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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FIGURE 1. Reduction in dissolved oxygen levels in the wines that did not receive SO2 (Fracassetti et al., 2013). FIGURE 2. Reduction in dissolved oxygen levels in the wines that did receive SO2 (Fracassetti et al., 2013). TABLE 1. Average variation in levels of different compounds measured at the beginning and end of the experiment (adapted from Fracassetti et al., 2013).

Results and discussion

In Figures 1 and 2 the oxygen consumption rate in the 13 wines can be seen. Figure 1 represents those wines to which no SO2 was added and Figure 2 those to which the 30 mg/ℓ SO2 was added just before the start of the experiment. It is clear that the rate of oxygen consumption differed to a large extent between the different wines. Where no SO2 was added, most of the oxygen was consumed after 40 days, but certain wines still had measurable levels of dissolved oxygen after 60 days. Where SO2 was added, the oxygen consumption rate dropped much quicker, with some wines having most of their oxygen being consumed after only 10 days. SO2 is added to wines to serve as an anti-microbial and anti-oxidative agent. SO2 is added to juices and musts to inhibit oxidation by inhibiting or destroying oxidation enzymes such as laccase. In musts the addition of SO2 would thus retard the consumption of oxygen, by inhibiting the oxidation enzyme which drives the oxidation process. However, in wines the opposite happens, where chemical oxidation takes place which is a much slower process than enzymatic oxidation that would occur in juice or must. In wines the oxidation of phenolic compounds leads to the formation of quinones, as well as hydrogen peroxide (H2O2). The function of SO2 is thus not to remove oxygen from wine, but to rather react with the H2O2, thereby removing it. The latter compound is a strong oxidant in wine and SO2 therefore protects the wine by removing this compound. SO2 also has a bleaching effect in white wines and juice by reducing oxidised quinones back to phenolics, thereby lowering the brown colour which could happen during oxidation. SO2 thus speeds up the consumption of oxygen in wine by removing the products of oxidation.

In Table 1 a summary of some of the main components of all 13 wines that were measured at the beginning and the end of the trail can be seen. The total oxygen consumption was a little bit higher in those wines to which no SO2 was added. Losses in glutathione (GSH), free SO2 and total SO2, was much higher in the wines to which SO2 was added. The formation of oxidised GSH, was lower in the wines that received SO2, probably due to the latter compound removing the oxygen before it could decrease the GSH. However, the total hydroxycinnamic acid concentrations (sum of caftaric, fertaric, coumaric acids etc.), were lower in the wines that did not receive any SO2 after fermentation, as these compounds are substrates for oxidation in wines. Increases in brown and yellow colours (420 and 440 nm) were also higher in the wines that did not receive any SO2, as expected. This is probably due to the bleaching effect of SO2 on brown quinones.

It is clear that oxygen consumption in white wines does not follow a strict set of rules and can be influenced by a number of different factors. SO2 is clearly playing a large role in rapidly reducing oxygen levels in wines. Oxygen levels in white wines with higher levels of SO2 and copper will therefore probably decrease faster. It is well known in the wine industry that around 1 mg/ℓ oxygen reacts with about 4 mg/ℓ SO2. This would mean that if a wine contains about 3 mg/ℓ O2, about 12 mg/ℓ SO2 will be removed from the wine. A study by Van der Merwe in 2013 have shown oxygen levels to range from 0.7 mg/ℓ to 7.5 mg/ℓ in South African white wines after bottling. This should be kept in mind by the producer on deciding possible ageing potential of his or her white wine.

Acknowledgements

Winetech and THRIP for funding this project and the cellars that donated wines.

References

Du Toit, W.J., Marais, J., Pretorius, I.S. & Du Toit, M., 2006. Oxygen in must and wine: A review. South African Journal of Enology and Viticulture 27, 76 – 94.

Fracassetti, D., Coetzee, C., Vanzo, A., Ballabio, D. & Du Toit, W.J., 2013. Oxygen consumption in South African Sauvignon blanc wines: Role of glutathione, sulphur dioxide and certain phenolics. South African Journal of Enology and Viticulture 34(2): 156 – 169.

Van der Merwe, H. Management of oxygen during bottling. WineLand, July 2013.

For further information contact Wessel du Toit at wdutoit@sun.ac.za.

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