Adding antioxidants to Sauvignon blanc musts (Part 2)

The addition of antioxidants to Sauvignon blanc must was investigated in two trials conducted by New Zealand researchers.¹ The main findings of this study are reported in this two-part article.

Introduction

In part one of the two-part article, the effect of the addition of antioxidants, sulphur dioxide and glutathione (GSH), as well as two commercial inactivated dry yeast products were discussed. The additions were made to the must prior to pressing and the volatile thiols of the resulting wines measured. In part two, the effect of varying concentrations of GSH, as well as the addition of ascorbic acid are reported.

Trial 2: Comparing sulphur dioxide (SO₂), GSH and ascorbic acid

Materials and methods

Hand harvested Sauvignon blanc grapes from two different sites located in the Marlborough region in New Zealand were used in the study. It was noted that, due to suboptimal weather conditions, the bunches were of relatively poor condition with berry damage and mould. Small scale experiments were conducted and four treatments were applied in the must phase prior to pressing. Sulphur dioxide (SO₂) (20 mg/L) were added prior to pressing for all treatments.

1. Control.
2. 20 mg/L GSH.
3. 100 mg/L GSH.
4. 20 mg/L GSH + 100 mg/L ascorbic acid.

The juices were inoculated with VIN7 and fermented to dryness.

Results: Glutathione content

• In both wines, adding 100 mg/L GSH delivered wines with the highest GSH content, followed by the wines made from juices to which both 20 mg/L GSH and 100 mg/L ascorbic acid were added together.
• For one of the wines, adding 20 mg/L GSH resulted in a wine with higher GSH content compared to the control, however, the concentration of these two treatments did not differ significantly for the second wine.

Results: 3-Mercaptohexanol (3MH) and 3-mercaptohexyl acetate (3MHA) content

• For one of the sites, adding 20 mg/L GSH in combination with 100 mg/L ascorbic acid increased the 3MH and 3MHA concentrations significantly compared to the other treatments. For this site, adding the combination of GSH and ascorbic acid to the juice resulted in around 8 500 ng/L 3MH and 3 000 ng/L 3MHA. This is a massive increase compared to the other three treatments which were less than 3 000 ng/L and 1 000 ng/L for 3MH and 3MHA, respectively.
• The other site delivered wines with lower thiol content in general, although a considerable amount was still present in the wine. For this site, there was no significant difference between the 20 mg/L GSH + 100 mg/L ascorbic acid and the 100 mg/L GSH treatments.
• For both sites, adding 100 mg/L GSH resulted in wines with significantly higher 3MH and 3MHA concentrations when compared to the 20 mg/L GSH and the control (no GSH addition).
• Adding 20 mg/L GSH resulted in a wine with significantly higher 3MH and 3MHA concentrations compared to the control (no GSH addition).

Conclusion

The results from the two trials (discussed in part one and part two) would suggest that the addition of antioxidants to Sauvignon blanc must can significantly affect the volatile thiol content in the resulting wines. Adding a combination of GSH with another antioxidant, such as SO₂ and ascorbic acid, shows even more potential compared to GSH alone and could further support the formation and/or preservation of 3MH and 3MHA. While GSH competes with the volatile thiols to react with the o-quinone, the addition of another antioxidant, such as SO₂ and ascorbic acid, could perhaps, in turn, compete with GSH, therefore protecting GSH from oxidation and making it available for thiol forming reactions.^1,2

When added alone, the amount of GSH added also had a significant impact on the amount of thiols formed with the 3MH and 3MHA concentration increasing as the GSH dosage increases. The current OIV dosage recommendation is 20 mg/L GSH. This recommendation should perhaps be revised to ensure optimal benefits are obtained.

It should be noted that the efficiency of the various treatments varied greatly between the different sites. The juice composition, therefore, plays an integral role in the amount of aromatic thiols ultimately formed. Some of the sites showed a marginal increase in thiol content, while others showed pronounced effects that would undoubtedly affect the sensorial perception of the wine.

The use of ascorbic acid without sufficient SO₂ is not advised and perhaps the addition of all three of these antioxidants are ideal and should be investigated. Future research should also include sensory analyses, as well as the quantification of reductive aroma compounds.

Abstract

Glutathione, sulphur dioxide, ascorbic acid and commercial inactivated dry yeast products were added either in isolation or in combination after which the volatile thiol content (among others) of the resulting wines were analysed. The addition of certain antioxidants led to significant and pronounced increases in 3MH and 3MHA, while others had no effect when compared to a control treatment.

References

1. Lyu, X., Del Prado, D.R., Araujo, L.D., Quek, S.‐ & Kilmartin, P.A., 2021. Effect of glutathione addition at harvest on Sauvignon blanc wines. Australian Journal of Grape and Wine Research 27(4): 1 – 11. https://doi.org/10.1111/ajgw.12495.
2. Clark, A.C. & Deed, R.C., 2018. The chemical reaction of glutathione and trans -2-hexenal in grape juice media to form wine aroma precursors: The impact of pH, temperature, and sulfur dioxide. Journal of Agricultural and Food Chemistry 66(5): 1214 – 1221. https://doi.org/10.1021/acs.jafc.7b04991.

– For more information, contact Carien Coetzee at carien@basicwine.co.za.