Oxidation in Winemaking

Typically, winemakers do what they can to carefully control, and often limit, oxygen exposure during the winemaking process. They “top off” wines in barrels and pump inert gases into steel tanks.

While a bit of fresh oxygen, as flows through the pores of oak barrels,  can help maturing wines, too much encourages the growth of unwanted yeasts and bacteria. Once bacteria like Acetobacter, get a hold of some fresh air, it will turn wine to vinegar. And long periods of aging in oak casks can lead to noticeable levels of acetic acid.

Yet, in some winemaking regions, oxygen exposure is intentional. We didn’t always have the ability to eliminate oxygen exposure with stainless steel tanks and nitrogen gas canisters. Once these technologies came onto market, they could have wiped out oxidized styles of wines. But some historic styles remain. Why? 

1. Jura’s Vin Jaune

Jura is a wine producing region in eastern France, between Burgundy and Switzerland. Vin jaune translates, literally, to “yellow wine”. Made predominantly from the white variety Savagnin, the wine turns a yellow color due to oxygen exposure. While red wines lose their luster and develop into tawny tones over time exposed to air, white wines often develop into yellow, gold and amber tones.
During years of oak cask aging, as the wine takes on these yellow tones, a “veil” of yeast develops on the surface. The yeast serve as a protective blanket and a sort of preservative. The veil allows for a slight amount of oxygen exposure, but contains it along the surface of the wine. Additionally, this veil prevents the development of high levels of volatile acidity. During long periods of aging in which oxygen is involved, volatile acidity is a significant risk.

The yeast also contributes to flavor. In Champagne, we often talk about smelling and tasting nutty notes or bready, brioche-y notes. These flavors result from a process called autolysis, in which the yeast cells break down (auto = self, lysis = cell disintegration by rupture) while still in the bottle of Champagne. This same process occurs in the film of yeast, with a continuous process of death and renewal contributing to the increasing complexity of flavors and aromas in the wine. This same process contributes to some of the aromatics and flavors of sherry wine, which we discuss below.

 

2. Madeira

Madeira is a volcanic island in the Atlantic Ocean, north of the Canary Islands and west of Morocco. They’ve been growing grapes since Prince Henry of Portugal sent colonists to conquer the island and clear the land for agriculture in the 1400s. Madeira wine is fortified, with styles ranging from dry to sweet. It once enjoyed great popularity but is more obscure today.

Where ships stop, there must be alcohol (hence, South Africa’s long wine history). Until the opening of the Suez Canal, the island of Madeira was a strategic stopping point for ships sailing from Europe to India and the New World. Madeira wine was popular with colonists and our Founding Fathers.

Once, wine made on Madeira were put on ships in casks for long voyages, during which the wine suffered heat, jostling and oxygen exposure…. and ended up tasting better than ever. These “vinhos de roda” or “round trip wines” were popular in the 1600s. While Madeira is no longer aged by sea voyage, there is now a bourbon that uses that process (Jefferson’s Ocean Bourbon). In 1833, Cyrus Redding wrote,

“They ripen and mellow their wines in stoves… by which they save six years of age; but a sea voyage gives a preferable quality to the wine”.

To ship a cask of wine to wherever and back was expensive and time-consuming, so winemakers tried to replicate those effects on the island. They developed a few ways to do this. One is to put the wine in large tanks and run hot water through surrounding pipes, heating the wine over several months, after which the wine is aged in cask. This quick and dirty way, called the estufagem process after the estufa vats, is mainly used for bulk production.

Alternatively, the Canteiro method ages wines in attics and typically produces the best wines. These will age in casks for several years, with the best wines aging for decades!

Fortunately for wine lovers with no great place to store wine (i.e. a place with controlled climate and humidity), Madeira is nearly indestructible. It’s  been fortified, heated and exposed to oxygen plenty already. There’s not much else you can do to abuse it.
3. Sherry 

Sherry is made in the Jerez (Sherry is the British corruption of the name) region on Spain’s southwestern coast. The chalky soils and maritime climate produce distinctive wines. By far, the main grape is Palomino. As a wine, Palomino is a bit boring on its own, but lovely when aged and fortified. Moscatel and Pedro Ximenez are also important grapes, particularly for sweeter styles of Sherry.

Sherry is a fortified oxidized wine, though not heated and abused like Madeira. There are a few different styles, but the ones with the most interesting aging processes are fino types and olorosos.

Fino type sherries are unique for their “flor,” a blanket of yeast that develops on the surface of the wine after primary fermentation – similar to the veil of the Jura’s vin jaune. The flor needs oxygen, so casks are not filled to capacity. The yeast blanket prevents excessive oxidation, allowing for a bit of fresh air to touch the layer of wine just beneath the surface and contributing to flavor and aromatic development through the yeast’s own renewal process. After some time under flor, wines are fortified to an alcohol level at which the yeast can not survive. Once fortified, some of the wine is moved through a continuous blending/aging system of barrels. This wine becomes an amontillado or palo cortado. As the wine ages in barrel, it takes on an amber hue. Amontillados, essentially aged finos, display this color, as well as nutty flavors. Palo Cortados, which spend even more time in barrel, are aged amontillados.

Another style, oloroso sherry, is not allowed to develop the flor of the fino types. With more time in barrel and a more oxidative aging process, these wines are more heavily fortified, nuttier and more aromatic in style.  

       
4. Tawny Port
Port is another styles of fortified wine. There are various types, including ruby and vintage, but tawny ports are the ones to talk about with regards to oxidation in wine. Tawny ports are aged in wooden casks. Over time, the wines lose the bright ruby hue of youth, taking on tawny tones and aromas and flavors of nuts, caramel, and dried fruits. A bottle of tawny port may display an indication of age, i.e. 10 years old, 20 years old, etc. The age is an average age of the blend, as tawny port is blended to achieve a certain consistent style. The age will tell you how tawny the tawny Port actually is.

A little bit of oxygen can be a good thing for most wines. And a lot of oxygen is a great thing for some wines. Breathe deeply as you try one of these four historic styles and appreciate the versatile effects of oxidation.

Sources

Jancis Robinson. Oxford Companion to Wine.

Karen MacNeil. The Wine Bible.

Cyrus Redding. A history and description of modern wines

1 thought on “Oxidation in Winemaking”

Leave a Reply

Your email address will not be published. Required fields are marked *