The measurement of alcohol content and the statement of this content on liquor bottle labels is regulated by law. The purposes of such regulations are to tax alcohol and to provide pertinent information to the consumer.
In the 18th century and until 1 January 1980, Britain defined alcohol content in terms of “proof spirit,” which was defined as the most dilute spirit that would sustain combustion of gunpowder.The term originated in the 18th century, when payments to British sailors included rations of rum. To ensure that the rum had not been watered down, it was “proved” by dousing gunpowder in it, then testing to see if the gunpowder would ignite. If it did not burn, the rum contained too much water — and was considered to be “under proof.” A proven sample of rum was defined to be 100 degrees proof; this was later found to occur at 57.15% alcohol by volume, which is very close to a 4:7 ratio of alcohol to total amount of liquid. Thus, the definition amounted to declaring that (4÷7) × 175 = 100 degrees proof spirit.
From this it followed that pure, 100% alcohol had (7÷7) × 175 = 175 degrees proof spirit, and that 50% ABV had (3.5÷7) × 175 = 87.5 degrees proof spirit.
The basic idea was that the percentage of alcohol by volume was multiplied by 1.75, which then gave the number of degrees proof spirit. From the 1740s until 1816, Customs and Excise and London brewers and distillers used Clarke’s hydrometer to measure degrees proof. Under the Hydrometer Act of 1818, the Sikes hydrometer was used to measure proof; it remained in use until 1980. The Customs and Excise Act of 1952 defined “spirits of proof strength” (i.e., 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.”
The European Union follows recommendations of the International Organization of Legal Metrology (OIML). OIML’s International Recommendation No. 22 (1973) provides standards for measuring alcoholic strength by volume and by mass [weight]. A preferred method to be used is not stated in the document, but if alcoholic strength by volume is used, it must be expressed as a percentage (%) of total volume, and the water/alcohol mixture must have a temperature of 20°C when measurement is done.
The document does not address alcoholic proof and the labeling of bottles.
In the United States, alcohol content is measured in terms of the percentage of alcohol by volume, (ABV). The Code of Federal Regulations (27 CFR [4-1-03 Edition] §5.37 Alcohol content) requires that liquor labels must state the percentage of alcohol by volume. The regulation permits, but does not require, a statement of the proof provided that it is printed close to the ABV number.
Alcoholic proof is twice the percentage of alcohol by volume when measured at a temperature of 60°F (15.5°C). Consequently, 100-proof whiskey contains 50% alcohol by volume; 86-proof whiskey contains 43% alcohol.
The terminology used in the United States is “n proof” (where n is a number, e.g., 86 or 100) — not “n degrees proof.” The term “degrees proof” is not used.
“In common with other EC countries, on 1st January, 1980, Britain adopted the system of measurement recommended by the International Organisation of Legal Metrology,a body with most major nations among its members. The OIML system measures alcoholic strength as a percentage of alcohol by volume at a temperature of 20°C. It replaced the Sikes system of measuring the proof strength of spirits, which had been used in Britain for over 160 years.”
Stronger drinks (i.e., spirits) are distilled after fermentation to increase their alcohol content. These form a very broad category of beverages whose alcohol content can range from 15% to 96.5% ABV.
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