, however, the rate of sugar consumption was sufficiently low after 20–21 days to suggest that complete fermentation would not occur, even with prolonged incubation. Effective treatment of the incomplete fermentation was therefore possible in the presence of up to 2 g acetic acid/L. Cell viability and fermentation kinetics were also strongly dependent on the strain of yeast.

Glycerol is a major fermentation product of Saccharomyces cerevisiae that contributes to the sensory character of wine. Diverting sugar to glycerol overproduction and away from ethanol production by overexpressing the glycerol 3-phosphate dehydrogenase gene,GPD2, caused S. cerevisiae to produce more than twice as much acetic acid as the wild-type strain (S288C background) in anaerobic cell culture. Deletion of the aldehyde dehydrogenase gene, ALD6, in wild-type and GPD2 overexpressing strains (GPD2-OP) decreased acetic acid production by three- and four-fold, respectively. In conjunction with reduced acetic acid production, the GPD2-OP ald6Δ strain produced more glycerol and less ethanol than the wild-type. The growth rate and fermentation rate were similar for the modified and wild-type strains, although the fermentation rate for the GPD2ald6Δ strain was slightly less than that of the other strains from 24 h onwards. Analysis of the metabolome of the mutants revealed that genetic modification affected the production of some secondary metabolites of fermentation, including acids, esters, aldehydes and higher alcohols, many of which are flavour-active in wine. Modification of GPD2 and ALD6 expression represents an effective strategy to increase the glycerol and decrease the ethanol concentration during fermentation, and alters the chemical composition of the medium such that, potentially, novel flavour diversity is possible. The implications for the use of these modifications in commercial wine production require further investigation in wine yeast strains. Copyright © 2002 John Wiley & Sons, Ltd.