Winetech Technical Yearbook 2022

SHUTTERSTOCK

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 H 2 S by reducing agents, enzymatic or otherwise, present in the grape juice could explain the increased thiol concentration observed. 2 The increased H 2 S plays an important role in the formation of volatile thiols through the addition of H 2 S 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 H 2 S 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 H 2 S 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 https://www.wineland.co.za/elemental-sulphur-residues-at harvest/

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

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WINETECH TECHNICAL YEARBOOK 2022