Oxygen is wine's mortal enemy. After all, oxygen oxidizes wine and degrades its quality. Whether during winemaking or after bottling, oxygen should be eliminated as much as possible—or so you might think.
In reality, oxygen is intentionally utilized at various stages of winemaking. Take hyperoxidation (also written as hyperoxygenation), for example, where oxygen is supplied to pre-fermentation must to force oxidation. This is a winemaking technique sometimes employed to remove components that might otherwise change through oxidation, thereby promoting wine stability.
Oxygen is also used when wines high in phenols and tannins feel hard and difficult to drink immediately after opening. Through swirling or decanting, oxygen is supplied to the wine, oxidizing phenols and bringing the wine to a more approachable, open state.
By the way, have you ever considered the specific meaning of the word "oxidation" that we've been using so casually? In the context of "wine oxidizes when it comes into contact with oxygen," we tend to understand oxidation as some phenomenon caused by oxygen. Strictly speaking, however, this is not entirely accurate.
This article explains what oxidation means in the context of wine and how oxygen is involved in the process.
Three Definitions of Oxidation
Not just in wine, but in the oxidation of many things—foods, iron products, and so forth—oxygen is involved in the process. This is why oxidation tends to be understood as the target substance combining with oxygen. However, when we attempt to define oxidation rigorously, three different approaches exist: definitions based on oxygen atoms, hydrogen atoms, and electrons.
The oxygen atom-based definition is the most familiar to us. Oxidation refers to a target atom combining with an oxygen atom, while reduction refers to losing an oxygen atom.
In contrast, the hydrogen atom-based definition states that oxidation is when an atom loses a hydrogen atom it was bonded to, while reduction is when it combines with a hydrogen atom. On one hand, "gaining" an oxygen atom is oxidation, yet in this alternative view defining the same thing, "losing" a hydrogen atom is oxidation—this apparent contradiction may seem difficult to grasp. However, there is actually a consistent rule underlying this, which is represented by the third definition.
Oxidation Is the Loss of Electrons
The third definition provides the most rigorous specification of oxidation: the electron-based definition.
In the electron-based definition, losing electrons is oxidation, and gaining electrons is reduction. In fact, both the oxygen atom-based and hydrogen atom-based definitions we examined earlier were simply expressing the same thing from the perspectives of oxygen and hydrogen, respectively.
Let's recall high school chemistry. When atom A chemically bonds with an oxygen atom, electrons are involved. The electrons that atom A and the oxygen atom each possessed form a shared electron pair, thereby bonding the two atoms. At this point, because the oxygen atom has a large electronegativity, the shared electron pair is drawn toward the oxygen atom. Atom A bonds with the oxygen atom by offering its electron to the oxygen atom—in other words, by losing an electron. When atom A bonds with a hydrogen atom, exactly the opposite occurs.
Because hydrogen atoms have low electronegativity, when a hydrogen atom forms a shared electron pair with another atom, that shared electron pair always shifts toward the other atom. From the perspective of atom A bonded to hydrogen, it is gaining electrons from the hydrogen. Conversely, when separating from the bonded state back into individual atoms, atom A must return electrons to the hydrogen atom, meaning atom A loses electrons. In other words, oxidation. When thinking in terms of electrons, it becomes clear why gaining a hydrogen atom is reduction and losing one is oxidation.
If you're feeling confused at this point, there's no need to force yourself. For now, simply understand that "oxidation is a phenomenon involving electrons, and oxygen is not necessarily required."
Why Understanding Oxidation in Terms of Electrons Is Necessary
All three definitions of oxidation are correct. The difference lies in how deeply they delve into the chemical phenomena occurring during oxidation. If you're not becoming a researcher and simply want to understand this as basic knowledge for enjoying wine—or as a bit of trivia—the oxygen atom-based definition alone might seem sufficient. Of course, that's fine, but it will make it harder to understand the somewhat deeper discussion of wine oxidation that follows. From here on, oxidation that does not involve oxygen will be relevant everywhere. There is a reason why I've spent so much time explaining the definitions of oxidation.
Wine is an extremely complex liquid containing a wide variety of components. For this reason, oxidation in wine does not necessarily involve oxygen. Rather, in chemical oxidation in wine, oxygen has almost no direct involvement.
There are two types of oxidation in wine: enzymatic oxidation and non-enzymatic oxidation. Of these, oxygen is directly involved only in non-enzymatic oxidation. However, non-enzymatic oxidation proceeds extremely slowly under typical wine pH conditions. As a result, its scope of influence is small and generally remains negligible. It's somewhat of a simplification, but this is why I state that "oxygen is not directly involved in wine oxidation."
Enzymatic Oxidation Is Central to Wine Oxidation
The core of wine oxidation is said to be enzymatic oxidation. What plays a crucial role here is not oxygen, but enzymes. Enzymes called phenol oxidases are what cause wine to oxidize.
There are several types of phenol oxidases, but the most important are tyrosinase and laccase. Tyrosinase is present even in healthy grapes, but laccase is detected in particularly high amounts in grapes infected with botrytis. Wine oxidation begins when these oxidative enzymes oxidize the phenols contained in wine into quinones. Notably, what is strongly involved at this stage is not oxygen, but rather metal ions contained in the wine.
