Wood, Whiskey and Wine: The Oak Barrel Affair, an Unbeatable TechnologyMay 22, 2018
A Comprehensive Guide to Barrel-Aging Cocktails at HomeJuly 24, 2018
Barrels are used to store and age spirits to give them added flavor. Let’s take The Glenlivet’s 50-year-old single malt scotch whisky as an example. When the whisky was freshly distilled in 1964, the new spirit was perfectly clear in color and tasted cereal-rich and slightly smoky from the smoked malted barley from which it’s made. But, it was allowed to mellow out in a barrel for fifty years, and fifty years later, the liquid became a deep amber color having richer baked fruit notes, like toffee, honey, caramel, and chocolate; and there is a tannic dryness to the finish. Much of the initial alcohol evaporated over the half-century in the barrel, leaving a mere 100 bottles available for sale.
Whisky, and any other spirit, is influenced by the environment inside the barrel, compounds in the wood, especially where it has been charred on the interior, and even the environment where the barrel is stored. Over time, these factors make a significant impact on a whisky’s color, flavors, and mouthfeel. Let us look at various spirits and what barrel aging does to them.
Bourbon must by law be aged in previously unused, charred wood vessels, whereas Scotch whisky is typically aged in barrels that previously held bourbon or sherry. Bourbon is aged in American oak barrels but Scotch is sometimes aged in European oak.
Barrels are either toasted or charred before whisky is aged in them. This charring creates charcoal inside the barrel that filters the spirit and partially removes undesirable flavors, much like a water filter. Freshly-distilled whisky would change a bit over time without exposure to oak if it were stored in a neutral container like stainless steel, but it turns out the barrel is still important. The environment outside the barrel matters as well. During changes of temperature between hot and cold seasons the liquid expands into and contracts out of the wood, accelerating those wood interactions. The wood itself adds flavors to the whisky, in the form of three main compounds: Degraded lignin in the wood helps add vanillin to the spirit; Lactones give rise to a buttery, coconut flavor; Wood spice gives the astringent flavors of tannins. European oak is richer in tannins than American oak, so again tasting experts can often guess in which barrel a spirit was aged based on this mouthfeel.
Barrels are used to age wine to vary the color, flavor, tannin profile and texture of wine. Oak barrels can impart qualities to wine through evaporation and low level exposure to oxygen. The porous nature of an oak barrel allows evaporation and oxygenation to occur in wine but typically not at levels that would cause oxidation or spoilage. The typical 59-gallon barrel can lose anywhere from 5 1⁄2 to 6 1⁄2 gallons in a year through evaporation. This allows the wine to concentrate its flavor and aroma compounds. Small amounts of oxygen are allowed to pass through the barrel and act as a softening agent upon the wine’s tannins. The chemical properties of oak can have a profound effect on wine. Phenols within the wood interact to produce vanilla type flavors and can give the impression of tea notes or sweetness. The degree of toast on the barrel can also impart different properties affecting the tannin levels as well as the aggressive wood flavors. The tannins present in wood, known as ellagitannins, are derived from lignin structures in the wood. They help protect the wine from oxidation and reduction.
Now let’s talk about rum. This alcoholic beverage is made from sugarcane byproducts, such as molasses or honeys, or directly from sugarcane juice, by a process of fermentation and distillation. The distillate, a clear liquid, is then usually aged in oak barrels. The chemistry of aging isn’t fully understood, but it’s universally accepted that aging mellows and improves the taste of freshly distilled rum. While the rums are sleeping in oak barrels, natural tannins in the wood impart a golden tint that yields to a rich brown cast after several years. The alcohol in the rum acts as a solvent and attracts the tannins in the porous wood as well as esters which will give rum a slight vanilla flavor as well as a smoky oak tone depending on the age of the spirit. Most rum producers age their rum at 70% to 80% alcohol. A few dilute their spirits to nearly bottle-strength, 40 to 45% alcohol by volume, before putting the barrels away for aging. A lower alcohol content during aging tends to leech slightly lighter esters and phenols from the wooden barrels while a higher alcohol content will attract heavier compounds and associated flavors. Most distilleries age their rum at a higher strength as this requires fewer barrels, but a higher alcohol content also contributes to higher evaporation losses, known as the angel’s share. Before the aged spirits are bottled, pure water is blended into the rum to reduce the alcohol content. Diluting the aged rum, however, also dilutes the color of the matured spirit. In most cases the rum is not as pleasing to the eye as the distiller would like, so caramel is added to adjust the color. Because not all barrels tint their contents to the same cast, caramel is sometimes sparingly added so that all rum bottled under a particular label will be the same color. In other instances, generous amounts of caramel are used to make the spirit black, as in the case of Myers’s from Jamaica or Gosling’s from Bermuda. Just as some wines are fermented to be drunk within a few years of bottling, some rums are distilled to be consumed as soon as they are bottled. In the case of French rhum agricole, the raw spirits are allowed to rest for a few months in large vats while the light gases formed during fermentation are released.