Wine and oxidation are a notoriously incompatible combination.
Consider a familiar scenario: you open a bottle of wine that tastes excellent, but you cannot finish it in one sitting. You reseal the bottle and store it in the refrigerator, intending to drink it the next day. However, a busy work schedule or feeling unwell prevents you from returning to the wine, and before you realize it, a week has passed since opening. When you finally pour a glass, the wine tastes different.
The culprit responsible for altering the wine’s flavor is oxygen. As wine was poured from the bottle, air entered to replace the dispensed volume. The oxygen contained in that air promoted oxidation, thereby changing the wine’s character.
Oxygen-induced flavor changes do not occur exclusively in bottles at home. Oxidation can also occur during the winemaking process itself. Once a wine has oxidized, the change is irreversible. Consequently, winemakers exercise meticulous care to prevent unnecessary oxygen contact with wine at all stages of production.
This description might suggest that oxygen is universally despised in winemaking—a quintessential villain. However, the reality is more nuanced.
Certain winemaking techniques actively promote oxygen contact, which would otherwise be strictly avoided. This article focuses on one such technique: hyperoxidation.
Two Oxidation-Promoting Techniques in Winemaking
Two techniques in wine production bear the term “oxidation” in their names: microoxidation (also referred to as microoxygenation) and hyperoxidation (also referred to as hyperoxygenation).
Despite their similar nomenclature, microoxidation and hyperoxidation are entirely distinct techniques. Paradoxically, while they are fundamentally different, their procedures and objectives share considerable similarities. The key differences lie in the type of wine targeted and the timing of implementation. Microoxidation is primarily applied to red wines after the completion of alcoholic fermentation. In contrast, hyperoxidation is applied to white wines before alcoholic fermentation begins.
Both techniques employ oxygen to induce intentional oxidation, yet the wine types and specific timing of application are diametrically opposed.
Microoxidation: Accelerating Wine Maturation
Microoxidation is, as its name suggests, a winemaking technique that artificially induces micro-scale oxidation.
Wine maturation is primarily driven by oxidation. Maturation refers to the cumulative changes in a wine’s flavor, aroma, and appearance over time. These changes occur predominantly through oxidative processes.
During maturation in oak barrels or in bottles sealed with cork, the material properties and structural characteristics of wood allow a certain quantity of air to enter the vessel over the storage period. Even when highly hermetic closures such as screw caps are employed, air remains present in the headspace—the volume above the liquid surface within the bottle. Oxygen contained in the headspace and in air that permeates from outside reacts with the wine, causing oxidation. This oxidative change is recognized as wine maturation.
However, such oxidative changes progress extremely slowly. The time required can extend from several years to several decades. In some cases, this extended timeline is acceptable; in others, it is not. Microoxidation was developed for situations where waiting is impractical.
In simple terms, microoxidation is a technique that artificially delivers, over a short period, the quantity of oxygen that wine would naturally absorb over years of aging. The result is an acceleration of maturation.
The bitterness and astringency characteristic of wine, particularly red wine, originate from phenolic compounds known as tannins. When phenols encounter oxygen, they undergo oxidation, structural modification, and subsequent polymerization, ultimately precipitating out of the wine. As phenol concentration decreases, the wine’s astringency diminishes, and the wine becomes smoother and more approachable. In broad terms, this constitutes maturation.
The fundamental objective of microoxidation is therefore to modify the structure of phenolic compounds in wine and reduce their concentration.
Hyperoxidation: Decelerating Maturation
While microoxidation aims to promote wine maturation, hyperoxidation—despite employing similar procedures—pursues the opposite objective. The purpose of hyperoxidation is, in a sense, to prevent wine from maturing.
Wine maturation and oxidation are essentially the same phenomenon. “Maturation” is simply the term applied to a favorable stage within the continuum of oxidative change. Oxidation that progresses beyond this favorable stage is termed “deterioration.”
Because maturation and oxidation are fundamentally identical, preventing oxidation necessarily inhibits maturation. Hyperoxidation is a winemaking technique specifically designed to suppress oxidation in wine. Viewed from another perspective, hyperoxidation can be considered a technique for restraining wine maturation.
For this reason, hyperoxidation is primarily employed in the production of white wines and styles particularly sensitive to oxidation, such as Blanc de Noir, even when produced from red grape varieties.
Hyperoxidation: Oxidizing to Prevent Oxidation
The intriguing aspect of hyperoxidation is that, to prevent oxidation in wine, the juice is oxidized before it becomes wine.
Wine oxidizes because it contains oxidation-susceptible components. Conversely, if wine were devoid of such components, it would be resistant to oxidation.
The fundamental principle of hyperoxidation is to oxidize and remove oxidation-susceptible components before the juice becomes wine, thereby suppressing the subsequent impact of oxygen on the finished wine.
The primary target for removal is phenolic compounds.
Phenolic Compounds: Central Components in Wine Maturation
Wine contains numerous oxidation-susceptible components, but among these, phenolic compounds exert the strongest influence on wine flavor and color. Within the phenol family, flavonoids are particularly significant. Flavonoids not only cause wine to brown through oxidation but also contribute to bitterness, astringency, and the sensation of astringency in wine.
Phenolic compounds in wine polymerize through oxygen-mediated reactions, progressively increasing in molecular weight. When sufficiently polymerized compounds exceed a critical molecular weight, they become insoluble in wine and precipitate. When this chemical reaction occurs in wine, it affects the wine’s flavor, aroma, and visual appearance including color. Conversely, if such phenolic compounds are absent from wine initially, oxidation-related changes will not occur.
