WINETECH Technical Yearbook 2021

OENOLOGY RESEARCH | OCTOBER 2021

Is adding copper the solution? (PART 1)

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.

It has previously been reported that copper can also partially remove varietal thiols in wines (e.g., 3-Mercaptohexan- 1-ol, shortened 3MH), which are major contributors of tropical aromas in wines, such as Sauvignon blanc and Chenin blanc (Ugliano et al. , 2011). Considering that researchers have also previously proved that copper is not completely removed by racking or filtering (Clark et al ., 2015a), copper residues from reductive off-odour fining, might therefore remove a portion of the varietal thiols. Consequently, the desirable aromas of “passion fruit”, “guava” and “grapefruit” in wines could be affected by copper fining. This research aims to clarify what are the short- and long-term chemical and sensorial effects of copper fining on wines with naturally high thiol levels, but with reductive off-odours present. MATERIALS AND METHODS • For this experiment, Chenin blanc 2019 from Worcester (South Africa),

naturally high in varietal thiol levels was used as a base wine. • The wine was sparged with nitrogen to reduce dissolved oxygen (to <1%) and free SO 2 was adjusted to 38-40 mg/L (the wine had a pH of 3.4 and an alcohol content of 12.73 vol %). • Various combinations of H 2 S, MeSH and copper (in the form of copper sulphate solution) were added to the base wine, resulting in eight different treatments. The concentrations of reductive compounds used in this experiment were at the high range of the spectrum normally found in wines (Siebert et al ., 2010): 40 µg H 2 S/L and 20 µg MeSH/L. The wines were treated with 0.3 mg copper per litre. • Wines were bottled in a reductive (anoxic) environment, under crown caps, and stored for 24 hours, six weeks and one year at 15°C.

IN A WINETECH-FUNDED PROJECT, researchers aimed to answer the question: How does copper addition influence the reductive compounds and varietal thiols in a wine after bottling? INTRODUCTION Copper is one of the most used winemaking remedies for removing residual reductive off-odours from finished wines. It effectively removes the “boiled egg” off-odour, by binding residual hydrogen sulphide (H 2 S) still present in a wine after fermentation (Clark et al ., 2015a & b). Moreover, copper is sometimes used when “cabbage” or “putrification” off-odours are present in wines, which arise from the presence of mercaptans, such as methanethiol (MeSH) (Vela et al ., 2017). However, copper binds methanethiol not as effectively as it does hydrogen sulphide, meaning the “cabbage” aroma is harder to remove from the wine than the “boiled egg” aroma.

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