This is not widely known, but wine oxidation—including the browning of white wines—primarily begins with the phenolic compounds present in wine. Components such as ethanol and tartaric acid are oxidized as a result of chemical reactions that follow the oxidation of these phenols. Alcohol is not being directly oxidized as a result of contact with oxygen.
The Chemical Oxidation Cycle
What follows is somewhat intricate. You don't need to understand everything. There's no need to memorize the specific substance names. What's important are these three points:
- In both white and red wines, oxidation begins with phenols
- The reactive oxygen species generated during the oxidation of phenols are extremely powerful and go on to oxidize virtually all components in the wine
- These reactions can be suppressed by sulfur dioxide (SO₂)
Phenol oxidases bind with metal ions to oxidize phenols into quinones, and oxygen exerts its influence here. Electrons from the metal ions bound to the oxidative enzyme transfer to oxygen atoms, causing the metal ions themselves to be reduced and further oxidize phenols into quinones. Additionally, at this stage, hydroperoxyl radicals are generated.
The generated hydroperoxyl radicals are reduced to hydrogen peroxide. Quinones and the reduced hydrogen peroxide oxidize substances in their vicinity, but some of the hydrogen peroxide binds with other metal ions to generate hydroxyl radicals, a type of reactive oxygen species. These hydroxyl radicals possess particularly strong reactivity even among reactive oxygen species.
The characteristic feature of hydroxyl radicals is their extremely strong oxidative power, known even in biological systems for causing continuous oxidation of lipids. These characteristics remain unchanged when present in wine, and hydroxyl radicals oxidize virtually all organic matter. This includes ethanol, tartaric acid, sugars, and glycerol.
Why SO₂ Prevents Oxidation
Sulfur dioxide (SO₂), also called sulfurous acid, is added to wine as an antioxidant. While many producers and consumers harbor strong resistance to its addition, SO₂ fulfills its role in preventing wine oxidation through multiple mechanisms.
First, it has a strong suppressive effect on the activity of phenol oxidases, which are the starting point of wine oxidation. It shows particularly strong effectiveness in inactivating tyrosinase, reportedly capable of suppressing its activity by up to 90%. Laccase, on the other hand, has high SO₂ resistance, and it is virtually impossible to suppress its activity through SO₂ addition within realistic dosage ranges.
SO₂ can also reduce both the phenols oxidized and converted to quinones by oxidative enzymes, as well as the hydrogen peroxide generated during the series of oxidation reactions. The fact that quinones can be reduced back to phenols—thereby recovering the residual phenol content in wine and maintaining the wine's antioxidant properties—and that hydrogen peroxide can be reduced—thereby suppressing the increase in hydroxyl radicals generated from it—holds great significance when considering wine oxidation as a whole.
Making wine exclusively from healthy fruit certainly allows you to limit the amount of laccase present, but tyrosinase exists in must even from healthy fruit. Since accurately assessing the quantity of these enzymes is difficult, it's important to know that carelessly reducing SO₂ additions can increase oxidation risk far more than imagined. Wine oxidation is not something that can necessarily be managed simply by keeping oxygen out.
What Can Be Done After Understanding Oxidation
We have examined what oxidation means in the context of wine. Understanding this process reveals that, contrary to popular belief, oxygen does not directly oxidize the various components contained in wine.
So what measures can be taken with this systematic understanding?
The fundamental approach is how to remove phenol oxidases—which trigger all the reactions—from must or wine, or how to inactivate them so they cannot function. Measures from this perspective can only be taken by winemakers. Several methods have been proposed for reducing enzyme content, but among them, the relatively accessible approach is appropriate SO₂ addition. Given that wine is made at normal temperature and pressure in ordinary atmospheric conditions, no matter how careful you are, some amount of oxygen dissolution is unavoidable. In that case, unless the target enzymes are precisely suppressed, oxidation will begin from there, ultimately allowing the generation of reactive oxygen species starting from dissolved oxygen. The target should not be oxygen, but enzymes.
Consumers, on the other hand, cannot fundamentally do anything to remove enzymes contained in wine. This might seem to mean that available measures must inevitably focus on oxygen, but actually, that's not the case. Knowing that the presence of enzymes is crucial allows you to consider creating an environment that prevents enzymes from functioning.
Consider temperature, for example. The amount of dissolved oxygen is generally higher at lower temperatures. According to one verification result, while the oxygen saturation level at 35°C is 5.6 mg/L, at 5°C that amount nearly doubles to 10.5 mg/L. Following conventional thinking that more oxygen intake leads to more oxidation, it might seem better not to lower the temperature too much. However, the oxidation rate of phenols accelerates with rising temperature. This is because enzyme activity increases. In other words, to prevent wine oxidation, it is better to keep the liquid in a low-temperature environment even if that means taking in more oxygen. Conversely, when the goal is to open up the wine, being conscious of an environment where enzymes function easily may be more effective than excessively swirling or decanting.
Oxidation is not necessarily bad for wine. Depending on the time and circumstances, a certain degree of oxidation is often desirable. In such cases, correct knowledge is essential for controlling oxidation more effectively. What is oxidation in wine, and what mechanisms does it operate on?—I hope this article serves as an aid in developing such understanding.
