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How does U.S. proof relate to alcohol percentage by volume?
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distilled spirit
The fermentation and distillation process for producing whiskey. The production of whiskey begins with grinding grain into a meal, which is cooked. Malt is introduced to the meal, which results in mash that is cooled and pumped into a fermenter, where yeast is added. The fermented mixture is heated in a still, where the heat vaporizes the alcohol. The alcohol vapours are caught, cooled, condensed, and drawn off as clean, new whiskey. This liquid is stored in a cistern room, and water is added to lower the proof (absolute alcohol content) before the whiskey is placed in new charred oak barrels for aging and later bottling.
In the United States, the relationship between proof and alcohol by volume (ABV) is straightforward: a liquor's proof is exactly two times its alcohol percentage by volume. For example, a beverage containing 50% alcohol by volume is 100 proof, while one with 30% ABV is 60 proof.123
This system differs from historical or international methods, such as those used in Great Britain or France, which utilize different standards for defining proof. In the U.S., the term "proof spirit" specifically refers to a liquor that is 100 proof, or 50% alcohol by volume.31