The Chemistry Of Using Wooden Oak Barrels To Age Spirits

The Chemistry Of Using Wooden Oak Barrels To Age SpiritsThe constitutional changes which occur to the spirit during the wood aging process are the result of three specific reactions which occur on a continuous and simultaneous basis within the wooden oak barrels used to contain the aging spirit.

1. The liquid extracts complex wood components from the oak barrels;
2. Oxidation of extracted wood components and constituents originally present in the liquid;, and
3. Cross-reactions between various organic substances already present in the liquid, which leads to the formation of new and additional congeners.

As in the case of wine, the particular type of wooden oak barrels used is extremely important, and most major brand-name spirits have very rigorous requirements. Bourbon whiskey regulations require the use of new oak barrels with inner surfaces that have been charred. This charring is accomplished by igniting the inside of the oak barrels until a layer of char develops. Most other whiskeys and non-whiskey spirits are not required to be used in new, unused oak barrels; nor do the oak barrels have to be pre-charred. The charring process does enhance and soften the final taste of the spirit, while imparting it with both color and body, however.

Spirit manufacturers determine the specific type of spirit to make, and a close relationship between aging and distilling strategies does exist. Any spirit that is distilled over 95 percent abv is legally considered to be a “neutral” spirit.

  • – Bourbon may not be distilled more than 80 percent abv
  • – Cognac cannot be distilled over 72 percent abv
  • – Rum must be distilled between 85 and 95 percent abv
  • – Scotch malt whiskey must be distilled between 70 and 71 percent abv
  • – Canadian whiskey must be distilled between 70 and 90 percent abv

The secondary products generated during the fermentation process of alcohol (called congeners), are made of esters, acids, aldehydes, fusel oils, mineral salt extracts, and minute traces of solid materials. Collectively, they do not add up to a very high percentage of the finished product, but they do factor into determining the character of the final product produced.

Esters are volatile elements that lend aroma to spirits. Aldehydes are produced by reactions between the air and alcohol and contribute to the spirit’s overall character.
Fusel oils are higher-level alcohols and yield complex mixtures. Not all of these are desirable compounds, and even those which are must be present only in specific amounts.


Copper stills are important to the process because the contact between the copper and liquid entraps sulphuric elements and fatty acids. Cognac regulations allow only French oak barrels. These oak barrels are not charred. Newly-distilled cognac is place inside of new wooden oak barrels for 16 to 18 months. Woden Oak barrels are considered to be “new” for the first three uses. The first of these initial three uses will last for just a few months, as the wood will convey excess amounts of tannin to the spirit. The second use may last for up to two years, and the third may last even longer. The oak barrels are kept full, unlike with malt scotch whiskey and bourbon. Blending is an essential factor. Both sugar syrup and caramel are additives that are both legal and highly-regulated. Using oak chips soaked in old cognac that has been left in casks for months or years is a common but unregulated practice. Purists consider the character from this as tannic and rather hard.

Bourbon whiskey

Charring of oak barrels used for aging is believed to impart bourbon’s distinctive caramel and vanilla flavors. Four separate grades of char exist, and every distiller has its own particular preference. Charring results in a layer of partially carmelized sugars forming just beneath the char. Charring of the oak barrels also imparts color to the spirit. Whiskey stored in new, charred white oak barrels takes on a distinctive amber-red coloring. No additives are permitted. The used oak barrels are sold to Scottish and Irish distilleries.

Scotch Malt Whisky and Irish Whisky

The majority of malt whisky is aged in bourbon barrels; only 4 percent is matured in sherry barrels. A primary distinction between Spanish oak barrels and American oak barrels is that the American variety is much harder. Therefore, the whisky matures more slowly. Spanish oak barrels contain resin that directly impacts the flavor. Bourbon and sherry both leach out the stronger oak extracts and tannins, thereby resulting in a lighter product.

1 Comment

  1. Chemistry World Blog » That’s the Spirit on July 19, 2016 at 6:28 am

    […] and other solids – a playground of chemical complexity. The esters are highly volatile and contribute much of the aroma. Acids often contribute the bulk of congeners, but are not necessarily the dominant flavours: we […]