SO₂ is one of the most commonly used additives in the food industry. In wines, SO₂ is used for its antimicrobial and antioxidant properties. The antioxidant properties of SO₂ play an essential role in controlling the evolution of the wine aromas, its color during the aging process, and the control of its shelf-life. Free SO₂ has these antioxidant propertiesthe rest of the total SO₂ is present in the form of combined SO₂ (SO₂ reversibly or irreversibility linked with several wine compounds) and molecular SO₂.

The direct reaction between SO₂ and molecular oxygen is slow and requires the presence of a catalyst such as iron or copper. Under enological conditions, free SO₂ can reduce quinones and oxidation products to phenols (Waterhouse et al. 2006, Danilewicz et al. 2010), which slows down the oxidation process (figure 1). It also reacts with a powerful oxidant, hydrogen peroxide, generated by the oxidation of phenolic compounds (Figure 1) and thus prevents the formation of ethanal (Danilewicz et al. 2010). These two mechanisms lead to a decrease in the concentration of free and total SO₂ when storing the wine.

Inadequate levels of free SO₂ cause the wine aromas and color to deteriorate rapidly. However, because of its toxicity, the SO₂ dosage is regulated, and the general trend is to reduce SO₂ during winemaking and in bottled wines.
Oxygen control is an important parameter that is becoming more and more relevant in the industry: technological means of measuring oxygen intakes are readily available, and known O₂ input tools exist during winemaking (micro-oxygenator, etc.) and bottled storage (closures with controlled oxygen ingress). However, a fundamental aspect of the role of SO₂ in wine remains to be defined: the relationship between the amount of O₂ consumed by the wine and the amount of SO₂ lost. This last aspect is crucial in predicting the decrease in SO₂ concentration over time, a key factor in the wine shelf-life.

Materials and methods

Various trials on different grape varieties and different winemaking conditions were performed in collaboration with research institutes including:

• 3 rosés from Grenache Noir (Wirth et al. 2012), 3 reds from Grenache Noir of traditional maceration and flash release extractionwith or without the use of micro-oxygenation (Wirth et al. 2010, Caillé et al. 2010) with the INRA of Montpellier
• 2 Syrah reds of traditional maceration with or without the use of micro-oxygenation (Ugliano et al. 2012) and a white Sauvignon Blanc treated or not with copper sulfate (Ugliano et al. 2011) with the AWRI
• A Cabernet Sauvignon red with or without the use of micro-oxygenation and fining agents (Han et al. 2015) and a white Chardonnay with or without aging on lees in stainless steel tanks or barrels (Waterhouse et al. 2016) with UC Davis
• 2 Riesling whites (Dimkou et al. 2011 and 2013) with the University of Geisenheim

All the wines underwent 3 or 4 degrees of exposure to O₂ after bottling by using co-extruded closures (Nomacorc) presenting different oxygen transmission rates (OTR), combined with the storage of bottles under atmospheres of varying oxygen concentrations. Oxygen penetration through the closure was measured (NomaSense O₂ P6000) in bottles filled with nitrogen, sealed with the closures used for the study, and stored in the same conditions as the experimental wines (Diéval et al. 2011). The resulting levels of oxygen exposure ranged from very low (0.2 mg/year, similar to those of a screw cap) to high (4 mg/year).
A total of 54 red wines, 46 white wines, and 20 rosé wines were followed during these trials.

The free and total SO₂ was regularly dosed by the Ripper method on each of the wines for 12 to 24 months according to the tests. At the same time, the dissolved and gaseous oxygen in the bottle was measured by luminescence (Dimkou et al. 2011) with the portable NomaSense O₂ P6000 oximeter.

The Total Consumed Oxygen (TCO) was calculated as the sum of the oxygen present at bottling, called TPO for Total Package Oxygen (oxygen located in the headspace + dissolved oxygen) and the oxygen entering the bottle through the closure during storage, from which the dissolved oxygen and the oxygen from the headspace measured at each stage are subtracted.

Effects of post-bottling oxygen exposure on the evolution of SO2 when storing wine

Figure 2 shows a characteristic pattern of the decline of free SO₂ in wines stored under different degrees of oxygen exposure. These degrees were created using the same closure for each method but by creating:

• a higher or lower oxygen level at bottling (0.5 to 1.5 mg/L of total O₂, the sum of dissolved O₂ and headspace O₂, also called Total Package Oxygen or TPO)
• and storage in atmospheres of different levels of O₂ (air at 21% O₂ and an inert atmosphere at 1% O₂)

The highest level of TPO at bottling results in a faster drop in SO₂ from the first measurement points, provided they are in equivalent storage conditions.
Given equivalent bottling conditions, storage at 21% of O₂ (air) allows for more O₂ transfers at a given time than the storage of 1% O₂, which is equivalent to the use of a lower OTR closure. A smaller decline in SO₂ is observed in the latter case.

