Have you ever had the unpleasant surprise, when opening a good bottle, to discover that the wine is harsh in the mouth? This unpleasant taste is the result of wine oxidation, a chemical reaction that transforms alcohol into acetic acid. But let's go into detail to understand where this taste of vinegar comes from that the oxidation of wine generates even in our best bottles.
Oxidation: what are we talking about?
The oxidation of wine takes place when it is exposed to contact with air. If, on an iron support, oxidation results in the appearance of rust, on wine, the chemical reaction ends up giving this famous vinegar taste.
To fully understand the phenomenon, you need to know that upon contact with air, part of the alcohol in wine, called ethanol , is first transformed into ethanal . Then, if the contact continues, this same ethanal, which is a toxic volatile compound, ends up transforming into acetic acid , this component which is mainly found in vinegar.
What is the effect of oxidation on wine?
The oxidation process has several consequences on wine. Initially, the chemical reaction will first alter the color of the coat . White wines will turn yellow, while red wines will take on a purplish, then brown color.
But oxidation also changes the balance of the wine . The presence of oxygen reduces the alcohol level and considerably reduces the freshness of the wine. Furthermore, particular aromas develop , such as nuts, green apple or curry, which are typical aromas of ethanal. The original aromas will thus disappear, and the aromatic profile of the wine will no longer be advantageous or pleasant at all. Excessive oxidation can then lead to the development of much harsher flavors, like those of vinegar.
Oxidation is an inevitable process in the life of wine, influencing its aging potential and its aromas. Oxidation mechanisms, such as yeast self-destruction, can alter the intensity of aromas and even cause cork taint. To control these effects, winemaking techniques, bottle filling level and cellar storage conditions are crucial. Wine quantity, acidity and yeast populations also influence the flavor profile and the appearance of off-odors, thereby affecting the final quality of the grape juice.
The oxidation of wine is therefore a natural process which, over time and when the phenomenon is poorly controlled, will denature the wine . The role of the winemaker is therefore to manage the oxidation of his wine bottles.
How to limit and control the oxidation of a wine?
To produce quality wine, the oxidation process is essential. It makes it possible to generate numerous chemical reactions which will develop all the qualities of the wines. But excessive oxidation can have harmful consequences on the wine, and winegrowers must therefore limit the contact of the wine, and in particular the must, with the air.
Throughout the winemaking process, techniques and materials are used to avoid oxidative aging :
- The stainless steel drum can be covered with a pneumatic cover to limit air contact;
- The topping technique helps prevent oxidation of the wine during barrel aging. The empty space left by wine which evaporates over time and is regularly filled with wine from another barrel;
- The addition of sulphites during aging and bottling helps limit oxidation, thanks to the antioxidant properties of sulfur...
Oxidation: an essential process for wines to age
As we have seen, the oxidation process can spoil a bottle of wine. But, conversely, it is also essential for producing good wines for aging. Indeed, good oxidation allows you to gain complexity and harmony. To do this, the winemaker can also facilitate exchanges between the air and the wine:
- By opting for aging in oak barrels , a porous material which allows a slow exchange between oxygen and the wine;
- By practicing artificial micro-oxygenation , in other words, by providing a small quantity of oxygen to the wine;
- By taking the time to chamber or decant the wine before serving, and letting the chemical reaction take place to release the aromas before tasting.
The particular case of oxidative wines
In certain very specific cases, the wine can undergo voluntary excessive oxidation , in order to obtain unique qualities. This is the case for yellow wine, sherry, or even certain natural sweet wines.
The yellow wine of the Jura
The yellow wines of Jura are none other than oxidative white wines made from grapes harvested over-ripe. They are then very rich in sugar, perfect for alcoholic fermentation. Once the sugars are transformed into alcohol, the wine is placed in barrels for 6 years. During this period, the wine evaporates, but the winemaker does not top up to fill the empty space, as he does for other wines. The objective is to allow the development of a new yeast (saccharomyces cerevisiae), which will form a protective layer on the surface of the wine, and limit oxidation, while developing typical aromas of curry or dried fruits.
Good to know: Sherry wines follow this same vinification process.
The Rivesaltes of Languedoc-Roussillon
To produce Rivesaltes , Languedoc producers use another technique, which consists of placing the wine in a Dame-Jeanne . This is installed outside, where it will be exposed to the sun, but also to temperature variations.
Other oxidative wines
Other wines using oxygen as a force include Tawny port , Madeira wine , Vernaccia di Oristano , and Tokaji Szamorodni .
Finally, oxidation should not be seen as a fault of the wine, but as a complex chemical reaction which can more or less work in the wine's favor. If, for some bottles, this process can be catastrophic, it can also be a real strength for other wines.
