oxidation is one of the three main processes that are responsible for the change in spirits as they are matured, along with extraction—where reactive compounds are leached out of the wood from which barrels are made—and evaporation, where volatile compounds burn off and less volatile flavor compounds are concentrated over time. See maturation. Technically, oxidation is narrowly defined as the loss of an electron from one molecule to another; it is inseparable from reduction, which is the gain of that electron; together, the process is known as a “redox” reaction. With spirits, as with food and drink, the redox reaction is the chemical basis for the transformations that occur when they are exposed to oxygen. These affect spirits just as they affect a piece of cheese or head of lettuce carelessly left out on the counter. Colors will darken, aromas will fade; flavors will change. In general, with foods and fermented beverages, these changes are negative. If managed carefully, however, oxidation can be used to remove harsh notes and create pleasant ones in distilled spirits.
Traditionally, distillers have managed this contact by storing their spirit in a watertight but slightly air-permeable container, whether it is the charred bourbon barrel, the ancient Armagnac barrel lying in a humid cellar, or the large, unglazed earthenware jars used to age pisco in Peru and baijiu in China. This process allows only the most volatile compounds to evaporate or to bond with the limited oxygen allowed into the container, forming new flavor compounds. Thus bright, fruity notes yield to ones characteristic of cooked or even dried fruit and nuts. In a wine, this would kill the freshness of the aroma; in a spirit, however, freshness can often translate into rawness. Hotter temperatures will hasten those changes; putting a beverage inside a barrel and sticking it in a hot Kentucky warehouse will really complicate things.
Understanding oxygen’s influence is still a work in progress, and while far more is understood today than even a few decades ago, traditional distillers did much right by instinct. Wood compounds are transformed by toasting and burning; the presence of these compounds such as lignins, furanes, phenols, and lactones offer material that in the presence of oxygen can combine with volatile compounds from the distillate to create completely new compounds. Particularly complex are the aldehyde phenols, which can offer a wide variety of flavors and aromas. We know they form in the presence of oxygen inside the barrel, but we’re still not sure precisely how it all is happening. It is safe to say, however, that through oxidation flavor compounds are created that are found neither in the wood nor in the spirit as it goes into the barrel, and oxidation can hence be seen as the process that turns the aged spirit into something greater than the sum of its parts.
Likewise, there is not total agreement on whether or not spirit can oxidize once it has been bottled, though traditional wisdom says it will not in a well-sealed bottle. It is generally agreed that open or partial containers will allow the escape of ethanol and particular aroma and flavor compounds through evaporation (once sufficient ethanol has evaporated, what is left in the bottle loses its antibacterial properties and becomes vulnerable to decay). But some spirits, particularly agave-based beverages, sometimes exhibit aromatic changes that cannot be sufficiently described by the loss of character through evaporation. More study will be required. Nonetheless, we are certain that oxygen participates in changes that offer hundreds or more aromas, flavors, and textures in spirits and alcohol beverages.
Buxton, Ian, and Paul S. Hughes. The Science and Commerce of Whisky. Cambridge: RSC, 2015.
Waterhouse, Andrew Leo, Gavin L. Sacks, and David W. Jeffrey. Understanding Wine Chemistry. Chichester, UK: Wiley, 2016
By: Doug Frost