Hyperoxidation as an Enzymatic Reaction
Hyperoxidation is a technique that utilizes oxygen-mediated oxidation reactions. Oxidation reactions in wine can be classified into two categories: enzymatic reactions and non-enzymatic reactions. Hyperoxidation initiates with enzymatic reactions.
Two enzymes are utilized in hyperoxidation. The first is tyrosinase, which is naturally present in grapes. The second is laccase, which becomes present in grapes primarily through infection by Botrytis cinerea.
Both tyrosinase and laccase belong to a class of enzymes known as polyphenol oxidases (PPO), which specifically catalyze the oxidation of phenolic compounds. The fact that neither enzyme is artificially added explains why hyperoxidation is characterized as “wine stabilization through native enzymes.”
The Oxidation Mechanism of Hyperoxidation
With polyphenol oxidase (PPO) serving as a catalyst, polyphenols are oxidized to quinones. In this reaction, laccase exhibits greater reactivity than tyrosinase, which is one reason why hyperoxidation cannot be applied to grapes infected with Botrytis.
The most abundant phenolic compound in white grapes is hydroxycinnamic acid, with caftaric acid being its principal derivative. Tyrosinase, a type of PPO, has high affinity for caftaric acid and oxidizes it to quinone during the initial stages of the oxidation reaction.
The caftaric acid quinone generated at this stage becomes the starting point for subsequent non-enzymatic reactions.
Caftaric acid quinone produced by PPO first combines with glutathione (GSH) present in grape juice or wine to form a colorless compound known as GRP (Grape Reaction Product). Once glutathione in the juice or wine is depleted, excess caftaric acid quinone begins oxidizing other constituents, including GRP and flavonoids.
Concurrently, a portion of caftaric acid quinone undergoes reduction back to caftaric acid. The regenerated caftaric acid then reabsorbs oxygen and reacts with PPO to form caftaric acid quinone again, establishing a cyclic reaction. Furthermore, caftaric acid quinone can react directly with caftaric acid to regenerate phenolic compounds, which are then re-oxidized via PPO.
Through this series of reactions, substantial quantities of oxygen are absorbed and consumed by the wine, driving oxidation forward.
The browning of wine results from quinones formed when flavonoids are oxidized by caftaric acid quinone. Quinones derived from flavonoid oxidation rapidly initiate polymerization, precipitating as brown-colored compounds. These brown compounds are moderately soluble in alcoholic media but insoluble in water. This difference in solubility explains why juice treated with hyperoxidation develops a far more intense color compared to oxidized wine.
Juice subjected to hyperoxidation, despite originating from pressed Chardonnay, occasionally develops such an intense dark brown color that it is referred to as “Black Chardonnay.”
Oxygen Requirements for Hyperoxidation
The term “hyperoxidation” might suggest the introduction of enormous quantities of oxygen to oxidize the juice. In practice, however, introducing excessive oxygen causes over-oxidation, which degrades the quality of the resulting wine.
The oxygen quantity required for hyperoxidation is strictly determined by the concentration of phenolic compounds—the oxidation targets—in the juice. This concentration varies by grape variety. Approximately 9 mg/L of oxygen is reportedly sufficient to precipitate flavonoids at concentrations below 100 mg/L (expressed as catechin equivalents). Even for juice with elevated flavonoid concentrations, such as that produced with skin contact, empirical values indicate approximately 20–30 mg/L is adequate.
A certain level of oxygen uptake occurs during standard winemaking processes. Consequently, depending on the phenolic compound concentration in the juice, results comparable to hyperoxidation may be achieved without deliberate oxygen introduction. In most cases, the oxygen quantity required for hyperoxidation is considerably less than the name would suggest.
Effects of Hyperoxidation on Wine
The sensory impacts of hyperoxidation on wine are largely antithetical to those caused by the presence of phenolic compounds.
Phenols cause wine to brown. Wines treated with hyperoxidation exhibit reduced color change over time. Moreover, the dark brown color that develops in juice during hyperoxidation results from particles that have grown to a size where they impart intense color while dispersed in the juice. These pigments are removed through fining and fermentation processes; consequently, the finished wine is actually lighter in color than conventionally produced wine.
Tannins impart astringency, bitterness, and the tactile sensation of astringency to wine. In wines where tannins have been oxidized and precipitated at the juice stage, these taste characteristics and textural sensations are substantially diminished. The resulting wines are more approachable, without the gripping sensation on the palate, and certain aromatic compounds become more perceptible.
Additionally, certain flavonoids have been identified as precursors to compounds implicated in off-flavors in white wine. Logically, if these causative flavonoids are removed, the resulting off-flavor compounds will not be present in the wine.
Brighter color, elimination of challenging astringency and bitterness, enhanced aromatic clarity, and stability that persists over time—these outcomes appear entirely beneficial. However, the temporal changes that would normally occur in conventional wines are either eliminated or substantially reduced. In other words, wines treated with hyperoxidation become resistant to maturation.
Furthermore, the loss of body and structure previously contributed by tannins creates a risk that the wine may appear excessively delicate or, at times, thin.
Excessive application of hyperoxidation degrades the quality of the finished wine. Certain grape varieties have also been identified as more or less suited to this treatment. Once negative effects manifest in wine, recovery is virtually impossible. Consequently, the potential risks associated with this winemaking technique are relatively significant.
Because hyperoxidation employs oxidation—arguably wine’s greatest adversary—careful implementation with appropriate dosing is essential.
