For barrel-aged spirits, the complex web of interactions that occur as the spirit rests in wood has one of the biggest—if not the single biggest—impact on the final character of the product.
The facets of barrel aging that help shape a spirit include evaporation—and with it, concentration—as well as extraction and transformation.
Here, we discuss aging and maturation largely in the context of new, charred white American oak (Quercus alba), the most common wood used for barrels in the United States, though there is considerable overlap with other types. Eventually, casks become fairly well-extracted and essentially become useful as neutral storage vessels—or for shipping salt cod across the Atlantic Ocean.
Aging vs. Maturation
When it comes to terminology, aging and maturation are sometimes used interchangeably. However, they’re not equivalent concepts, and the distiller needs to understand the difference.
Aging provides a simple, straight-line measurement of how much time a whiskey, rum, or other spirit has spent in a barrel. Maturation, by comparison, implies progress toward a target.
The latter is a subjective, relative concept: Is the spirit mature, based on our standard profile for a certain product? Does it check the particular boxes we use to define a mature spirit at three, five, 10, or more years?
For most spirits, age can be an indicator of maturity, but it is not the last word. Given the number of variables, a spirit that is fermented cleanly, distilled with tighter cuts, and aged in a warmer environment for three years can easily taste more mature than a six-year-old spirit with a funky fermentation and wider cuts, aged in a cool rickhouse.
Evaporation
In some cases, spirits improve during aging because of the losses that occur—addition by subtraction.
The porosity of oak allows for evaporative action that creates the angel’s share, preferentially encouraging volatile heads compounds to escape as the barrel rests. While the primary angel’s share loss is in ethanol and water—because of their overwhelming concentration in the aging spirit—the evaporation of minute amounts of undesirable volatiles also leads to an overall improvement in quality.
Operating in parallel with the positive impacts of evaporation is the concentration of components that remain behind. Because most of the congener load in a spirit is heavier than ethanol and water, these compounds increase in relative concentration as water and ethanol escape the barrel, creating a richer, more intense product.
While distillers may lament the angel’s share as an unfortunate loss, it is in fact a valuable process that helps to improve the spirit.
The Wood’s Impact
Another avenue for undesirable compounds to be removed is via the char layer.
The creation of a char layer has a complex impact on barrel flavor chemistry. Charring destroys some flavor-active compounds in the wood itself, but it also creates a layer of activated carbon that binds up undesirable components—particularly sulfur compounds—effectively helping to clean up the spirit.
While the char itself doesn’t lend flavor to the spirit, the charring process does produce additional toasted layers to the wood—between the char itself and the untouched wood, farther from the heat—increasing or modifying flavor compounds.
Present in the oak itself are a large number of volatile, flavor-active oak compounds that can impact the flavor, aroma, and texture of a barrel-aged spirit—different sources put the number in the range of about 75 to 100. Extraction occurs when such compounds infuse into the spirit, changing its character.
Oak lactones come from lipids in the oak, especially when it’s toasted or charred. These compounds lend oaky and coconut notes to a spirit.
One of the first groups of compounds to dissolve into a barrel-aged spirit is tannins, a group of phenolic compounds. Particularly with new oak, the level of tannins and color compounds will rapidly increase in the first six to 12 months of aging, slowing down afterward. (That, paired with the elevated relative surface area of the barrel, is why whiskey aged in small barrels, such as five and 10 gallons, can quickly become unbalanced and over-oaky.) Older, well-extracted barrels can take much more time to provide much noticeable tannin and oak character.
Tannins can offer a lot of flavor upside as the spirit ages and other components come into balance, but they initially create a very dry, astringent profile, leading to an unbalanced, overly oaky spirit. Tannins are polyphenols that bind to the proteins in saliva, creating the drying and even coarse textural experience in the mouth when consumed. The oak staves that are used for barrels are generally seasoned for years before assembly to reduce the level of tannins and begin some breakdown of other oak compounds.
More Complex Reactions
A variety of other major flavor and aroma compounds result from oak extraction or from reactions between oak compounds and the resident spirit, often facilitated by the micro-oxidation allowed by oak storage.
The primary structural components of oak are lignin, cellulose, and hemicellulose. These are fairly stable compounds—ethanol doesn’t dissolve barrels, even after decades of storage—but they’re also not completely inert.
Cellulose and hemicellulose are both built of sugars—glucose, in the case of cellulose, and a variety of sugars in the case of hemicellulose. While cellulose is a highly stable polymer, generally not considered to lend flavor and aroma compounds to a spirit, hemicellulose degrades during charring, creating caramelized sugars that can dissolve into the spirit.
Lignin is another key source of soluble compounds that can impact a whiskey or other barrel-aged spirit. By-products of lignin degradation and transformation in the spirit include vanillin (vanilla character), eugenol (clove), guaiacol (smoky, woody, phenolic), furfural (almond), and coniferaldehyde (bready).
Lignins also take part in a complex series of reactions whereby they break down into aldehydes. These aldehydes slowly oxidize into acids, which then react with alcohols to produce fruity, floral esters. (The oak itself also contributes acids, such as acetic and linolenic.)
Some of these reactions are balance-driven: As particular compounds rise to a certain threshold, the catalyzation of those compounds slows down until they’ve reacted further, changing into other compounds, and their content in the spirit has dropped. This is, in part, responsible for the inconsistent, sine-wave form in spirit quality during maturation. A barrel will often improve over time, then take a step backward in flavor and aroma as a bottleneck occurs in unpleasant compounds, then improve again and find a new plateau.
On an even longer timeline than regular esterification, there is transesterification. In this process, the esters that have already formed in the barrel—or previously, during fermentation or distillation—continue to evolve. During transesterification, existing esters combine with other alcohols to form different esters and add new flavors and aromas to the mix.
While barrel aging and barrel maturation are not synonymous, one is a valuable indicator of the other. The process of aging spirits in barrels is complex. It’s an art that can take a lifetime to master, and one not yet fully understood from a chemistry perspective.
However, distillers should have at least a general idea of where they’re going and how they’ll get there—what to expect from the twists and turns, the hills and valleys that stand between new-make and a beautiful, mature product.