During the aging process, the whiskey stored in barrels undergoes evaporation, with a portion of the spirit lost to the atmosphere each year. This phenomenon, known as the “angels share of whiskey”, is a tribute to the celestial beings who watch over the aging process. In fact, it is not uncommon to see a statue of an angel in the distillery’s storage room during a whiskey tour.
As barrels are made of wood, It allows the whiskey to breath. It takes in oxygen from the atmosphere and ‘exhales’ moisture out. While this seems like a loss to the distillery, it’s actually a vital part of the aging process. It allows the character of the whiskey to change by interacting with the environment that the casks are stored in. Probably the most obvious form of this are those coastal whiskies that take on a marine or salty character. The Angels Share whiskey is directly attributable to a couple of factors, including:
1) The thickness and porosity of the wood used in the cask
2) The humidity level in the surrounding area.
Typically losses are ~2% but could be as high as 5-6% depending on the factors above. Master distillers are constantly trying to find the perfect environment to age their whiskey and to slow the aging process. Some have even tried to store their whiskey in the ocean, or abandoned mineshafts. Still others, try to find the environment that will impart the perfect finish to their whiskey.
While distillers lament the loss of precious whiskey, they also acknowledge the role of the angels in creating the unique flavors and aromas of aged whisky.