Oxygen and Wine: a complex relationship

Wine, a cherished beverage across cultures and time, undergoes a multitude of processes before reaching our glasses. One such process, oxidation, has been both a foe and friend to winemakers. Though typically considered harmful, under certain conditions, oxidation can prove beneficial, imparting unique characteristics to the wine.

The Two Primary Mechanisms of Wine Oxidation

1. Biochemical or Enzymatic Oxidation: This involves enzymes that act as catalysts, accelerating the oxidation process. Predominantly occurring in the prefermentative stage, which is either in the must (juice) or when the grape reaches the winery, this type of oxidation can also take place in newly made wines.
2. Chemical or Non-enzymatic Oxidation: Unlike its enzymatic counterpart, this form of oxidation happens without catalysts. Characteristically slower, this oxidation typically appears in matured wines and is most evident during the aging process.

When oxygen contacts and dissolves in wine, it activates reactions with substances present, notably with polyphenols. However, other substances, such as ascorbic acid, metals like iron and copper, sulfurous anhydride, ethyl alcohol, or tartaric acid, also play a role.

Winemakers strive to prevent harmful oxidation. Yet, during barrel aging, a slow oxidation can occur. Under specific conditions, this oxidation is deemed beneficial. It brings about physicochemical transformations in the wine, particularly influencing the polymerization phenomena between anthocyanins and tannins. This imparts stability and enhances the wine's sensory attributes.

Factors Influencing Wine Oxidation

The oxidation potential of a wine, expressed as "mV" (millivolts), determines the amount of dissolved oxygen in the wine. Typical values range between 200 to 300 mV, although these can differ based on the wine type, handling in the winery, and storage conditions.

• Red versus White Wines: Red wines utilize oxygen faster than white wines due to their richer content of oxidizable or metabolizing substances, mainly phenolic compounds. In simpler terms, white wines retain oxygen longer, leading to a higher oxidation potential, which is why they oxidize faster than red wines.
• Alcohol and Acidity: Contrary to popular belief, the wine's alcohol content doesn't significantly impact its oxidation potential. However, higher acidity decreases this potential. Alcohol does serve as a protective layer, preventing oxygen from entering, especially at higher concentrations.
• Wine Color: Darker wines with a high concentration of phenolic compounds, particularly anthocyanins, reduce the oxidation potential.
• Winery Operations: Activities that cause wine movement in the presence of air lead to increased dissolved oxygen. This raises the oxidation potential, consequently promoting wine oxidation. Specifically, operations like filling tanks from the top, using non-hermetic pumps, and bottling without vacuum or inert gas greatly elevate the risk of wine oxidation.
• Storage Conditions: Storage conditions, like ambient humidity and temperature, significantly influence a wine's oxidation potential, with lower values being ideal.
• Storage Containers: The type of vessel housing the wine plays a crucial role in its oxidation. For instance, a sealed steel tank minimizes air entry, while an aging barrel permits more air contact.

In essence, understanding the nuances of wine oxidation is crucial for both winemakers and connoisseurs. It not only impacts the wine's flavor profile but also its longevity and overall quality.