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Alcoholic proof is a measure of how much ethanol is in an alcoholic beverage, and is approximately twice the percentage of alcohol by volume (ABV, the unit that is commonly used at present).

The European Union follows the recommendation of the International Organization of Legal Metrology (OIML) which measures percentage of alcohol by volume at 20 °C.

In Britain alcohol by volume is now used. This replaced the Sikes hydrometer system (based on proof spirit) which was used since 1816, although officially the Customs and Excise Act of 1952 defined "spirits of proof strength" (or proof spirits):

"Spirits shall be deemed to be at proof if the volume of the ethyl alcohol contained therein made up to the volume of the spirits with distilled water has a weight equal to that of twelve-thirteenths of a volume of distilled water equal to the volume of the spirits, the volume of each liquid being computed as at fifty-one degrees Fahrenheit."

Previously, Clarke's hydrometer had been used since the 1740s when Customs and Excise and London brewers and distillers began to use it.

In this system, spirit of 100 degrees proof contains 57.1% alcohol by volume or 49.28% alcohol by weight at 51 deg F (10.6 deg C) and is equivalent to 114.2 proof in the USA. Pure ethanol is 175 degrees proof. [1]

In the definition current in the United States of America, the proof number is twice the percentage of the alcohol content measured by volume at a temperature of 60 °F (15.5 °C). Therefore "80 proof" is 40% alcohol by volume (most of the other 60% is water), and pure alcohol would be "200 proof". If a 150-proof beverage is mixed half-and-half (by volume) with water, the product is close to 75 proof (not exactly because of a small volume change when alcohol is mixed with water). US proof numbers are properly cited as, for instance, "86 proof," not "86 degrees proof." The use of the word "degrees" in this context is incorrect.

US Federal regulation (CFR 27 5.37 Alcohol Content) requires that liquor labels state the percentage alcohol by volume (sometimes abbreviated ABV). The regulations permit (but do not require) a statement of the proof as long as it is right next to the percentage alcohol by volume. [2]

Alcohol is produced by yeast during the process of fermentation. The amount of alcohol in the finished liquid depends on how much sugar there was at the beginning for the yeast to convert into alcohol. In beer, the alcohol is generally 3% to 12% (6 to 24 proof) and usually about 4% to 6% (8 to 12 proof). Depending on the strain of yeast, wines top out at about 14% to 16% (28 to 32 proof), because that is the point in the fermentation process where the alcohol concentration denatures the yeast. Since the 1990s, a few alcohol-tolerant 'superyeast' strains have become commercially available, which can ferment up to 20%. [3]

Fortified wines, such as sweet and dry vermouths become fortified by adding alcohol. These added alcohols are typically distilled wines such as brandy, port, sherry, marsala, and madiera. This second source of alcohol is added before the wines are finished fermenting, as the added alcohol kills the yeast in fermenting wine, leaving an abundance of sugar.

Stronger liquors are distilled after fermentation is complete to increase their alcohol content. This way, the alcohol content can be raised up to a maximum of 95.6% (191 proof), the concentration of the water/alcohol azeotrope. When 100% ethanol (absolute alcohol) is required for scientific or industrial purposes, other methods must be used. 100% ethanol is hygroscopic, and if it is left open, it will absorb water out of the air, reducing the concentration.

The term originated in the 18th century, when payments to sailors included rations of brandy. To ensure that the brandy had not been watered down and was of good quality, it was "proved" by dousing gunpowder in the liquor, and testing to see if it would ignite. If it did not, the solution contained too much water—and the alcohol content was considered low or "underproof".

A "proven" solution was defined as 100 degrees proof. This has since been found to occur at 57.15% ethanol. The British still use this definition, although only the ABV system is used to mark bottles for sale. A simpler ratio to remember is 7:4, i.e. 70 degrees proof is approximately 40% alcohol by volume. Thus pure alcohol is approximately 175 degrees proof.

• History of the Sikes Hydrometer