The process of fermenting grape juice into wine or distilling corn mash into whiskey can be a complicated one. In the beginning, there is a fair amount of commotion as the ingredients for a mash are blended together or the juice is extracted from the grapes. At some point, however, the process enters a very slow and deliberate phase, as the newly fermented wine ages or the mash converts to alcohol. It's at this point a phenomenon called the Angel's share first comes into the picture.

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.

Over time, some of the alcohol seeps through the grain of the oak staves and evaporates into the open air. The evaporating alcohol is called the Angel's Share, alluding to the belief that guardian angels watch over the product as it ages. Barrels of wine or distilled spirits may remain stacked in large warehouses for years, each one rendering its own Angel's Share over time. Needless to say, these warehouses filled with oak barrels and aging spirits can be quite fragrant.

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 2% per barrel. This means that a distilled whiskey stored in an oak barrel for 7 years could be expected to lose 14% of its total alcohol content. Because 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.