In American whiskey, proof is simply twice the alcohol by volume percentage – i.e. 50% alcohol by volume is 100 proof. Legend has it that the term “proof” comes from proving the whiskey by gunpowder. On the frontier, to prove the strength of your whiskey, they would mix it with a small amount of gunpowder and light it. If it burned in an uneven manner, with lots of sputtering in the flames, then it was considered “under proof”. If it burned quickly with a hot, blue flame, it was considered “over proof”. If it burned with a steady, even flame, it was considered “proved”. This even burn comes with a whiskey of about 50% alcohol thus it would be “100% proved”. 

This trick was mostly a sales pitch – used to impress a potential buyer of a distiller’s whiskey. Another such trick was “checking the bead”. Place some whiskey in a clear glass bottle and shake the bottle, then observe the bubbles. Low proof whiskey produces small bubbles that dissipate quickly. Higher proof whiskey will have larger bubbles that last longer. Both of these tricks can be used to impress buyers of your whiskey, but the distillers did not depend upon them while making their whiskey. Most of them would depend upon a hydrometer and science to check the proof of their whiskey.

The hydrometer is a tool used to measure the percentage of alcohol in whiskey. It consists of a copper cup with a thermometer along the side and a glass device with lines of measurement on it that would be dropped into the whiskey. Where this device floated in the whiskey could then be measured and compared to the temperature of the whiskey to give the ratio of alcohol to water. The use of the hydrometer dates back at least to the 18th century. The first whiskey tax in the United States, the tax that caused the Whiskey Rebellion, was based upon a proof gallon as measured by a hydrometer. A proof gallon is a gallon of spirit at 50% Alcohol By Volume (ABV), at 60 degrees Fahrenheit. In the 1790s, the government gauger would assess the tax owed to the government using a hydrometer. The distillers more than likely had their own hydrometers to measure the proof of the whiskey they were producing. 

James C. Crow took the use of the hydrometer to the next level. While most distillers were using it mostly to check the proof of their final product, Crow used it to measure proof at the various stages of distilling. He would measure proof of the beer and the proof of the distillate at various times during the distillation process. He wanted to know where the best proof was for distilling the best whiskey. More importantly, he wrote down his results. These notes allowed future generations to learn how to duplicate his whiskey making process, from fermentation to distillation.

Proof is an important part of making whiskey. Even today, variations in distillation proof and barrel entry proof can make whiskey with a wide range of flavor variables. In Crow’s day, the barrel was the main package for distillers selling their whiskey. Consumers would purchase their whiskey from third parties directly from the barrel. That is why most distillers entered their whiskey into the barrel at a proof ranging between 90 and 105 proof. That range of proof is what most people wanted to drink. When bottling became economically feasible, the proof range was still in that range and the standard for Bottled-in-Bond whiskey became 100 proof. 

Today, the proof range is much higher. The low end is 80 proof. That is the standard for all whiskey internationally. However, today the upper range is much higher. The maximum barrel entry proof in the United States is 125 and most big distillers use that as their standard. Overseas, there is no maximum limit on barrel entry proof and many Scotch whiskies go into the barrel in the 130 range.  Proof has an impact on the flavor of the whiskey as it ages in the barrel with sugars from the barrel being better extracted from the wood at lower proof. 

The next time you purchase a bottle, think about proof – not only the bottle proof, but distillation and barrel entry proof. They all have an effect on the flavor of the whiskey you are purchasing.

Photos Courtesy of Rosemary Miller