As spirits age in barrels, the porosity of the wood results in a certain amount of evaporation. In a standard distillery barrel (i.e. 53 gallons) about 10% of the remaining spirit in the barrels evaporates each year.
Depending on the temperature and humidity conditions the evaporation will result in an increase or decrease in the percent alcohol. In low humidity conditions, the loss to evaporation may be primarily water. However, in higher humidities, more alcohol than water will evaporate, therefore reducing the alcoholic strength of the product.
New, unaged, whiskey is totally clear, and it cannot be called bourbon until it has aged for at least two years. In its new state, it is called ‘white dog’ and its aroma is often described as ‘green,’ ‘sharp’ and ‘grainy.’ Straight from the still, whiskey at this stage has yet to be influenced by its charred white oak barrel.
7 Year Old Bourbon
After seven years, whiskey turns the deep, ruddy caramel of classic bourbon, and its aroma holds oak and vanilla undertones. It has soaked into the barrel to a depth of ¼” to ½” and about 32% of it has evaporated. There’s less in the barrel, but what remains has a more distinctive, elegant flavor.
12 Year Old Bourbon
After 12 years, bourbon has been absorbed into the wood even more deeply, with roughly 47% lost through evaporation. We call this loss the “angels’ share” since those vapors float into the heavens. Flavors and aromas are deep and refined, with a rich mouth-feel that spells satisfaction to the connoisseur.