By Meistermann Eric¹, Jean Baptiste Diéval²

The role of oxygen in the ageing of wine has been known since Louis Pasteur. However, it is only in the last fifteen years, thanks to improvements in measurement of total oxygen in bottles (Diéval, Vidal, & Aagaard, 2011) and developments in closure, that significant progress has been made in controlling oxidation phenomena in bottled wines. The current trend of reducing sulphites in wines and the need to ensure proper conservation of wines has given an even greater importance to this issue.

Firstly, it has been shown that the amount of oxygen picked up at bottling affects the future development of the wine (Dimkou, Ugliano, Diéval, Vidal, & Jung, 2013; Dimkou et al., 2011), highlighting the practical value of controlling oxygen intake at bottling. Additionally, a large number of studies have highlighted the impact of closures’ measured permeability on the development of bottled wine (Caillé et al., 2010; Dimkou et al., 2013; Dimkou et al., 2011; Ugliano, 2013; Ugliano et al., 2012; Ugliano et al., 2011; Wirth et al., 2012; Wirth et al., 2010). These studies have shown that by modulating the amount of oxygen entering the bottle, the closure can direct the wines towards different sensory profiles ranging from reduction to oxidation.

The aim of this study was to evaluate the impact of closures’ oxygen ingress on the profile of the wines made from two Alsace grape varieties depending on management of sulphiting during their production. The ultimate goal of this work is to adapt the closure to the potential of the wine and marketing objectives.

Experimental protocol

The experimental system included three factors and twelve methods in all:

• Factor 1 = Grape variety: the study was conducted on Riesling and Gewurztraminer, two emblematic grape varieties of the Alsace vineyards with rather different oenological characteristics.
  • Optimised sulphiting incorporates the use of S0₂ as practised in the region enabling the production of wines with free SO₂ contents consistent with customs.
  • Reduced sulphiting aims to lower sulphiting by approximately 50% in relation to the above.
• Factor 2 = Control of SO₂: the winemaking was carried out using a same must following two sulphite management strategies:

The analytical characteristics of the wines after bottling (Table 1) showed that the reduction in total SO₂ content was 50% for Riesling and 40% for Gewurztraminer. The total SO₂ content was higher for the latter grape variety which is less acidic. The wines were dry and did not undergo malolactic fermentation.

Change in SO₂ levels and colour of the wines

During conservation in the bottles, the levels of free SO₂ rapidly decreased during the first year (Figure 1) due to consumption of total oxygen (dissolved and gaseous in the headspace) that was picked up at bottling. The decrease was lower subsequently and this is where the differences between the closures became evident in accordance with their permeability to oxygen. In the reduced sulphite methods, there was no longer any free SO₂ after 4 years storage in bottles. The results were comparable for both the grape varieties. The colour intensity, measured by optic density at 420 nm, increased fairly regularly over time (Figure 2). It was higher when sulphiting was reduced and when the permeability of the closure was higher. The variations between the two sulphiting control methods were greater than between the three types of closures. These results were in line with what is generally observed during the analytical monitoring of bottled wines.

Change in the organoleptic characteristics of the wines

Impact of the sulphiting methods

The wines of the two grape varieties, each produced using two different sulphite management methods, developed in different ways (Figure 3). The “quality” descriptor for the wines was chosen to evaluate the wines for this test: it typifies the balance between the profile tasted and the profile expected by the jury for each of the grape varieties, i.e., a reducing profile with mineral notes for the Rieslings and a more open profile with floral notes for the Gewurztraminers.

With optimised sulphiting, the quality of the wines was maintained during the first 4 years; it improved a little for the Riesling during the first two years with the emergence of mineral notes. It was only after 5 years in bottles that the wines displayed signs of change and an increase in the intensity of oxidation (Figure 4).

With reduced sulphiting, the quality of the wines was fairly close to those for the reference wines on the first tasting. For the Gewurztraminer, it deteriorated quite rapidly subsequently, due to more oxidising notes. For the Riesling, the variation was less significant because the oxidative nature was attenuated by the appearance of more mineral notes that are typical of this grape variety.

