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 are the sources of glutathione?
Glutathione is present in many natural products. In freshly pressed grapes GSH concentrations have been reported to be as high as 50 mg/L, with some proportion of that concentration still present at the end of fermentation. The concentration of GSH in grape juice is dependent on the conditions under which grapes are processed. Other potential sources of glutathione include yeast-derived products, some of which are marketed as glutathione-enriched inactivated dry yeast preparations. Compared to grape processing interventions, these additives contribute relatively little to the overall concentration of glutathione in juice.
Can small concentrations of GSH at crush make a difference to finished wine?
OIV resolution OIV-OENO 445-2015 recommends the addition of no more than 20 mg/L of GSH to must or wine. The effect of such an addition to freshly pressed grape juice was evaluated in a pilot-scale trial producing Chardonnay sparkling base and Chardonnay table wine. Glutathione additions were made immediately after pressing and two yeast strains were used to make the wines. Analysis of wine composition after six months in bottle indicated very few differences between the wines. Standard wine chemical measures and low molecular weight volatile sulfur compounds did not vary with treatment. Polyfunctional thiol concentrations did, however, vary, both with winemaking style (sparkling base vs table wine) and wine yeast used.
The polyfunctional thiols 3-MH and 3-MHA are typically associated with ‘passionfruit’ aroma in Sauvignon Blanc wine but more recently have been shown to be important contributors to aromatic complexity in other varieties. When GSH was added to Chardonnay juice from grapes harvested to meet a sparkling base specification, the concentrations of 3-MH and 3-MHA were not different from the controls, regardless of the yeast strain used. However, when GSH was added to Chardonnay juice from grapes harvested for table wine, the 3-MH and 3-MHA concentrations in the finished wine were higher than in the controls and the degree to which their concentrations increased was dependent on yeast strain. Surprisingly, for one of the yeast strains, the concentration of 3-MH in particular was almost double that of the no treatment wines and substantially above its aroma threshold. Whether these results translate into perceptible sensory differences remains to be determined. While it initially seemed unlikely that such a small addition of GSH to a freshly pressed juice would result in measurable differences in finished wine, the 3-MH results of this trial suggest otherwise. It should also be noted that the addition of yeast products that contain glutathione may have additional effects beyond those of pure glutathione.