Governments use proof as a fiscal instrument to levy excise tax or a customs duty on the volume of alcohol in a beverage. For drinkers, alcoholic proof is the potent and pleasurable psychoactive effect that leads to inebriation and intoxication if consumed to excess. Understanding the different measurement standards and praxis for alcoholic proof numbers can be perplexing, even opaque.
At the most elementary level, alcoholic proof divides into a series of numerical thresholds. At the top of the scale is the azeotropic point, 95.6% Alcohol by Volume (ABV) where ethyl alcohol molecules need water to bind to remain in liquid form. In Scotch whisky, the next threshold is the regulatory maximum for ‘grain spirit’ Proof at 94.8% ABV and American whisky 95% ABV for use in blended whisky classifications. National regulations on product identity standards protect the flavour compositions of different grain mashes and distilling apparatuses, with Scotch malts and straight American whiskeys distilled at lower proof to capture more of the congeners that transform into flavour compounds during cask maturation. Only America sets a maximum distilling Proof of 80%, while most malt and straight distilleries yield spirit in the low 70s to mid-60s. The next threshold is proof entered into the cask, where American straight whisky must not exceed 62.2% ABV, and Scotch malt distillers commonly fill at 63.5% ABV. Next is bottling strength with Scotch, Irish and North American whiskies at a minimum 40% ABV; below this strength, the whisky risks collapsing its flavour matrix. For purists who avoid chill-filtering, the whisky’s proof needs to be above 46% ABV to prevent fatty proteins precipitating in liquid at temperatures under 3° Celcius, causing flocculation and reducing appetite appeal. Finally, the drinker can reduce the proof to taste, which happens in many drinking occasions, through dilution in cocktails, carbonated mixers, water and melting ice cubes. Blood alcohol levels that legally impair judgement span 0.05 and 0.08% ABV (or five parts of alcohol for every 1,000 parts blood), and over 0.4% causes fatal blood poisoning.
The empirical measurement of proof began with the invention of the alcoholic hydrometer. In April 1787, the Board of Customs and Excise adopted Clarke’s hydrometer, replacing it with the Sikes instrument with the 1818 Hydrometer Act. When Britain joined the EU, the Guy-Lasac metric system became compulsory from 1980, used in France since 1824 (coincidently, the same year Britan modified weights and measures from the English unit to the Imperial system). The United States adopted the Dicas hydrometer in April 1790, moving to the Tralles system in January 1825 for gauging imported liquors. The Sikes, Lussac and Tralles systems use different methods of measurement. Sikes and Dicas measure the specific gravity (or relative density) of liquids, while Tralles use the 200-point-scale and Lussac 100 point-volumetric measurement. As ethanol is lighter than water, Sikes proof strength equates to 57.1% ABV, Lussac 50% ABV and US 100 proof – all containing 50% water and 50% alcohol, with less than one per cent contribution from flavouring and colouring chemicals.
While the global consumer trend is towards drinking lower proof alcoholic beverages, the small counter-trend amongst the cognoscenti is moving to cask strength whisky. At the beginning of the 19th century, the equivalence of cask strength at 48% ABV to 65% ABV was how all whisky was purchased. In 1824, new British proof regulations varied by spirit class and region; whisky was ‘never below’ 12 Sikes under proof (50%) for consumer sale, but distilleries were allowed to ship puncheons at 25 over proof from the still (71%) and 11 OP diluted in cask (63%). At retail, whisky sold above 10 UP (+50% ABV), and the 1879 Sale of Food & Drug Amendment set at 25 UP (43%). During World War I, to appease the temperance movement and extend limited stocks, bottling lowered to 35 UP (37.2%) in 1916, which remained until 1980 when the new minimum strength of 40% ABV was mandated.
Whether it’s defined by production or legal standards, the strength of proof is always in the numbers.