Correlation between the O₂ content of wine and the evolution of SO₂.

The correlation between the loss of free SO₂ and the decline in dissolved oxygen is the highest in the first few days after bottling (Table 1), even though the correlation coefficient remains around 0.8.

Measuring dissolved O₂ does not, therefore, correctly determine the amount of total O₂ that was consumed by the wine. For this, a fullassessment must be considered for all sources of O₂: Dissolved O₂ and O₂ from the gaseous headspace taken during bottling as well as the O₂ entering the bottle through the closure. For that, it is necessary to substract the dissolved and the gaseous headspace O₂ that were not consumed to reach the total consumed O₂ (TCO) at each measurement point.

Knowing the total amount of oxygen that can react with wine, it is then possible to establish a correlation with the decline of SO₂, one of the most easily measurable antioxidants in wine. It has been demonstrated in the literature that the antioxidant activity of SO₂ in wine is not due to a direct reaction between oxygen and HSO₃^–, the main form of SO₂ in wine (Danilewicz 2010) according to equation 1:

2 HSO₃^– + O₂ ———–> 2 SO₄^2- (1)

The reaction mechanism brings into play more complex mechanisms involving quinones and hydrogen peroxide.
It is customary in the industry to consider that the consumption of 1 mg of O₂ causes a drop of 4 mg of SO₂, in reference to the stochiometry of equation 1.

For the 54 red wines, 46 white wines, and 20 rosé wines studied, a linear correlation was observed between the concentration of free SO₂ and the TCO. Figure 3 aggregates this correlation of the 16 methods of one of the Riesling wines, followed by the University of Geisenheim. The resulting correlation has a slope coefficient of -2.2, indicating that consumption of 1 mg of O₂ causes the loss of 2.2 mg of free SO₂. This ratio is well below 4, commonly accepted in the industry.
Of the 120 wines followed, the average loss of SO₂ is 2.5 mg per 1 mg of O₂ consumed. Ratios higher than 4, sometimes up to 10, have, however, been measured in wines bottled with low OTR closures, suggesting the involvement of SO₂ in mechanisms other than oxidation and possibly reduction mechanisms.

Development of a calculator to estimate the wine shelf-life

The average ratio of 2.5 was selected to develop a wine shelf-life calculator, without the appearance of oxidative defects. For that, it was accepted that when the level of free SO₂ is less than 10 mg/L, the risk of appearance of aroma oxidation is high, as described by Ferreira (2010). This was corroborated by the tasting of the trials by a trained expert panel.
This calculator (Figure 4) estimates the time it takes to reach the 10 mg/L threshold of free SO₂ by providing:

• the free SO₂ content of the wine at bottling – the TPO (O₂ dissolved + O₂ from headspace) at bottling
• the total oxygen contribution of the closure (closure oxygen ingress): the release of oxygen from the closure and the entry of oxygen through the closure (OTR).

the model has been refined by incorporating additional parameters such as:

• the total SO₂, an indication of the oxidation that the wine underwent during winemaking and thus the reservoir of oxidative aromatic compounds that can be released during storage
• the wine storage temperature, influencing reaction speeds
• the level of polyphenols measured by linear scanning voltammetry (PhenOx index obtained with the NomaSense PolyScan P200 analyzer), these compounds being largely involved in oxidation reactions.

Conclusion

At first, this study was able to demonstrate that the simple measurement of dissolved oxygen does not predict the evolution of wines and that it is necessary to consider the oxygen contribution as a whole (dissolved, headspace, contribution of the closure systems) to be able to find any correlation with changes in the concentrations of wine constituent molecules. In addition, this study of the link between consumed O₂ and the drop in free SO₂ in bottled wines showed that, on average, 1 mg of O₂ consumed results in the loss of 2.5 mg of free SO₂. This ratio has been integrated into a calculator that estimates the potential losses of free SO₂ in the wine during its life in the bottle and thus estimates the wine shelf-life before the development of oxidation taints. However, this ratio can vary from one wine to another, so the result of the calculator provides only an estimate, and above all has an educational purpose of showing the importance of certain factors in the evolution of wines and of guiding improvements in production.