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The use of yeasts other than Saccharomyces cerevisiae to produce wines with novel aroma and flavour profiles is gaining increased attention. The present study is concerned with the sensory impact on wine of fermentation with two selected Saccharomyces bayanus strains. The S. bayanus strains AWRI 1176 and AWRI 1375 were compared to S. cerevisiae strain AWRI 838 for their ability to affect the aroma profile and chemical composition of Chardonnay wine. Wines that were made with the S. bayanus strains contained more glycerol, succinic acid, acetaldehyde and SO 2 than reference wines made with the S. cerevisiae wine yeast, but less acetic acid, malic acid and ethyl acetate. There were significant differences in the aroma profile of wines made with each yeast, as quantified by formal sensory descriptive analysis. Wines made with S. cerevisiae AWRI 838 were rated highly in the attributes 'estery', 'pineapple', 'peach' and 'citrus'. Wines made with S. bayanus AWRI 1176 were rated lower than AWRI 838 in each of these attributes, but higher in the attributes 'cooked orange peel', 'yeasty', 'nutty' and 'aldehyde'. The aroma profile of wine made using AWRI 1375 was different from that of wine made with the other yeasts, showing intermediate scores for the 'estery', 'citrus', 'nutty' and 'aldehyde' attributes, and no significant difference from the AWRI 838 strain in the attributes 'pineapple' and 'peach'. These results demonstrate that, in addition to producing wine which has a different chemical composition, S. bayanus strains can produce a different sensory profile in wine when compared to a widely used S. cerevisiae wine yeast.

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.

The formation of sulfur dioxide (SO2) by the related Saccharomyces cerevisiae strains AWRI 838, Lalvin EC11 18 and Maurivin PDM has been determined in white wines produced by laboratory-scale fermentation. Three different preparations of these strains, available for vinification in 1995, were indistinguishable based on production of SO2. None of the physico-chemical factors investigated could stimulate an over-production of SO2, and any changes in its concentration were independent of juice pH, sugar content, exposure to air, clarity, initial concentration of SO2, and grape variety. Only fermentation without exposure to air could affect the concentration of SO2, reducing it by 13–15% in the final wines. Under the conditions tested, changes in the concentration of SO2 in wines produced with AWRI 838 and related strains would not account for wines which resist malolactic fermentation.

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.