WINETECH Technical Yearbook 2021

• After each time point, a sensory analysis with a trained panel and a chemical analysis of the bottled wines were performed. RESULTS In this article, the results of short- to mid-term effects of copper fining are presented (after 24 hours and six weeks of wine storage). This can be translated to the situation when a winemaker sees the immediate effect of copper addition (after 24 hours) and once the wine is bottled and potentially already sold (after six weeks). The experimental results showed that copper immediately bound the free H 2 S, which resulted in the “boiled egg” off-odour being drastically reduced after 24 hours. Chemical analysis showed that free H 2 S levels after six weeks were also reduced in the wines without copper treatment. This is very likely due to the binding of other components in the wine (such as oxidised polyphenols – quinones) to free H 2 S. In winemaking conditions after fermentation, quinones also bind a portion of reductive compounds (Nikolantonaki & Waterhouse, 2012). However, if there is excess free H 2 S in wines, this process alone might not completely remove all the remaining H 2 S. Finally, the presence of copper alone seems to increase the bound H 2 S in wines.

3MH - after 6 weeks (Graph 1) Graph 1: 3MH – after 6 weeks

However, that bound H 2 S did not cause “boiled egg” or “reductive” off-odours in the wine samples. Copper addition also had an important contribution to MeSH off-odour removal, although a portion of “cabbage” off-odour related to the presence of mercaptan MeSH was retained in the wine. When the treatment containing MeSH and the treatment containing MeSH and copper were compared, there were no differences between them in the levels of free or bound MeSH after six weeks. The sensory analysis confirmed that the “cabbage” off-odour derived from MeSH was equally present in both wine treatments. Unlike H 2 S, the increase of newly created MeSH in wine was not copper related, as it was occurring slowly in all the wine treatments. With regard to varietal thiols in this exper- iment, 3MH levels were decreased mostly in the Chenin blanc wine that was copper treated in the absence of added reductive compounds (graph 1). A less obvious de - crease happened in the copper-treated wine with H 2 S present. This could be due to the greater affinity and faster reaction of cop - per with H 2 S, than with 3MH. The results lead the researchers to the conclusion that copper should only be added to wines with H 2 S present and observed “boiled egg” off- odour, to prevent the reaction of varietal

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Control

C + Cu C + MeSH C + MeSH + Cu

C + H2S C + H2S + Cu C + H2S + MeSH

C + H2S + MeSH + Cu

thiols with the copper. Interestingly, anoth- er varietal thiol 3MHA was not influenced by copper additions. Sensory analysis showed that high levels of H 2 S and MeSH suppressed the “passion fruit” aroma, characteristic for the varietal thiols 3MH and 3MHA (Coetzee & Du Toit, 2012) (graph 2). Therefore, copper was beneficial in removing this suppressing effect of reductive aromas (especially in the case of H 2 S). Surprisingly, copper addition to Chenin blanc base wine seemed to boost the “passion fruit” aroma after 24 hours. This might be due to a short- term effect of removing any residual H 2 S, which was slightly suppressing the varietal 0 10 20 30 40 50 60 Control

thiol aroma perception in the base wine (Franco-Luesma et al ., 2016). However, after six weeks, the copper-treated base wine displayed lower levels of “passion fruit” aroma, which corresponds to the loss of varietal thiol 3MH, also confirmed through chemical analysis. CONCLUSION As a winemaker you might ask yourself, should I use copper if I have a wine with reductive off-odours? The best practice would have been to have prevented reductive compounds from forming in the first place, through optimised fermentation management. However, if you are past that stage and you are left with a finished C + H2S C + H2S + Cu C + H2S + MeSH C + H2S + MeSH + Cu

Passionfruit - after 24 hours vs 6 weeks (Graph 2)

C + Cu C + MeSH C + MeSH + Cu

24 hours 6 weeks

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