What does oxidation do to wine?

If you look at and smell an opened bottle of wine after a week, it may be a little brown and smell like coins and vinegar. These are the consequences of the destructive effects of oxidation: the same process that turns a sliced apple brown or causes the flavor of an avocado to change.

However, oxidation is not always a bad thing. In fact, it is an important part of most wine-making processes. Long before you uncork a bottle, the wine has likely undergone some form of oxidation, either from contact with the air during the winemaking process or during aging from the oxygen entering the bottle through the cork.

Oxidation can be the reason for the flat taste of wine that is over the top, but it can also be the element that opens up the range of flavors that wine grapes can offer. It can soften tannins, give nutty, earthy and roasted flavors, and is responsible for breaking down primary flavors into deeper secondary and tertiary notes.

Controlled oxidation is a requirement for almost any good wine, but how exactly does that process happen?

Open-tank fermentation, over-pumping, and bâttonage (lees stirring) are all processes that introduce oxygen into a wine. There is also oxidative aging, which includes any wine that has spent time in a porous vessel, such as a wooden barrel or clay amphora. Bottle aging under the cork, which is also porous, is another controlled way of oxygenating wine over time.

Oxidation can be responsible for the flat taste of wine that is over the top, but it can also be the element that opens up the range of flavors that wine grapes have to offer.

Under these conditions, oxygen slowly seeps in to initiate reactions during aging. New oak barrels allow more oxygen to penetrate the wine than used barrels, as does the decision not to refill wines that have evaporated in barrels. Oxidation also occurs, albeit faster, when you open a wine. It releases the wine into an environment full of air, which contains 21% oxygen. Decanting and rolling accelerate these reactions, allowing wine to "open up" faster with new flavors. But even if it is re-corked, wine will continue to change rapidly after opening and can quickly become over-oxidized.

Reductive winemaking usually takes place in hermetically sealed steel tanks and is widely used for white wines, as well as for some red wines. A popular form of this technique is carbonic maceration, in which the tank is flooded with inert carbon dioxide gas. The process blocks contact with oxygen while whole grape clusters ferment. This method is perhaps best known among wine enthusiasts for the production of Beaujolais Primeur, and is known for producing particularly fruity aromas.

"Wine is a redox system," says Dr. Federico Casassa, assistant professor of oenology at Cal Poly San Luis Obispo. "This means that when one compound or compounds are reduced, other sets of compounds are oxidized and vice versa." This is why a wine produced in a reducing style often oxidizes much faster in another environment, such as a barrel or open bottle.

The difference between oxidative aging and biological (reductive) aging

Organic aging, or aging "under flor," is another example of reductive winemaking, although this is often mistaken for oxidation. Flor, known in France as voile, is a yeast layer that forms on the surface of some wines as they age in barrels.

Flor is often associated with oxidation because of its use in certain styles of sherry and nutty flavors, but its presence actually indicates the opposite. Because the yeast feeds oxygen and nutrients to the wine, they actually form a barrier that prevents oxidation.

It is true that many sherries are characterized by heavy oxidation. This is especially true of oloroso, which forgoes the use of flor in favor of 100% oxidative aging, giving the wines their typical deep brown color and richness. Fino Sherry, which includes Manzanilla from Sanlúcar, is an organically aged wine that is protected from oxygen by its flor.

It is not difficult to identify a wine that was made in a reducing environment. Think of it as a very fresh and fruity style of young wine.

Finos (fall under fortified wines) are pale to yellow, completely dry and rich in acid. Their flavor profile represents autolysis, or flavors obtained when yeast is broken down. These occur in sparkling wines using the traditional method, or wines with a lot of lees contact. It produces nutty and savory notes, as do flavors obtained by oxidation, adding to the confusion.

Organically aged wines can also become oxidative, such as amontillado Sherry. It is produced when the protective flor on a fino Sherry disappears, exposing the wine to oxygen. Amontillado Sherry is darker in color and has a different set of flavors than fino, although it is often inaccurately labeled "medium-dry.

"An amontillado sherry is dry by definition," says Andrew Mulligan, Spanish portfolio manager for Skurnik Wines. "People taste amontillados and they think they are sweet, but there is no sugar in the wine. There is less sugar in it than in the average bottle of California Chardonnay, but there is a perceived sweetness."

"If the voile does not form naturally, we will already get aromas that are not pure and flavors that can really damage even the structure of the wine," says Joseph Dorbon, who produces wine under his own name in the Arbois appellation. "The most important aspect for long-term aging under voile is to have non-oxidized juice with good potential structure and alcohol content."

Dorbon stresses the importance of using wood correctly in the process. "The barrel cannot be neglected either," he says. "A good barrel for oxidative aging is an old barrel that over the years has collected these famous yeasts and bacteria from the wine and the ambient air in the winery that are needed to form the voile.

• Tawny Port: A style of port is aged in wooden barrels to encourage oxidation and evaporation, which gives the wine its distinct nuttiness.
• Madeira: This famous wine with longevity develops oxidative flavors from both heating and aging.
• Vernaccia di Oristano: A grape grown in Sardinia, used to produce wine in a traditional oxidative solera system, or aged in uncapped barrels.
• Tokaji Szamorodni: This rare dry style of Tokaji is made from botrytized grapes and aged under a veil of yeast. Oxidative Tokaji were more common in the past.

In addition to these wines, many table wines can show oxidation, although there is no clear rule to identify them. Some oxidative white wines are intentionally bottled in clear glass, where the darker color of the wine will be the biggest hint. Otherwise, check the vintage - the older a wine is, the more likely it is to show oxidative characteristics.

Instead of dismissing oxidation as a fault, look first at the wine and how it was produced. Oxidation is a complex effect. It can lead to the eventual decay of a wine, but it is also the process by which wines can reach their best form.

By Tammie Teclemariam / @WineEnthusiast / Illustration by Rebecca Bardley