Oxidation and reduction—opposite but intimately related phenomena—are a central (and complex) part of winemaking. Small nuances that shift the balance in one direction or the other can wind up making a significant difference in the finished wine. That’s why it’s important to define these phenomena—and their nuances—properly.
Getting terms straight helps us frame situations or conditions to let us think clearly about what is actually happening with the wine. Maurizio Ugliano, of Nomacorc’s enology department, wonders if frequently used terms like “aerobic” and “anaerobic” are always used exactly as they should be. “When fermentation is taking place—because there are yeasts and other organisms at work—it is appropriate to talk about aerobic or anaerobic conditions, since the term really applies to living organisms, as when we refer to types of physical exertion. If we’re talking about finished wines or what happens in the bottle, where biological phenomena should be much less a factor, it might be more appropriate to talk about oxic or anoxic conditions, even though the terms are less common, because then it’s more a question of the presence or absence of oxygen in and around the wine.”
Is it just a question of terminology? Yes and no. To take a different kind of example, one might take an oxidative approach in making a certain wine—meaning that the wine is exposed to oxygen at one point or another of the process. But this does not necessarily result in an oxidized wine, or even a wine with strongly oxidative characteristics on the sensorial level. Saying that the wine is “oxidized,” on the other hand, would mean that it has been exposed to an excessive amount of oxygen and has lost the flavors and aromas it should have presented.
Extremes, and Everything in Between
When a wine is said itself to be oxidized or reduced, it’s not exhibiting a stylistic expression: It is a wine that’s completely gone. Its expected aromatics are drowned out by unpleasant characteristics: rancid and cooked at one end, swampy and burnt at the other.
We apply the terms oxidative and reductive to various factors to describe respective characteristics along a continuum. One can speak of reductive or oxidative storage conditions—stainless steel (reductive); barrel (oxidative); topping up barrels (oxidative); blanketing with neutral gas (reductive); etc. There are also reductive and oxidative biochemical processes that happen during fermentation and in the finished wine. An finally, many aromas found in wine can be described as oxidative or reductive.
All of these different usages of oxidative and reductive are not necessarily linearly connected. Sometimes an oxidative process can help avoid an oxidative character in the finished wine. Browning the juice of white grapes at the press, for instance, is very much an oxidative process, but it actually removes a number of components (phenolics, chiefly) that could affect the wine in the long run and give it an oxidative character. So in this case, oxygenating the juice at the beginning of the process is one way to avoid an oxidized wine at the end.
The rate at which the reactions happen can influence the results greatly. In the case of oxidation, a slow process can provide something desirable (in some white wines from the Jura, for instance) while in other cases, it can become quite undesirable (premature oxidation in some white Burgundies).
Generally speaking, we tend to believe that protecting a wine from oxygen is a way to preserve maximum expression of its fruity aromas, because oxidation is the enemy of fruit flavors. However, in this in-between zone we are describing, enough oxygen is present for oxidation to happen chemically, but without obvious aroma deterioration. Under these conditions, expression of fruity aromas can actually be increased dramatically. Many wines have a tendency to accumulate reductive sulfur compounds, and these can mask fruit expression. A gentle oxidation will remove these compounds, resulting in cleaner and more intense fruit aromas.
Consumers, In Another Way
The example shown here of “brown juice” goes to another important concept in oxidative and reductive chemistry: oxygen consumption. This notion has to do with the amount of oxygen that is “used” by various components present in the wine when the wine comes in contact with air. This can vary greatly, depending on each individual wine’s composition and the presence of components like sulfur dioxide, glutathione, or ascorbic acid on the reductive side, or certain phenolic compounds on the oxidative side. While some will divert the oxygen from other reactions that can damage wine aromas, other components will create a chain reaction that will accelerate the oxidation of the wine.
How these reactions happen can also depend considerably on how certain chains of molecules have formed during the winemaking process, and how stable these chains are. Here, we are talking mainly about tannins and anthocyanins, which can bind together to produce long molecular chains that are more stable and will resist oxidation longer as a wine ages. This process, called polymerization, also binds tannins and anthocyanins. It is encouraged by the presence of oxygen during the winemaking process, once again showing that an oxidative process doesn’t necessarily result in an oxidized wine. On the opposite side of the scale, protecting wines from oxygen in the early stages may sometimes result in wines that are less resistant to oxygen and will age more quickly in bottle.
Looking at all these phenomena, one might think that oxidative-reductive vocabulary could almost be summed up in one essential word: paradox.