Elemental sulphur residues at harvest – boosting both the good and the bad

A recent study conducted by New Zealand researchers investigated the effects of the addition of antioxidants (a combination of ascorbic acid and sulphur dioxide) and elemental sulphur during grape processing on the sensory and chemical composition of the resulting wines.

Introduction

Elemental sulphur is an effective, durable and economical fungicide for the management of powdery mildew in vineyards. But sulphur residues present on the grapes at harvest can have adverse effects and concentrations exceeding 10 µg/g in musts are associated with increased hydrogen sulphide (H₂S) formation during fermentation.¹ Growers and winemakers are therefore cautious when applying elemental sulphur closer to the harvest date for fear of increased residual elemental sulphur being transferred into the juice. However, studies have shown that H₂S during fermentation could significantly contribute to the fruity volatile thiol formation resulting in increased intensity of desirable attributes in the finished wines.²

The reduction of elemental sulphur to H₂S takes place in the presence of antioxidants.¹ In juices that lack antioxidant protection, H₂S is expected to be rapidly removed by reaction with oxidised o-quinones.³ This is important given the ability of H₂S to combine with certain C6 alkene compounds present in the juice to form 3-mercaptohexanol (3MH) and 3-mercaptohexyl acetate (3MHA) during the early stages of fermentation.⁴

Materials and methods

Sauvignon blanc, Pinot gris and Chardonnay grapes were collected from different sites in New Zealand during the 2018 harvest. Grapes were obtained from three different sites for each variety. The following additions were made after destemming and crushing:

• No addition.
• 100 mg/L ascorbic acid + 50 mg/L SO₂.
• 100 mg/L ascorbic acid + 50 mg/L SO₂ + 100 mg/L elemental sulphur.

The grapes were pressed, juice cold settled, racked into 1.5 L glass bottles and inoculated with Anchor VIN7 yeast whereafter fermentation took place at 15°C. Sensory evaluation was performed using descriptive analysis by a trained sensory panel.

Results

Results differed significantly not only between the different treatments, but also between the varieties, as well as between the different sites. However, the same trend was observed in all the experimental lots.

Processing the grapes in the absence of antioxidants resulted in wines with no measurable volatile thiol concentrations, while the presence of antioxidants during the pre-fermentative stages led to wines with increased 3MH and 3MHA content. In some lots, this increase was pronounced and the highest concentration was reported for one of the Sauvignon blanc sites, which delivered 3MH and 3MHA concentrations of approximately 27 500 ng/L and 5 000 ng/L for 3MH and 3MHA, respectively. The importance of antioxidants should therefore not be underestimated and it is advised to ensure that sufficient antioxidant protection is present, not only during processing in the pre-fermentative stages, but also after fermentation to ensure the preservation of the formed volatile thiols.

The addition of elemental sulphur during grape processing in the presence of the antioxidants led to wines with even higher thiol concentrations. For the wine mentioned earlier, additional supplementation of elemental sulphur increased the concentrations of 3MH and 3MHA to around 47 000 ng/L and 10 000 ng/L respectively. Similar trends were seen for the other sites and even the other varieties included in this study.

The reduction of elemental sulphur to H₂S by reducing agents, enzymatic or otherwise, present in the grape juice could explain the increased thiol concentration observed.² The increased H₂S plays an important role in the formation of volatile thiols through the addition of H₂S to certain C6 compounds resulting in the formation of the odorous compounds.

Sensory evaluation using descriptive analyses showed that the wines made from juices to which elemental sulphur was added had a higher intensity of “3MHA” and “4MMP” (the panel was trained using the reference standards), as well as “burnt rubber” and “canned asparagus”. The presence of elemental sulphur thus not only led to increased desirable aroma compounds, but also increased the undesirable compounds responsible for the reductive aroma. The intensities of the good attributes vs the bad attributes will depend on the concentration and the type of compounds present.

Practically, it is difficult to control the amount of reductive compounds formed due to elemental sulphur and until the exact mechanism(s) of formation is better understood, it could be too risky to apply this treatment in a commercial setup. Elemental sulphur residues on the grapes can be minimised by reducing skin contact and by clarifying the juice. Juice clarification have been shown to lower the sulphur residues by over 95%, while fermentation of grapes on the skins after a long maceration led to a two- to three-fold increase in H₂S production.¹ Longer withholding periods between fungicide applications and the grape harvest also led to a drop in residual sulphur levels and values below a critical limit of 10 μg/g was achieved when the spraying ceased over 35 days from harvest.¹

Conclusion

This study demonstrates that elemental sulphur can have an important role as a sulphur donor in the formation of volatile thiols. The pathway by which the thiols form starts with the reduction of elemental sulphur in the early stages of juice processing to produce wines with very high levels of volatile thiols, contributing to increased tropical fruit-associated attribute intensities. However, the increase in thiol concentrations was accompanied by an increase in reductive aroma, which at higher concentrations contribute to undesirable aroma characteristics. Further studies are needed to investigate how to control the formed H₂S in order to produce volatile thiols, while minimising unwanted aroma compounds.

Abstract

Elemental sulphur residues present on the grapes at harvest can lead to increased formation of fruity volatile thiols during fermentation. However, while the formation of the desirable compounds is boosted, an increase in reductive aroma is also observed, which could have an adverse effect on the sensory composition of the wine.

References

1. Kwasniewski, M.T., Sacks, G.L. & Wilcox, W.F., 2014. Persistence of elemental sulfur spray residue on grapes during ripening and vinification. American Journal of Enology and Viticulture 65(4): 453 – 462. https://doi.org/10.5344/ajev.2014.14027
2. Araujo, L.D., Vannevel, S., Buica, A., Callerot, S., Fedrizzi, B., Kilmartin, P.A. & Du Toit, W.J., 2017. Indications of the prominent role of elemental sulfur in the formation of the varietal thiol 3-mercaptohexanol in Sauvignon blanc wine. Food Research International 98: 79 – 86. https://doi.org/10.1016/j.foodres.2016.12.023
3. Nikolantonaki, M. & Waterhouse, A.L., 2012. A method to quantify quinone reaction rates with wine relevant. Journal of Agricultural and Food Chemistry 60: 8484 – 8491.
4. Harsch, M.J., Benkwitz, F., Frost, A., Colonna-Ceccaldi, B., Gardner, R.C. & Salmon, J.A., 2013. New precursor of 3-mercaptohexan-1-ol in grape juice: Thiol-forming potential and kinetics during early stages of must fermentation. https://doi.org/10.1021/jf3048753

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