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During the process of making wine or whiskey, producers often use oak barrels to store their drink products; when alcohol is stored at 60 percent humidity or higher, for long periods of time, some of it will evaporate out of the barrel. The evaporating alcohol is called the angel's share, alluding to the belief that guardian angels watch over the product as it ages. The beverage's alcoholic level can be determined in part by how much alcohol evaporates out of the oak barrels. The angel's share is also the name of a popular bar in New York City and an ale made by a company called the Lost Abbey, based in California; both of these uses of the name are derived from the process of evaporating alcohol.
Many distilled spirits and wines are virtually undrinkable after their sugar content has fermented into alcohol. In the case of distilled spirits, the alcoholic content of a "fresh" batch may be nearly 190 proof or better, and the flavor of the beverage would not be very appealing. To reduce the percentage of alcohol and to intensify the subtle flavors of the wine or distilled spirit, producers often store their products in oak barrels. In the case of distilled spirits, such as whiskey, the staves of the oak barrels are often heavily charred before assembly to give the drink a distinct taste.
Over time, some of the alcohol seeps through the grain of the oak staves and evaporates into the open air, and the evaporating alcohol becomes the angel's share. Barrels of wine or distilled spirits may remain stacked in large warehouses for years, each one rendering its own angel's share over time. If the barrels are stored in a low-humidity area, more water will evaporate out of the barrel, giving the beverage higher alcoholic content; if the barrels are stored in higher-humidity areas, then more alcohol will evaporate out, leaving the beverage less alcoholic.
Although much depends on the materials and methods used for barrel-making, many wine and distilled spirit producers estimate that the angel's share of alcohol lost annually is around two percent per barrel. This means that a distilled whiskey stored in an oak barrel for seven years could be expected to lose 14 percent of its total alcohol content. As a result of the angel's share, a formerly undrinkable, pure-grain alcohol can mellow to a more palatable 86 proof over time. Wines aged in oak barrels may not require nearly as much storage time, but the Angel's Share phenomenon can reduce the amount of alcohol enough to allow the wine's more subtle flavors and textures to emerge.