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The role of oxygen in winemaking is complex and multifaceted. In one regard, oxygen is vital in building a viable population of yeast cells needed to start fermentation as well as assisting in the polymerization of pigmented tannins and other phenolic compounds in the wine which contribute to its complexity and aging potential. Additionally oxygen assist in the volatilisation of esters and aroma compounds in the wine that contribute to its bouquet. Too little exposure to oxygen can lead to several reduction wine faults such as the rotten egg smell that is associated with the reduction of sulfur dioxide into hydrogen sulfide. It is for reasons like this that wines are often aged in oak barrels which allow limited amounts of oxygen and why some wine drinkers will swirl the wine in their glass or decant it so that some oxygen will aerate the wine.

But oxygen, by its nature, is a destructive force which leads wine on a path towards oxidation and eventually to becoming vinegar. From a visual level this path can be observed in the browning of the wine's color with both red and white wine eventually degrading in color to the same shade of brownish-orange. Within the grape must are several oxidase enzymes that destroy many of the compounds that contribute to fruity aromas and flavors in the wine. Exposure to oxidation can lead to several wine faults such as excessive acetaldehyde production which gives the wine aromas of dried fruits and leftover nuts. While in some wines, such as Sherry, these aromas are desirable, they can overwhelm the natural fruity aroma found in other wines such as Riesling and Sauvignon blanc.

White wine, in particular, is very susceptible to the negative impacts of oxidation due to the low amounts of phenolic compounds that red wines receive from the skins of the grape during maceration. These compounds offer some anti-oxidant benefits by utilizing the oxygen molecules in polymerization reaction. In their absence, winemakers have developed several techniques of anaerobic winemaking that severely limits the wine's exposure to oxygen. Among these techniques are enclosed wine presses and stainless steel fermentation tanks that have been pre-flushed with gases such as carbon dioxide, nitrogen and argon to remove the oxygen before it comes into contact with the wine. Anti-oxidants such as sulphur dioxde (usually in the form of potassium metabisulfite) and ascorbic acid are added to the wine to bind to oxygen molecules and keep them from attacking the fruit flavor and aroma compounds. Many of these techniques were developed in New World wine regions during the 20th century and while they have helped to create wines with less faults, they have also lead to criticism of producing "bland wines" that are "technically perfect" but without some of the complexity that some exposure to oxygen can bring. This has led winemakers throughout the globe to look for a balance between completely anaerobic winemaking and limited exposure to oxygen.

How oxygen gets into wine

Positive influence of oxygen

Negative influences of oxygen

Anaerobic winemaking