What is glutathione?
It occurs naturally in grapes with grapes and yeasts as the potential sources.2 It is an anti-oxidant which protects white wine against oxidation and consequently the shelf life of the wine is promoted.

Glutathione is defined as a tri-peptide because it consists of the three amino acids glutamic acid, cysteine and glycine. Cysteine is a sulphur-containing amino acid and glutathione can consequently also be defined as a thiol. Other well-known thiols are 4MMP (4-mercapto-4-methylpentan-2-one), 3MH (3-mercapto-hexan-1-ol) and 3MHA (3-mercaptohexylacetate). These compounds are responsible for guava, passion fruit and grapefruit aromas in Sauvignon blanc, Chenin blanc and Colombar wines. Glutathione, which is the best-known thiol in grapes, occur in an oxidised form (consisting of only one glutathione molecule) or in a reduced form (consisting of two glutathione molecules linked between its two sulphur groups). The oxidised form is the active form. The content of the oxidised form increases with the ripening of the grapes and concentrations between 10-30 mg/l occur.

The glutathione content in wine is always lower than in grape juice because yeasts utilise glutathione during alcoholic fermentation. Yeast can however due to autolysis secrete glutathione after fermentation. Glutathione is like the most thiols sensitive to oxidation, which will also contribute to the lower glutathione contents of wine in comparison with juice.

Research confirmed that the addition of 10 mg/L glutathione prior to bottling assures that the fruitiness of wines is retained for a longer period, the atypical maturation character is decreased and the browning of the wine is prevented. Table 1 summarises these results.  

 Table 1: The influence of glutathione (GSH) addition on the contents of 3-mercapto-hexanol (3MH), 2-amino-aceto-phenone (2AAP) and optical density (OD) of a Sauvignon blanc wine, three year after glutathione addition. (Dubourdieu et al, 2004)

Glutathione addition protects white wines in three ways by preventing the oxidation of the aromatic thiols, suppressing the formation of the atypical maturation character and preventing phenolic oxidation. The maturation potential of wines is consequently improved by increasing glutathione concentrations in white wines. Although comprehensive research has been done regarding the protection of wines against oxidation, limited information is available regarding the utilisation of glutathione by yeasts during alcoholic fermentation.

Yeast and glutathione:

Research was done at the University of Stellenbosch, South Africa, to ascertain the influence of different yeast strains on the glutathione content of Elgin Sauvignon blanc wines made under oxidative or reductive conditions (Du Toit, 2007). The results are summarised in table 2.

Table 2: The influence of different yeast strains on the glutathione content (mg/l) of Sauvignon blanc wines.

The influence of reductive winemaking on the glutathione content of wine is very obvious. Cross Evolution and VIN 7 caused the highest glutathione concentrations after fermentation and in the case of oxidative winemaking it seems that VIN 7 formed some glutathione during fermentation. Further research is however necessary to ascertain why the differences exist amongst different yeast strains.  

Additional information was also obtained regarding the effect of oxidative and reductive treatment of grape juice on the fermentation rate.  In general all the yeast strains fermented slower under reductive conditions. VIN 7 had the highest fermentation rate irrespective of the conditions. Cross Evolution had the second highest fermentation rate under reductive conditions. This indicates that these two strains have a low oxygen demand. Contrary to that NT 116 had the second highest fermentation rate under oxidative conditions, but was the slowest strain under reductive conditions. This confirms that NT 116 has a high oxygen demand.

Ways to increase the glutathione concentrations of wine:

1. Create and maintain reductive conditions. These conditions must however be maintained during the entire winemaking process prior to bottling and not only during alcoholic fermentation. Reductive winemaking can however increase the potential pinking of wines.  
2. Use yeast strains which cause higher glutathione concentrations in wine.   
3. Add an inactivated yeast product which is enriched with glutathione to the juice prior to fermentation.  (3)

References:

1. Du Toit, Wessel. Glutathione report of different yeast strains conducted for Anchor Yeast, 2007.
2. Guerrand, David. Build up and protect your wine after fermentation, 2005.
3. www.lallemandwine.com