It is well known that wines are negatively affected by oxygen during the aging process, whites even more so throughout winemaking. Oxidation causes color to turn to a brownish color and convert desirable aroma and flavor compounds into flaws, possibly making the wine undrinkable. Sulfur dioxide, or SO₂, is used in winemaking partly because of its anti-oxidant power; it also has anti-oxidasic (fights oxidative enzymes) properties, and, albeit much less effective, anti-bacterial and anti-fungal properties. There are no other known winemaking preservatives that have the same scope and efficacy of SO₂.

Oxidation reactions and how SO₂ protects wine from such spoilage reactions are complex, but we have learned a great deal about these reactions and behaviors in wine over the last 25 years. However, we still have much to learn about the kinetics and dynamics of how these reactions interplay in such a complex medium as wine. We know what reactions take place, but we cannot predict in which order they will occur or at what rate. There are just so many factors that it is impossible to develop a model.

Here, we will look at a very simplified view of a model of chemical oxidation mechanisms in red wines, those being the most complex (compared to white wine, for example), and the role of sulfur dioxide in protecting against oxidative effects.

Major Wine Components

The major compounds or classes of compounds (shown in red bold font) in red wine include: ethanol (CH₃CH₂OH), the main alcohol; anthocyanins, which are part of the class of polyphenols (flavonoids) and responsible for imparting the red color; tannins, also part of the class of polyphenols (flavonoids) and responsible for the taste of bitterness and the sensation of astringency, or what is often referred to as mouthfeel; other polyphenols, both flavonoids and nonflavonoids, such as phenolic acids, that play an important role in color changes during the aging process; dissolved molecular oxygen (O₂), which increases during such operations as racking and pumping, and which becomes consumed during oxidative reactions; and naturally occurring iron (Fe) and copper (Cu), which exist in their ionic forms and act as oxidation catalysts.

Source: Schmidtke, L.M., Clark, A. C., Scollary, G. R. (2011). Micro-Oxygenation of Red Wine: Techniques, Applications, and Outcomes. Critical Reviews in Food Science and Nutrition. 51: 115–131.

Oxidation Reactions

The first reaction (A) is that of molecular oxygen being reduced to its major radical, hydrogen peroxide (H₂O₂), under the catalytic effects of iron (Fe²⁺) and copper (Cu⁺) ions. Hydrogen peroxide is a very strong oxidizer; it can easily oxidize ethanol into acetaldehyde (CH₃CHO) (reaction E) under the catalytic effects of iron ions. Water (H₂O) is a byproduct of this reaction.

There are already small amounts of acetaldehyde in wine, the result of yeast fermentation, but which have no olfactory impacts. But when acetaldehyde from ethanol oxidation becomes excessive, the wine takes on a distinctive bruised-apple smell or nut-like aromas reminiscent of Sherry-style wines; this is considered a flaw. As long as there is oxygen available, hydrogen peroxide will be produced and ethanol converted into acetaldehyde. Acetaldehyde can go on to polymerize with flavonoids and anthocyanins (reaction F) and form pigmented polymers that will give color some orange/brown hues.

Reaction A is actually a coupled reaction: as oxygen is reduced into hydrogen peroxide, colorless polyphenols (those with an o-diphenol component if you want to know) are oxidized in their brown-colored quinone forms, in what is known as phenolic browning. These quinones can go on to polymerize with flavonoids and anthocyanins (reaction D) and form pigmented polymers, much like acetaldehyde. Quinones are also involved in what is termed thiol trapping by condensing (reacting) with those sulfur-containing aroma compounds and negatively impact the aroma profile.

The Role of SO₂

At wine pH, SO₂ exists primarily (94–99 %) in its bisulfite form (HSO₃^–) and a much smaller amount in its molecular form. These two forms are what constitute free SO₂ (sulfite ions, HO₃^2–, which theoretically are part of free SO₂, are essentially non-existent at normal wine pH). Bisulfite ions are those involved in protecting wine from the oxidation reactions described above.

When potassium metabisulfite (simply referred to as sulfite) is added to wine, it ionizes into its bisulfite and molecular forms, and bisulfite ions get down to work and accomplish several things, the order of which is random although some reactions are much favored over other types of reactions. This randomness depends on pH, temperature, and other wine chemistry factors.

In one reaction (H), bisulfite ions intercept those hydrogen peroxide molecules and prevent ethanol oxidation; it can happen very quickly, hence the importance to maintain proper SO₂ levels in wine. This is a well-known reaction; winemakers are often told to use hydrogen peroxide to reduce excessive sulfite additions. As long as there is free SO₂ protecting the wine, hydrogen peroxide is intercepted and “neutralized.” The reaction products are water and sulfate ions (SO₄^2–), i.e., ionized sulfuric acid.

It can be seen from reactions A and H that, as is often incorrectly stated, SO₂ does not react directly with molecular oxygen.

Bisulfite ions are also able to reverse the polyphenol-to-quinone reaction (A), in what is known as diphenol regeneration (reaction B), therefore reversing the browning effect. Sulfate ions are byproducts of this reaction. Bisulfite ions can also bind to those brown-colored quinones (reaction C) to form bisulfite addition products, a reaction that increases bound SO₂ and reduces free SO₂ and therefore protection against oxidation. Bound SO₂ has no protective properties.

And bisulfite ions can also intercept and bind to acetaldehyde (reaction G) to form non-volatile bisulfite addition products; that’s why sulfite is added as soon as acetaldehyde is detected in wine. This reaction is reversible and will therefore exist and continuously re-establish itself to an equilibrium state.

These reactions demonstrate the need for proper SO₂ management to ensure graceful aging.