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.
Winetech and THRIP for funding this project and the cellars that donated wines.
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 email@example.com.