It’s no secret that the wood casks or barrels in which whisky ages contribute color, aroma, flavor, and complexity, but it takes practice to sort the oak notes from more obvious cask influences such as spices, caramel, and vanilla. Know the wood, know the whisky; look for key information on the label to identify subtleties in flavor such as the oak species, coopering methods, filling history, and cask size (barrel being a specific size, different from a hogshead, butt, and others).

Bourbon is matured in barrels of new charred American oak (Quercus alba) and the cooper plays an important role in unlocking the flavors of the barrel by choosing air over kiln drying to season the wood, and applying a light or heavy toast with heat. The toasty oak character in whisky comes from the caramelized sugars in the uppermost layers of the oak, though toasting releases a complex array of other flavorsome compounds, including vanillin and oak lactones during the breakdown of lignin, cellulose, hemicellulose, and tannins. The barrels must also be burned to produce the desired char level.

Why and How Oak Matters in Whisky

After bourbon barrels are emptied, they are deployed as first-fill bourbon casks in other whisky nations. Each subsequent filling depletes the ability of the wood to contribute flavor, much like trying to make a second, third, or fourth cup of tea from the same teabag. Tired casks can be rejuvenated to prolong their useful life by scraping down to a fresh layer and preparing the new surface; this works, but never replicates the original qualities of the first-fill bourbon cask.

Sherry casks are also repurposed for aging whisky and both European oak (Quercus robur) and American oak are used. Beyond sherry, casks that have previously contained wine, rum, or port may be used, although these are generally applied to add layers of additional flavor through a stage called finishing, when whisky is re-casked into a secondary vessel for a final period of maturation.

As you swirl your glass, search for aromas of little mounds of pencil shavings, the heady scent of fresh-sawn oak, Spanish cedar cigar boxes, Creamsicle sticks, varnished speedboats, polished walnut antiques, or the old seasoned lumber on a boardwalk.

Describing something as simply woody or oaky can be a pejorative term, meted out to very old bourbons and whiskies past their prime left stewing too long in cask. Too much oak can deliver a hot, spicy mouthful of drying tannins, chewy leathery notes, and earthy mustiness. The oak may be mighty, but we much prefer a more subtle oak influence in our drams.