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What is oxidation in wine? – Ask Decanter

Sylvia Wu reports on how oxygen can be both friend and foe to winemakers, and how it affects the taste of different wines.

Oxidation, in wine terms, refers to a group of chemical reactions that occur when wine comes into contact with air.

It can result in a wine losing its primary fruit characters and developing a brown hue.

But for certain styles of wine, controlled oxidation – and also what some would call ‘micro-oxidation’ – encourages desirable characteristics to develop, such as more complex flavours and less astringency.

When the wine is exposed to the air for too long, alcohol is first oxidised to acetaldehyde, which smells like bruised apples at high concentration levels.

It finally becomes acetic acid, when the wine is literally turned into vinegar.

The enemy of wine?

Over-exposure to oxygen in air is a constant threat throughout winemaking.

It begins when the grape berries are crushed; the freshly released phenolic compounds from the seed, skin and stem are ‘among the most readily oxidised wine constituents’, according to research on Oxidation mechanisms occurring in wines, published in 2011 in Food Research International.

Oxidised phenolic compounds form deep-coloured polymers that give the wine an amber hue. This deepens with time, just like what happens to an apple when it’s peeled and left aside. 

White wine turns to a browner shade, whereas red wine becomes less vibrant and paler in colour, before gradually developing a tawny character.

The oxidation process also reduces the phenolics compounds available to develop varietal aromas and enhance the body of the wine at a later stage.

It’s generally considered that oxidation spoilage is a bigger danger for white wines. Red wines contain higher amounts of phenolic compounds, thanks in part to the pigments and tannins extracted from skin maceration.

In order to preserve the fresh colour and varietal aromas of a white wine, winemakers may choose to take protective measures during winemaking. That might include using stainless steel to minimise the exposure to oxygen, or adding SO2 or other inert gasses. 

However, for non-aromatic white grape varieties, such as Chardonnay, some producers might deliberately give the juice a ‘shot’ of oxygen prior to fermentation to help generate more complex flavours. 

The double-edged sword

Despite the risks, oxygen isn’t entirely an enemy to wines. For example, allowing some red wines to be in contact with small amounts of oxygen during winemaking (e.g. via pumping over) can help to stabilise colour and to soften the bitterness. 

This is due to a series of complex chemical reactions happening between oxygen and the phenolics in wine. One of them is called polymerisation, where small molecules of tannins and anthocyanins join together to form larger chains – or polymers.

The binding effect of tannins and the protein in our saliva is believed to be one of the key reasons for the astringency we feel when we taste a young wine. But salivary proteins cannot interact with large molecules, thus aged or slightly oxidised wines appear to be smoother in texture.

Moreover, as oxygen speeds up the degradation of wine, even by small degrees, it also encourages more complex aromas and flavours to arise.

As wines slowly oxidise with time, you may notice whites wines starting to pick up a honeyed, waxy smell, while reds may develop an earthy, dried fruit nose. When present at lower levels, these aromas add an extra layer of complexity to the primary fruit characters of the wine.

However, over time, these tertiary flavours eventually take over to the point at which a wine completely loses its fruit character. 

Racking and ageing in porous vessels, such as oak barrels, are among the methods that provide gradual exposure to oxygen during winemaking and maturation.

Producers can choose different types of closure to determine the desired rate of oxidation after bottling.

Wine lovers may also wield the power to oxidise, as we swirl the wine in the glass or let it ‘breathe’ in a decanter

By speeding up the oxidation process, it’s commonly thought that we enhance a wine’s aroma and soften the texture of young wine, in particular. Old wines tend to be much more fragile, thus should be aerated with caution.

The ‘oxidative’ styles

Winemakers who prefer their wines to maintain a youthful, fresh and fruit-forward style may take extra steps to minimise oxygen contact.

Green-hued New Zealand Sauvignon Blancs and flinty Chablis are typical examples of such ‘reductive’ winemaking. 

At the other end of the spectrum, you’ll find wines that display clear hallmarks of deliberate oxidation. Oloroso Sherry, Tawny Port and Madeira are among the most classic examples. 

In the case of Madeira and aged Tawny, the oxidation is combined with heat during storage, making them almost indestructible in the bottle.

The prolonged oxidative ageing in barrels often creates rich aromas of caramel, toffee, roasted nuts and dried fruits.

‘Premature’ oxidation

Finally, it’s perhaps worth a quick note about the risk of premature oxidation (‘premox’) in your precious old bottles.

Premox is arguably one of the most mysterious risks haunting the premium end of the wine world.

This phenomenon essentially refers to a wine growing old before its time; it may have been built for long years in the cellar but it has developed oxidative characters associated with ageing in a relatively short period. 

In the late 1990s, people identified the issue in white Burgundies but it was also discovered that premox could affect other age-worthy white wines. In 2014, Jane Anson wrote about whether the problem was becoming an issue for some red wines in Bordeaux.

At first, some people associated premox with poor cork quality at the time or insufficient level of SO2 in the bottle. 

Renowned Bordeaux oenologists, the late Prof Denis Dubourdieu and Dr Valérie Lavigne-Cruege, later suggested that it may also be linked to insufficiency of glutathione, a natural antioxidant produced in vines, due to lack of water and nitrogen supply in vineyards.


References:

Oxidation mechanisms occuring in wines
June 2011, Food Research International, 44(5):1115-1126
DOI: 10.1016/j.foodres.2011.03.050

Salivary Protein-Tannin Interaction: The Binding behind Astringency
By Alessandra Rinaldi and Luigi Moio
DOI: 10.5772/intechopen.93611

The Oxford Companion to Wine
By Jancis Robinson and Julia Harding


More ‘ask Decanter‘ articles: 

What is orange wine?

How long should you keep wine open? 

How do wine yeasts affect flavour? 

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