(n.) Wooden barrels, or casks, have been used for millennia to transport liquids throughout Asia and Europe, falling out of favor only relatively recently with the advent of modern packing materials. But casks continue to play an integral role in the aging of wine, beer and brown spirits such as whiskeys and cognacs, as time spent in a wooden cask can add distinct color and flavor.
Usually made from oak, new barrels are charred to different degrees, which can impart flavors of vanilla, caramel and smoke to the liquid. The wood will also impart some tannins to the spirit, which can add astringency and texture. Factors affecting the transfer of flavors include the percentage of alcohol of the spirit, the temperature and humidity of the storage room, the amount of char on the barrel and the type of wood. Barrel size can also play an important role: the larger barrel, the less flavor imparted to the spirit. Old barrels, usually used for three or more years, transfer less flavor, sometimes to the point of acting almost neutral. Some spirit makers search out gently-used barrels to achieve a flavor profile, such as Scotch producers who age their product in sherry barrels for a nuttier finish. Casks, especially new models, tend to be quite expensive, so some producers skirt the process by adding oak chips or caramel coloring to the liquid in lieu of properly aging, both of which create an inferior product.
Regional types of wood (such as American or French oak) can affect the flavor of the liquid stored in casks, and some producers are experimenting with other varieties like maple wood or wood that has already aged wine or fortified wine like port.