Using proof as a measure of ethanol alcohol content is a predominantly historic practice presently replaced by determinations of alcohol content as a percentage of alcohol by volume, commonly known as ABV. This statement of content is regulated by law in most countries. In the United States, requirements from the Alcohol and Tobacco Tax and Trade bureau demand a consumer always sees the alcohol percentage, with proof permitted but not required. Some drink menus and bottle labels still provide the alcohol content as proof (rarely seen on wine and beer bottles if ever) and on bottle labels in the United States, that number must be close to the required ABV. A quick way of calculating the proof is to double the ABV. For example, a liquor with 50% alcohol content is a 100 proof liquor. Regulations in the United States allow an actual alcohol content variation of 0.15% of the bottle’s stated ABV.
Proof’s use dates back to 16th century England, with spirits taxed at rates determined by their alcohol content. The testing of spirits by soaking a pellet of gunpowder in them determined ratings above or below proof, with spirits above proof taxed at a higher rate. In practice, if the wet gunpowder ignited, the liquor was rated above proof and if it did not, the liquor was rated below proof. Gunpowder does not burn soaked in a liquor containing less than 57.15% ABV, so liquors passing the test were given, at the time, an arbitrary designation of 100 degrees proof. Slowly, England began to use specific gravity as a criterion for measuring the dual concepts of alcohol content and proof. With specific gravity’s high sensitivity to temperature, problems with standardization arose. Finally, in 1816 the standard moved to a more precise definition of 12/13th the specific gravity of pure distilled water at an equal temperature. Of note, many sources claim the gunpowder test dates only back to the 18th century, however, this claim remains uncited and its historical basis remains tenuous. The United Kingdom and the European Union presently use a system close to the American’s ABV definition, one important distinction is the EU provides standards for both mass and volume and remains silent on a preference of one versus the other. The historical evolution in the United States is much less complex, with the proof system established around 1848 and based on the percent of alcohol by volume, not specific gravity.
A couple of loose ends to wrap up:
- A common mistake in writing is stating “80% Proof,” what the misguided author likely means is 80 Proof or 40% ABV.
- A common question for US readers is “why is the bottle always 40% ABV or 80 Proof?” This is actually a great question and the answer is a combination of law and economics. First, the questioner is likely referring to “spirits” like gin, vodka, rum, and tequila. The answer is simply that US law requires these products to have a minimum ABV. Since a higher ABV product is more expensive in the market, manufacturers generally dilute and bottle at the legal minimum. The actual law can be found here.
- For the rest of the world, legal requirements differ. For example, the UK and the EU require 37.5% ABV while South Africa requires 43.5% ABV. As a result, bottles of mainstream brands found across the world differ. For labeling information around the world check out: The International Alliance for Responsible Drinking
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