WHAT DOES OXIDATION DO TO WINE?
By Tammie Teclemariam / @WineEnthusiast / Illustration by Rebecca Bardley
If you look at and smell an opened bottle of wine after a week, it may be slightly browned and smell like coins and vinegar. Those are the consequences of the destructive effects of oxidation: the same process that turns a sliced apple brown or changes the flavor of an avocado.
However, oxidation is not always bad. It is an important part of most winemaking. Long before you uncork a bottle, the wine has likely undergone some form of oxidation. Either through contact with air during the winemaking process or during maturation, due to oxygen entering the bottle through the cork.
Oxidation can be the reason for the flat taste of wine over the top, but it can also be the element that opens up the array of flavors that wine grapes can offer. It can soften a tannin, impart nutty, earthy, and toasty flavors, and is responsible for breaking down primary flavors into deeper secondary and tertiary notes.
Controlled oxidation is a requirement for almost every fine wine, but how exactly does that process work?
The science behind wine oxidation
Oxidation occurs when the exposure of wine to air triggers a series of chemical reactions that convert ethanol (what we commonly call alcohol) into acetaldehyde. It concentrates color and creates aromas and flavors that are generally considered to be; grassy, nutty, or apple-like.
Open-tank fermentation, pumping-over, and bâttonage (dregs stirring) are all processes that bring oxygen into a bottle of wine. There is also oxidative aging, which includes any wine that has spent time in a porous vessel, such as a wooden vat or clay amphora. Bottle aging under the cork, which is also porous, is another controlled way of converting wine with oxygen 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 array of flavors that wine grapes have to offer.
Under these conditions, oxygen slowly seeps in to initiate reactions during maturation. 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. By decanting and rolling, these reactions accelerate, allowing the wine to “open up” faster with new flavors. However, even if re-corked, the wine will continue to change rapidly after opening and quickly become over-oxidized.
What is a reductive wine?
Reduction is the opposite of oxidation. Many wines made with limited exposure to air show features of reducing winemaking.
It is not difficult to identify a wine made in a reducing environment. Just think of a very fresh and fruity style of a young wine.
Reductive winemaking usually takes place in hermetically sealed steel tanks and is widely used for white wines (and some red). 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 bunches of grapes ferment. This method is perhaps best known to wine enthusiasts in the production of Beaujolais Primeur and is known for producing particularly fruity aromas.
“Wine is a redox system,” said 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.” It is why a wine produced in a reducing style often oxidizes much faster in a different environment, such as a cask or open bottle.
The difference between oxidative aging and biological (reductive) aging
Biological aging, or 'under flor' aging, 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 mature in barrels.
Flor is often associated with oxidation due to its use in certain styles of sherry and nutty flavors, but its presence indicates the opposite. As the yeast feeds oxygen and the wine's nutrients, they form a barrier that prevents oxidation.
What is sherry and is it oxidized?
The difference between oxidative aging and biological aging is critical to understanding Sherry. This style is misunderstood, and "sherry-like", is often used the wrong way to describe oxidation in other wines.
Many sherries are indeed characterized by heavy oxidation. This is especially true of oloroso, which forgoes the use of flor in favor of 100% oxidative aging. This gives the wines their deep brown color and richness. Fino Sherry, which also includes Manzanilla from Sanlúcar, is an organically matured wine protected from oxygen by its flor.
It is not difficult to identify a wine made in a reducing environment. Just think of a very fresh and fruity style of a young wine.
Finos, or fortified wines, are pale to yellow, completely dry, and rich in acid. Their flavor profile represents autolysis, or flavors obtained when the yeast has broken down. These occur in sparkling wines made according to the traditional method or wines with a lot of contact with the lees. It produces nutty and savory notes, as do the 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 has a darker color and has a different range of flavors than fino. Although, it is often inaccurately labeled as 'medium-dry'.
“An amontillado sherry is by definition dry,” said 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. It has less sugar than the average bottle of California Chardonnay, but there is a perceived sweetness.”
The oxidative white wines of Jura
The famous oxidative white wines of the Jura region of France are also aged organically but under drastically different conditions than those of Spain. In the climate of the region, it can take weeks or months for the yeast layer to form. The wines, such as Vin Jaune, have intense flavors due to the combination of oxidative winemaking and biological aging.
“If the voile does not form on its own, we will already get aromas that are not pure and flavors that can even really damage the structure of the wine,” says Joseph Dorbon, who produces wine under his name under the Arbois designation. “The most important aspect for long-term aging under voile is to have unoxidized juice with good potential structure and alcohol content.”
Dorbon emphasizes 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 is needed to form the voile.
Other types of oxidative wines
In addition to non-fino sherries and sous voile (under the layer of yeast and bacteria) wines from the Jura, there are several other styles of oxidative wine. Most come from areas where traditional winemaking methods are used.
Tawny Port: A Port-style, it is aged in wooden barrels to encourage oxidation and evaporation, which gives the wine its distinct nuttiness.
Madeira: This famous longevity wine develops oxidative flavors through 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 untopped barrels.
Tokaji Szamorodni: This rare dry style of Tokaji is made from grapes with botrytica and aged under a veil of yeast. Oxidative Tokaji was 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, with the darker color of the wine being the biggest hint. Otherwise, check the vintage – the older a wine is, the more likely it is to exhibit oxidative properties.
Rather than dismiss oxidation as a flaw, look at the wine and how it was produced. Oxidation is a complex effect. It can lead to the eventual decay of wine, but it is also the process by which wines can reach their best shape.
Source: WineEnthusiast