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Oxidation

ossidazione (I)
oxidación (ES)
oxidation, redox (GB)
redoxreactie (N)
oxydation, réaction d’oxydo-réduction (F)

Term for a chemical reaction in which an atom or molecule releases electrons (oxidises) and another substance accepts these electrons (reduces). 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. The term was later expanded to include reactions in which hydrogen atoms are removed from a compound (dehydrogenation).

Oxidation - Eisen, Gestein, Sherryglas

Oxidising agent

Oxidising agents are substances that can easily accept electrons, such as oxygen, chlorine and fluorine. Oxidising agents are reduced during the redox reaction itself. And reducing agents are those substances that easily release electrons, e.g. hydrogen, carbon and base metals such as iron, magnesium and sodium. The reducing agents are oxidised during the redox reaction itself. In winemaking, oxidation should only take place in as precisely dosed and controlled a form as possible, as uncontrolled wine defects often occur. It is thanks to Emperor Napoleon III (1808-1873), so to speak, 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 spoilt. The influence of microorganisms was still completely unknown at the time. Pasteur discovered that too much air favours the spread of acetobacter (acetic acid bacteria) and thus turns wine into vinegar. However, he also discovered that a small amount of oxygen can have a positive influence on development.

Phases of oxidation

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 oxidisable wine components. This bound oxygen can no longer be measured. Oxygen can be absorbed again in the same quantity of bound oxygen. In phase 3, the bound oxygen is further transferred to substances that cannot be directly oxidised.

Possible wine defects

There it leads to sensory and analytically perceptible consequences. In wine, oxidation is usually disruptive or only desirable under controlled conditions. If used uncontrolled, it attacks aromatic substances and often leads to negative changes in taste and colour, which are expressed by ageing tones and brown breakage, as well as undesirable sherry tones. The colour scale can range from yellow to amber to dark brown. The wine loses freshness and ages more quickly. Contact with oxygen can also cause acetic acid bacteria and mould (yeasts) to multiply, which can result in a wine defect and, in extreme cases, even make the wine undrinkable.

Normally, oxygen access is prevented or at least severely restricted during winemaking. This is achieved by spraying the mash with ascorbic acid and/or 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 prevents oxygen from entering. Another option is to fill the empty space of the fermentation tank with inert gas (protective gas such as carbon dioxide).

Controlled oxygen influence

The influence of oxygen is very desirable for special wines, for example sherry, Malaga or Vin Jaune. With certain white wine varieties such as Chardonnay, some producers encourage a certain oxidation of the must before fermentation because this releases certain flavours. The controlled addition of oxygen or air to the must or wine at various stages of winemaking is called aerate (aeration). The addition of very small amounts of oxygen to the wine during bottle ageing through the closure is known as nano-oxygenation (individual atoms) or, in slightly higher quantities, micro-oxygenation. Methods with controlled oxidation are called oxidative ageing. Preventing or restricting this is known as reductive ageing. See also oxygen management.

Further information

For the production of alcoholic beverages, see Champagne (sparkling wines), distillation (distillates), spirits (types), winemaking (wines and wine types) and wine law (wine law issues).

Iron: from ChadoNihi on Pixabay
Rock: from Taken on Pixabay
Sherry glass: By I, Hashashin, CC BY-SA 3.0, Link

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