If you've ever looked at the label on a liquor bottle or heard connoisseurs discussing their favorite drinks, you may have encountered the term "proof." In the United States today, proof is calculated simply by doubling a bottle's percentage of alcohol by volume (abbreviated ABV). Thus, vodka that contained 35 percent alcohol would be 70 proof. Even though 70 proof vodka has slightly less alcohol than standard vodka, there's usually a tasty addition thrown into the mix.
The Scoop on 70 Proof
Seventy proof vodka is slightly lower in alcohol content than most vodka, which is typically 40 percent ABV, or 80 proof. This is the lowest level of alcohol content allowed by law for vodka in the U.S. Vodka below 80 proof is usually flavored in some way, diluting it to a lower strength; the regulations governing vodka's alcohol content do not apply to flavored vodkas. As a result, 70 proof vodka, unlike its 80 proof counterpart, can sometimes freeze if stored in a freezer.
During the 18th century, British sailors were served rum as part of their daily rations. They were concerned that military suppliers might try to cheat them by serving watered-down rum. In order to "prove" the rum, or demonstrate that it was labeled with the correct alcohol content, officials would douse gunpowder in it and then attempt to set it alight. If the powder would not burn, there was too much water in the rum. The level at which the rum would burn was said to be "proof" -- that is, proof that the rum was not diluted.
The level of this early "proof" was 57.1 percent alcohol by volume. Other drinks were measured by their relationship to this figure. For instance, a drink that was 48 percent alcohol would be 84 percent of the proof level. This level was known as "84 degrees proof" or simply "84 proof." The measurement of alcohol content was done with a measuring instrument called a hydrometer, but the earlier "proof" standard remained customary. The U.S. proof system simplifies the traditional measurement, setting 100 proof at 50 and making it much easier to calculate proof from ABV. Britain remained on the older, more complicated proof system until 1980.
The Last Relic of Tradition
The use of the proof number is part of a tradition dating back to the British Navy in the 18th and 19th centuries. Many vodka distillers still label their bottles with the proof of their products. In some cases, this can even be the name of the product. Absolut's "Absolut 100" vodka is 100 proof, or 50 percent ABV, while Bacardi 151 rum is named for the fact that it is 151 proof, or 75.5 percent ABV. However, proof labeling is not a legal requirement in the U.S. Like most countries, the United States regulates alcohol by ABVor ABW (alcohol by weight), with ABW being more common for beer.
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