In theory, barrels can be made of nearly any kind of wood, and on occasion, one will discover one that has been coopered oddly, but there is a good reason that oak barrels for aging whiskey have been the choice in production for many years. In whiskey production, American oak barrels for aging are the most common choice. Although wine producers prefer barrels made of French or Hungarian origin, their flavor and effects are different. This may lead a connoisseur to wonder why oak barrels for aging and specifically why American oak.
American oak, Quercus alba, grows well in the Eastern USA. It is found in the US from the Canadian border to Northern Florida and as far west as the state of Missouri. Since Kentucky and Tennessee are some of the primary states for distilleries and lie in the center of the growing region, these native trees were the natural choice for transformation into barrels by coopers.
However, the story goes beyond just convenience. Oak and particularly American oak, maintains the qualities both coopers and distillers find desirable. The wood is durable and strong, but with the right tools can be shaped easily. Unlike other wood, oak is low in rein that could cause a negative flavor of or aroma. No one wants their whiskey to taste or smell like pine sol.
So, oak is a wood that is easily shaped into barrels and does not impart a bad smell. That makes it great for collecting rainwater, but for booze, oak still does not create the interest distillers want to impart in their product. This brings the cooper to the final important consideration for great oak barrels for aging: fire. For the American oak barrels for aging to impart the characteristic flavor to whiskey, the inside must be toasted or charred.
Charring is the process in which the cooper burns the inside of the oak barrel for aging using a direct fire. The longer the flame remains in contact with the wood, the greater the amount of charcoal that is formed. No one is sure how distillers began charring the barrels and there are numerous legends around. One favorite that you may want to share at your next whiskey tasting goes as follows:
A backwoods bootlegger had been working all day long to create his special recipe for whiskey and was about to fill barrels with whiskey, when he realized the last use of the barrels had been to carry fish to market. In order to eliminate the fishy smell and taste from his oak barrels for aging, he flamed the inside of each to remove the fishy remains. Months later, as the spirits were decanting the finished product, he took note of the change in both color and flavor. Soon the entire whiskey industry followed suit and all drank happily afterwards. And that friends, brings the end to a fishy story about oak barrels for aging your favorite whiskey.
No matter what the origin, the charring of oak barrels for aging offers a distinct chemical effect on the flavor of whiskey as it matures. Fire breaks down compounds known as hemicelluloses into simple sugars and caramelizes them to add sweetness in aroma and flavor. The caramelized sugars also add color to the clear liquid that is emitted from the still.
Oak barrels for aging also contain lignins that are changed as the wood is charred. When these compounds are heated, they form vanillins, that smell and taste like vanilla. Additional heating of the oak lignin releases the phenols that give a smoky, medicinal or burnt smell.
Charring of American oak barrels for aging also increases the lactones in the barrel. While naturally present in any oak, the lactones in America oak are more abundant. These compounds provide a coconut and woody flavor to the spirits aged inside the barrel and are prominent in many malt whiskeys or bourbons.
Finally, charring oak barrels for aging breaks down some of the tannins in the wood. These tannins normally would add a bitter or astringent flavor to the whiskey aged in oak. However, by charring the effects of the oak are more palatable for the average individual Tannins help to oxidize the spirits and provide a more mature and delicate aroma. However, the process takes time to achieve and whiskey that has been stored in oak barrels for aging properly is worth the wait.
American oak barrels for aging have many effects on whiskey that are not to this day completely understood and far more complex than can be addressed here. Variables such as the humidity and temperature of the barrelhouse, the thickness of staves used to create oak barrels for aging, the geometry and size of the barrel and even whether the barrel is standing up straight or lying on its side all have great effects on the finished whiskey in the bottle.