WINETECH Technical Yearbook 2021

OENOLOGY RESEARCH | NOVEMBER 2021

Is adding copper the solution? (PART 2)

MATIJA LESKOVI Ć , JEANNE BRAND & WESSEL DU TOIT: South African Grape and Wine Research Institute, Department of Viticulture and Oenology, Stellenbosch University, Stellenbosch KEYWORDS: Volatile sulphur reductive compounds, varietal thiols, wine copper fining, wine storage.

The aim of this Winetech-funded project is to improve the understanding of winemakers of the influence of copper fining on wine quality.

INTRODUCTION YOUNG WHITE WINES made from Sauvi- gnon blanc and Chenin blanc grapes can attribute their signature tropical aromas of “guava”, “passion fruit” and “grapefruit” to the high natural presence of varietal thiols (Coetzee & Du Toit, 2012; Wilson, 2017). Winemakers are aware that varietal thiols can decrease in the presence of oxygen and therefore to preserve varietal thiols, the up- take of oxygen is oftenminimised from grape crushing, through vinification and wine storage (Coetzee & Du Toit, 2012). These reductive winemaking techniques, if not carried out optimally (e.g., incorrect yeast nutrient management during fermentation), can sometimes result in the production of unwanted reductive off-odours, resembling “boiled-egg” and/or “cabbage” (Kreitman et al ., 2019). They are the two most common off-odours appearing in reductive wines. “Boiled-egg” off-odours are derived from the presence of excess hydrogen sulphide (H 2 S)

and “cabbage” off-odours are derived from high levels of methanethiol (MeSH) (Siebert et al ., 2010). Copper fining is usually done to remove excess reductive off-odours from wines before bottling. The “boiled egg” off- odour is rapidly removed after the copper addition, due to copper binding to H 2 S ren- dering it non-volatile. The copper-H 2 S com- plex is not completely removed from wine by precipitation or filtering, as it was pre - viously believed, but it stays in wine (Clark et al ., 2015). On the other hand, removing “cabbage” off-odour present in wines after bottling from wine by copper addition, is only partially effective. Unfortunately, copper present in wines after bottling can also partially remove varietal thiols (e.g., 3-Mercaptohexan-1-ol, shortened 3MH) (Ugliano et al. , 2011). In the previous article, we have reported that in our experiment the addition of copper to Chenin blanc wine, decreased 3MH levels after six weeks, but only slightly decreased

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