Term for a chemical reaction in which an atom or molecule gives off (oxidizes) electrons and another substance takes up (reduces) these electrons. Oxidation is therefore always associated with a reduction. Both processes together are partial reactions of a so-called redox reaction. Originally, oxidation was exclusively understood as the chemical reaction of a substance with oxygen. Later the term was extended to include reactions in which hydrogen atoms are removed from a compound (dehydrogenation).
Oxidizing agents are those substances that can easily take up electrons, such as oxygen, chlorine and fluorine. The oxidizing agents are themselves reduced in the redox reaction. And reducing agents are substances that easily give off electrons, for example hydrogen, carbon and base metals such as iron, magnesium and sodium. The reducing agents themselves are oxidized during the redox reaction. During vinification, oxidation should only take place in as precisely dosed and controlled a form as possible, as uncontrolled wine defects often occur.
It is so to speak emperor Napoleon III. (1808-1873) that the mostly harmful influence of too much oxygen was clarified. In 1863 he commissioned the chemist Louis Pasteur (1822-1895) to investigate why so much wine spoils. The influence of microorganisms was still completely unknown at that time. Pasteur discovered that too much air supply favours the spread of Acetobacter (acetic acid bacteria) and thus turns wine into vinegar. But he also discovered that low oxygen influence can have a positive effect on development.
The oxidation of wine takes place in three phases. In phase 1, oxygen is absorbed from the atmosphere up to a maximum possible concentration of 9 mg/l at a temperature of 20 °C and can be measured as a gas dissolved in the wine. In phase 2, the dissolved oxygen combines with easily oxidizable wine components. This bound oxygen can no longer be measured. The same amount of bound oxygen can be absorbed again. In phase 3, the bound oxygen is transferred to substances that are not directly oxidizable. There it leads to sensory and analytically perceptible consequences.
In wine, oxidation is usually disturbing or only desired under controlled conditions. With uncontrolled application or undesired oxygen ingress, aroma substances are attacked and negative changes in taste and colour often occur, which are expressed by different age tones and brown breakage, as well as undesired sherry tone. The colour scale can range from yellow to amber to dark brown. The wine loses freshness and ages faster. Through contact with oxygen, acetic acid bacteria and yeasts can also multiply, which ultimately make the wine undrinkable.
Normally, oxygen access is prevented or at least severely restricted during vinification. This is already achieved in various stages of winemaking by spraying the mash with ascorbic acid and/or by adding sulphur. Fermentation usually takes place in the absence of oxygen(anaerobic condition). This is achieved by closing the fermentation tank with a fermentation cap, which allows the carbon dioxide produced to escape, but does not allow oxygen to enter. Another possibility is to fill the empty space of the fermentation tank with inert gas (protective gas). For special wines, the influence of oxygen is very much desired, for example sherry, Malaga or Vin Jaune.
Some producers promote a certain oxidation of the must before fermentation for certain white wine varieties such as Chardonnay, because this releases certain flavourings. The metered addition of oxygen or air to the must or wine at various stages of winemaking is called aeration (aeration). The supply of smallest amounts of oxygen to the wine during bottle aging through the cap is called nanooxigenation (single atoms) or in slightly higher amounts microoxigenation. Methods with controlled oxidation are called oxidative ageing. The prevention or restriction is called reductive ageing. See also oxygen management.
Complete lists of the numerous vinification measures and cellar techniques, as well as the types of wine, sparkling wine and distillate regulated by wine law are included under the keyword vinification. Comprehensive information on wine law can be found under the keyword wine law.