by Wessel du Toit, Daniella Fracassetti, Carien Coetzee, Andreja Vanzo & Davide Ballabio – for Wineland Magazine

Introduction

Oxygen plays an important role in wine production. The exposure of red wines to small amounts of oxygen can be beneficial to the wine’s development in terms of colour stability and the softening of tannins during barrel ageing. However, in general the addition of oxygen in white wine is not wanted. This is due to the development of a brown colour, a decrease in fruitiness in the wines and an increase in acetaldehyde levels. Factors affecting the consumption of oxygen in white wines are not completely clear, which is probably due to the large chemical differences existing between white wines from different cultivars, areas and vintages. The main aim of this work was thus to follow the decrease in dissolved oxygen concentrations in a number of white wines and to try to link these with the chemical composition of the wines.

Materials and methods

We obtained 13 young Sauvignon blanc wines from the 2010 vintage just after the completion of alcoholic fermentation. These wines were collected from different commercial wineries before any SO2 additions were made after alcoholic fermentation and transported to the Department of Viticulture and Oenology, Stellenbosch University. The wines were collected in 20 ℓ canisters into which N2 gas had been previously blown. The pH of these wines ranged from 3.2 to 3.5 with alcohol levels ranging from 12.3% to 13% v/v. Each wine was then divided into two treatments, one that received no SO2 addition, with the other half receiving 30 mg/ℓ SO2. All the wines were then saturated with oxygen and the wines placed at 37°C for 60 days to enhance the oxidation process. Oxygen levels were monitored daily during this period and wine samples drawn at the beginning and end of the experiment for chemical analyses. Analyses included free and total SO2, glutathione (GSH) analyses, oxidised GSH, grape reaction product, range of phenolic compounds such as caffeic acid, caftaric acid, catechin, coumaric acid, ferulic acid etc., Cu, Fe, as well as absorbencies at 280 nm (total phenolics) and 420 and 440 nm (brown and yellow colour).

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