5.5.1.b What effect does oxygen have on wine?

Impact of oxygen on tannins and anthocyanins

Oxygen can have a positive or negative impact depending on the quantity and timing of the addition. The type of tannins present in the wine will determine the reaction to oxygen.

	Oxygen	Astringency
Oligomeric tannins	High reactivity	A lot
Polymerized tannins	High reactivity	Little
Tannins combined with polysaccharides	Low reactivity	None

	Polymerized form %	Tanins associés aux polysaccharides %	Oligomeric tannins %	Reaction with regard to oxygen
Skins	85	15	0	Slow and difficult reaction
Stems	60	13	27	Intermediate
Pips	40	5	55	Quick and efficient response

It can be seen from these tables that grape variety, harvest date and vinification have a considerable impact on a wine's capacity to withstand oxygen. Depending on the initial composition of the environment, the wine will benefit from varying amounts of oxygen during ageing.
The key characteristic of the grape variety is the proportion of skin tannins, seeds tannins and anthocyanins.

For example, Cabernet Franc is low in skin tannins compared to Cabernet Sauvignon. Colour intensity will be less impacted by an addition of oxygen to Cabernet Sauvignon than Cabernet Franc.

There are numerous reactions between tannins, anthocyanins and oxygen. The following diagram represents the majority of these interactions and their consequences on the wine's colour and taste profile.

An excess of oxygen will lead to the anthocyanins breaking down and tannin oxidation, resulting in an increase in the yellow colour of the wine. Similarly, tannin polymerization will lead to the appearance of very condensed tannins which will initially increase the yellow colour and then precipitate as they are unstable in solution.

A controlled supply of oxygen can help stabilize the colour via the tannin-anthocyanin bond by increasing the red colour. The bond is often made by means of an ethanal bridge. Ethanal is produced by the reaction of oxygen with ethanol.
The reactions of the polyphenols with oxygen, in addition to modifying and stabilizing the colour, will improve the wine's taste profile by softening the tannins.

The impact of oxygen on aromas

If the wine is reductive, the addition of oxygen via racking can prove beneficial. It eliminates the molecules responsible for the reduction such as H2S which is highly volatile.

However, an excess of oxygen can cause significant olfactory deviations. The impact is different depending on the wine's aromatic molecules and the quantity of oxygen. Thiols for example are very sensitive to oxygen. Too much oxygen leads to the destruction of these aromas and therefore to a loss of aromatic typicity. This is a crucial point for many white wines.
An excess of oxygen also produces oxidation molecules such as ethanal (musty or over-ripe apple odours). Generally speaking, excessive oxygenation leads to the standardization of wine aromas.
For red wines, odours of prunes and dried figs, due to γ-nonalactone and MND are markers of premature oxidation. However, this is not solely due to oxygenation during ageing. The degree of ripeness, vinification and pressing are also factors.
A typical marker of premature oxidation in white wines is sotolon, which has notes of curry and walnuts.

Microbiological impact

Exposure to oxygen during ageing favours the development of microorganisms responsible for spoilage. Excessive oxygen levels lead to organoleptic deviations induced by different microorganisms. See the chapter on Microbiological monitoring.

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