Effect of the closures

During the first tasting, 5 months after bottling, there was no difference between the closures, due to the impact of the TPO (intake of O₂ during bottling) at this point of conservation. Subsequently, the impact of the closure depended on both the grape variety and sulphiting management method.

For the Riesling, the quality of the wine had already deteriorated by the second tasting (Figure 5) with the closure that is more permeable to oxygen. This was especially evident in the case of reduced sulphiting. The wines displayed more intense oxidative notes and much lower floral notes for this method. Conversely, with the least permeable closure, the quality of wine improved after 12 months in bottles and remained stable over time. With intermediate permeability, the wine fluctuated between a reducing profile (mineral notes) and an open profile (oxidative notes) depending on the sulphiting management method used: a change appeared already in the first year in the case of reduced sulphites but was delayed until after three years with optimised sulphiting.

For the Gewurztraminer, despite slightly higher oxidisation intensity, the wines sealed using the most permeable closure were preferred during the first three tastings (Figure 6). They displayed fruitier and spicier aromatic notes that disappeared subsequently, leading to a loss in quality and especially in the case of reduced sulphiting. After 4 years in the bottle, the intermediate closure was preferred when sulphating was optimised and the least permeable closure when sulphating was reduced.

The reduction of SO₂ during the winemaking process led to a quicker change in the wines which were logically more sensitive to oxidation. However, differences in behaviour emerged between the two grape varieties studied. The results obtained confirmed the observations made by the Chambers of Agriculture of Alsace during a comparison of different closures (Ansen & Pinsun, 2009; Pinsun, 2010, 2012). This study demonstrated that the Riesling was more sensitive to oxidisation and that it could tolerate a slight reduction. The least permeable closures adapted better to this grape variety, especially for long-keeping wines. However, they were not suitable for the Gewurztraminer because they tended to cause tastes related to reduction or at least aromatic closure of the wines. This grape variety is more sensitive to reduction and tolerates slight oxidisation better.

The choice of optimal closure for each of the four wines can be summarised as follows:

• Riesling – optimised sulphiting:
  • Very low or low permeability for keeping more than 4 years.
  • Moderate permeability is acceptable for keeping for less than 4 years.
• Riesling – reduced sulphiting: very low permeability is imperative.
• Gewurztraminer – optimised sulphiting:
  • Moderate permeability for keeping for 2 to 3 years.
  • Low permeability for keeping for longer.
• Gewurztraminer – reduced sulphiting: low or very low permeability.

Conclusion

The importance of oxygen at the time of bottling and the role of closures regarding conservation of wines in bottles no longer needs to be demonstrated. The ageing of the wines also depends on their intrinsic qualities, their grape varieties and how they are produced. There are differences in behaviour between Riesling and Gewurztraminer wines. The Riesling is sensitive to oxidisation and its organoleptic properties decrease when there is an increase in the quantity of oxygen to which it is exposed in the bottle. It loses its floral and mineral notes. The least permeable closure is the best adapted to this grape variety. This type of closure must be avoided for the Gewurztraminer which is, on the contrary, sensitive to reduction. In such a case, the wines lose their fruity and spicy notes. Management of SO₂ during the winemaking process and on bottling plays a major role in the wines’ capacity for ageing and keeping. For the wines in this study, reduced sulphiting led to more developed wines regarding the Gewurztraminer rather than the Riesling. The difference in pH can influence this difference in development. Use of a closure that is less permeable to oxygen makes it possible to attenuate some of the consequences of the reduced level of SO₂ in the wines.

Knowledge of these factors should enable the choice of the closure to be adapted, according to its oxygen permeability, to the grape variety, the winemaking process and particularly, the management of sulphiting, as well as the desired duration of conservation of the wine in the bottle.

Know more about Nomacorc closures

¹ Institut Français de la Vigne et du Vin – Pôle Alsace – Colmar – France
² Vinventions – Enology Team – Rodilhan – France