Adding antioxidants to Sauvignon blanc musts (Part 1)

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

Introduction

Sauvignon blanc aromas are often characterised by attributes such as tropical fruit, passion fruit, grapefruit, guava and citrus. The aroma compounds mainly responsible for these attributes are the fruity volatile thiols (or polyfunctional mercaptans), particularly 3-mercaptohexanol (3MH) and 3-mercaptohexyl acetate (3MHA) and winemakers often adjust winemaking processes to maximise the formation and preservation of these compounds. The volatile thiols are sensitive to oxidation and the decrease in these compounds has been linked to polyphenol oxidation, catalysed by metals. The addition of antioxidants can thus be valuable by preferentially reacting with the oxidation compounds, thereby protecting the fruity volatile thiols.¹

How can glutathione contribute to the preservation and formation of the fruity volatile thiols?

Glutathione (GSH) is tripeptide with its biological significance mostly related to its free sulfhydryl moiety which confers unique redox and nucleophilic properties.² During winemaking, GSH plays an integral role in the oxidation of musts, where it traps o-quinones, formed during oxidation, to limit the amount of browning pigments and loss of aroma in wine.^3-6 GSH, therefore, competes with the fruity volatile thiols to bind the o-quinones and serves as a sacrificial compound.

Other than protecting the already present volatile thiols from oxidation reactions, GSH can potentially also contribute to the formation of 3MH and 3MHA. GSH may be a potential source of hydrogen sulphide during the early stages of fermentation, which, in turn, can react with C6 compounds, particularly trans-2-hexenol and trans-2-hexenal resulting in the formation of 3MH and 3MHA.^7,8 GSH can also react with C6 compound, trans-2-hexenal to form thiol precursors.⁹

Recommendations and regulations

The International Organisation of Vine and Wine (OIV) advises adding GSH to the musts or at the start of alcoholic fermentation while ensuring that, prior to and during fermentation, the assimilable nitrogen level is sufficient to avoid the metabolism of GSH by the yeast. They also recommend that the GSH be in a reduced form and comply with the prescriptions of the International Oenological Codex while not exceeding a dosage of 20 mg/L.¹⁰

According to the Liquor Products Act 60 of 1989, the addition of pure GSH is not currently permitted in South Africa (it is also not included in the European Union list of authorised oenological compounds). However, commercial inactivated dry yeast (IDY) products naturally rich in GSH has been developed and are allowed with conditions for use during winemaking. For more information, please consult the respective regulations.

Testing the efficiency of antioxidants to form/preserve fruity volatile thiols

New Zealand researchers recently conducted two studies to investigate the effect of antioxidant additions to Sauvignon blanc must on the chemical composition of the resulting wines.¹¹ The main antioxidant under investigation was GSH, however, the use of inactivated dry yeast preparations, sulphur dioxide (SO₂) and ascorbic acid were also included for comparison. The study mainly focussed on the effects on 3MH and 3MHA concentrations.

In part one of this two-part article, the effect of the addition of antioxidants, sulphur dioxide and GSH, as well as two commercial inactivated dry yeast products, are discussed. In part two, the effect of varying concentrations of GSH, as well as the addition of ascorbic acid, will be reported.

Trial 1: Comparing SO₂, GSH and IDY products

Materials and methods

Machine harvested Sauvignon blanc grapes from three different sites located in the Marlborough region in New Zealand were used in the study. Small scale experiments were conducted and five treatments were applied in the juice phase prior to pressing:

1. 30 mg/L SO₂ (Control).
2. 100 mg/L GSH.
3. 30 mg/L SO₂ + 100 mg/L GSH.
4. 30 mg/L SO₂ + 30 g/hL of IDY product A.
5. 30 mg/L SO₂ + 30 g/hL of IDY product B.

The juices were inoculated with Lalvin EC-1118 and fermented to dryness.

Results: Glutathione content

• Results showed that the wines made from grapes to which 30 mg/L SO₂ + 100 mg/L GSH was added, had by far the highest GSH content measuring between 80 and 120 mg/L GSH.
• Compared to the above treatment (30 mg/L SO₂ + 100 mg/L GSH), the wines made from grapes to which 100 mg/L of GSH was added alone, had much less residual GSH with typically half (and for one site a quarter) of the added concentration still present in the wine.
• The control samples (to which only SO₂ was added) contained very little (< 10 mg/L) GSH.
• The addition of either IDY products to juice before pressing did not deliver wines with increased GSH content and the concentrations were not significantly different from the control samples.

Results: 3MH and 3MHA content

• Results showed that the wines made from grapes to which 30 mg/L SO₂ + 100 mg/L GSH was added, had, again, by far the highest 3MH and 3MHA concentration. For these wines, 3MH concentration ranged from around 3 000 to 4 000 ng/L. 3MHA concentrations measured around 750 to 1 300 ng/L. These are significant amounts and will undoubtedly have a sensory effect on the wine.
• For two of the three wines, the treatment delivering the second highest volatile thiol content was the controls (30 mg/L SO₂ only) measuring around 2 500 and 450 ng/L for 3MH and 3MHA, respectively. For these two wines, the thiol content in the 100 mg/L GSH treatment was about half the concentration found in the corresponding control samples. This would suggest that the antioxidant effect of 30 mg/L SO₂ was occasionally greater than the protective effect of 100 mg/L GSH.
• The addition of IDY products did not result in wines with elevated volatile thiol content under the condition of this study. The 3MH and 3MHA concentrations were significantly lower for the IDY treatments when compared to the control. It should be noted that the products were added to the juice phase (just before pressing) and not right before fermentation as recommended by the suppliers. A different outcome could have been possible if suppliers’ recommendations were followed.

Conclusion

The results from trial 1 would suggest that the addition of SO₂ is more effective in preserving the volatile thiols compared to adding GSH alone. However, adding a combination of GSH together with SO₂ was the most effective and should be considered.

The two commercial IDY products tested in this study showed little impact on wine aroma compounds when added before pressing. More research is needed to investigate the exact benefits of these products. Especially when added just before fermentation as recommended by the suppliers, as opposed to adding the product prior to pressing where a large percentage of the product can potentially be removed with the grape skins.

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

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8. Harsch, M.J., Benkwitz, F., Frost, A., Colonna-Ceccaldi, B., Gardner, R.C. & Salmon, J.M., 2013. New precursor of 3-mercaptohexan-1-ol in grape juice: Thiol-forming potential and kinetics during early stages of must fermentation. Journal of Agricultural and Food Chemistry 61(15): 3703 – 3713. https://doi.org/10.1021/jf3048753.
9. 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.
10. OIV, 2015. Treatment of must with glutathione. Resolution OIV-OENO 445-2015.
11. 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.

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