Oxidation (chemical reaction) and Oxidative Conditions
Technically speaking, oxidation is a chemical reaction in which a molecule, atom or ion loses electrons. Oxidation is a half reaction which cannot occur on its own. It must always be accompanied by reduction, which is a chemical reaction in which a molecule, atom or ion gains electrons. Together they form what is known as a reduction-oxidation, or redox, reaction. Hence, reduction and oxidation are complementary processes: as one compound is oxidised, another is reduced. These reactions happen in wine all the time, both during winemaking and also during maturation.
Oxidation is a destructive process: it makes iron rust, fruit go brown, humans grow old, and wine turn into vinegar. It is for this reason that bottles are tightly sealed and ideally kept on their side, such that the cork does not dry out and shrink, letting oxygen into the bottle. Fortunately, oxidation is a slow process, and a wine will at first only gradually start to lose its fruit flavours. Unfortunately, this process begins to take its toll from the beginning of the winemaking process.
In modern day winemaking, exposure to oxygen is avoided at nearly all stages of the winemaking process in order to preserve the fruit characteristics of the wine being made. Indeed, most modern-day winemaking is classified as anaerobic, which means that it is carried out in the relative absence of oxygen. The few exception to this include sherry and Tokaj, where wine is deliberately oxidised to achieve the oxidative character which is part of the traditional style.