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.
Predictability of glutathione consumption
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. The capacity of different yeasts to consume glutathione and the metabolic endpoints are also not clear. To address this, the effects of nitrogen concentration on the consumption of glutathione during fermentation were evaluated in defined medium and freshly prepared low YAN must. While consumption of glutathione was reduced in ferments with higher nitrogen concentrations, substantial glutathione consumption was observed at all nitrogen concentrations. The consumed glutathione did not alleviate growth limitations in low nitrogen conditions, indicating it was not used as a nitrogen source in these experiments. Finally, increased glutathione consumption in low nitrogen ferments was not associated with changes to the volatile sulfur compound profile of the finished wines. In all cases the volatile profile was driven by nitrogen concentration rather than glutathione concentration.
In parallel work, the effects of inert pressing and aerobic pressing on the consumption of glutathione added prior to inoculation were evaluated. Inertly processed must has, in some reports, been shown to have much higher concentrations of glutathione, with the glutathione preserved following disruption of the berry. It was predicted that the addition of glutathione under this condition would raise concentrations to levels higher than are usually encountered. Conversely, it was predicted that addition of glutathione to aerobically pressed juice would result in additional glutathione oxidation and in the formation of glutathione adducts. In this work, however, differences in glutathione concentration in pre-fermentation musts did not relate to pressing treatment. Glutathione was consumed during ferment similarly in both oxidatively and anaerobically prepared ferments, resulting in similar final glutathione concentrations in both wines. Analysis of aroma compounds revealed little interaction between pressing and glutathione treatments; however, unlike in the nitrogen work, a small number of volatile sulfur compounds, specifically hydrogen sulfide and methanethiol, were found to have higher concentrations in wines made from glutathione-treated musts.
Effects of glutathione on Chardonnay and Riesling wine
In the 2017 vintage the effect of glutathione addition pre- and post-inoculation was evaluated in combination with standard and low SO2 concentrations at bottling. In total 36 wines were produced. Sensory evaluations of those wines were completed this year. Overall, glutathione additions to both Riesling and Chardonnay had a detrimental effect on wine sensory qualities, with control wines rated higher in fruit-related attributes and glutathione-treated wines rated higher in ‘dank’/‘drain’ and ‘capsicum’ attributes. Correlation of sensory and chemical data is in progress. The same wines are due to be re-assessed one year after the initial sensory evaluation.