Winemaking interventions to modulate glutathione status
The OIV has recently passed two resolutions allowing the addition of glutathione to must and wine. (See OIV-OENO 445 addition of glutathione to must.pdf and OIV-OENO 446 addition of glutathione to wine.pdf.) An Australian evaluation of its potential benefits and risks is necessary to make a case for its inclusion in the Australia New Zealand Food Standards Code, and for the industry to subsequently make informed decisions about its use.
The concentration of glutathione in wine can be enhanced by direct addition, or indirectly as a consequence of winemaking practices. Given its role as aroma precursor, it’s not known what the effect of glutathione is on the expression of terroir and varietal characters; and glutathione has also been reported to act as precursor to a range of desirable and negative sulfur aroma compounds. Much of the previous work associated with glutathione manipulation has targeted Sauvignon Blanc, aiming to preserve varietal thiols and colour in bottled wine. While thiols also contribute to the character of wines made from other varieties, the impact of glutathione addition on wine quality and style is relatively unknown. This project aims to extend current understanding about the effects of glutathione additions in white juices and wines, in other varieties as well as in Sauvignon Blanc.
This project is a continuation of the existing four-year Wine Australia Project AWR 1502 that commenced in June 2016. The research plan includes studies of protective juice treatments, the effects of direct glutathione addition, and an assessment of microbial consumption of glutathione. The fate of added glutathione will be determined through use of labelled compounds, which will aid in the determination of the degree to which glutathione turnover contributes to H2S production. Finally, small-scale winemaking trials will be used to assess the sensory and chemical impact of glutathione treatments. This work will elucidate how glutathione in combination with other factors, can be used during grape processing to manipulate or preserve grape aroma compounds and wine quality, typicity and regional characters.
What happens to glutathione when it is added to a ferment?
OIV resolution OIV-OENO 445-2015 provides limited guidance on the addition of glutathione to must, advising only that practitioners should ensure that the assimilable nitrogen level is sufficient to avoid the metabolism of glutathione by yeast. While the resolution limits the addition of glutathione to 20 mg/L, the conditions under which glutathione consumption by yeast is likely to occur are not specified and have not yet been determined.
Previous work in this project assessed the effects of nitrogen concentration on the consumption of glutathione during fermentation in defined medium and freshly prepared low yeast assimilable nitrogen (YAN) must. While glutathione consumption was observed at all nitrogen concentrations in these experiments, the additional glutathione did not alleviate growth limitations in low nitrogen conditions, indicating it was not utilised as nitrogen source in these experiments.
The consumption of nitrogen during fermentation was further investigated using yeast strains with different capacities for small molecule uptake. It is known that different yeast strains have different capacities to take up small molecules such as glutathione due to different complements of transporters. The consumption of glutathione by two near-identical yeasts was evaluated at three nitrogen concentrations (150, 320 and 430 mg/L YAN) and four glutathione concentrations (0, 20, 100 and 250 mg/L). Residual glutathione concentrations were found to vary with strain at low YAN and low glutathione concentrations, but as either the initial glutathione concentration or the initial YAN concentration increased, the strain-specific differences in glutathione concentration were no longer evident. For both strains the consumption of glutathione was highest at the lowest YAN and highest glutathione concentration.
Yeast strain and nitrogen concentration effects were also observed in the production of low molecular weight sulfur compounds. At low YAN, the addition of glutathione 24 hours post-inoculation was associated with increased post-fermentation hydrogen sulfide (‘rotten egg’ aroma) concentrations. This was not the case at moderate (320 mg/L) or high (430 mg/L) nitrogen concentrations. A similar but more pronounced effect was evident for methyl thioacetate concentrations (‘sulfurous’, ‘cheesy’ aroma), but in addition to nitrogen suppression of a glutathione-mediated response at higher nitrogen concentrations there were also yeast strain-related differences in production of this sulfur compound.
In summary, this work shows that at low glutathione doses, such as those recommended by the OIV, glutathione consumption by yeast can still occur at low must nitrogen concentrations. Some production of glutathione by yeast does also occur and contributes to the overall final glutathione concentration. Pre-ferment glutathione addition can contribute to low molecular weight volatile sulfur compound production (i.e. ‘reductive’ aroma compounds), but the degree to which this occurs can be dependent on yeast strain and nitrogen concentration.