Whiskies are almost as varied as the countries that produce them, from Canada's light and dry ryes to peaty Scotch and Irish whiskies and sweet American bourbons. Even within a given family of spirits, such as bourbons, distinctions often exist in how they're aged and in their potency. For example, the difference between 80 proof and 100 proof bourbon is their respective alcohol content.
Why It's Bourbon
The definition of whiskey changes between countries, but the term "bourbon whiskey" has a very specific legal meaning. At last 51 percent of the grain used to make the whiskey must be corn, distilled to no more than 80 percent alcohol by volume and then diluted down to no more than 62.5 percent alcohol when it's placed in barrels for aging. The barrels must be new, never before used, and must be charred on the inside to give the bourbon its characteristic flavor and color. As with any other whiskey, long aging in oak smooths the bourbon and gives it better flavor.
The alcohol content of whiskey in its oak barrels is relatively high to begin with, though it decreases over time as alcohol and water evaporate through the porous wood of the barrel. This evaporation -- traditionally known as "the angels' share" -- reduces the volume of whiskey in a barrel, and it's part of the reason aged whiskies sell for a premium price. Some brands sell small quantities of whiskey at the relatively high alcohol level found in their barrels, referred to as "cask strength." More often, it's diluted to a standardized percentage that's more pleasurable to drink.
Retail Percentages and Proof
The standard concentration of alcohol in bourbon and most other spirits is 40 percent ethanol by volume. That's high enough to still pack a concentrated punch, but low enough to reduce the alcohol's distinctive burn. Some varieties are sold at 50 percent alcohol by volume, allowing each drinker more leeway to add water without altering the whiskey's distinctive flavor. In the United States, these percentages can also be expressed in "proof" measurements. Proof is approximately double the percentage of alcohol, so bourbon at 40 percent alcohol is described as 80 proof, while the same spirit at 50 percent alcohol is called 100 proof.
The Back Story
Like many other valuable substances, alcohol is very easy to adulterate. "Proving" or "proofing" a spirit was a simple test of purity that could be performed anywhere, in the days when lab-quality hygrometers weren't readily available. A prudent purchaser used a sample of the whiskey to moisten a sample of gunpowder, then ignited the paste. A high, flickering yellow flame indicated too much alcohol, while no flame or a sputtering flame showed that the spirits had been diluted. A low blue flame marked the expected percentage of alcohol, which in those days was approximately 55 to 60 percent.
- On Food and Cooking: The Science and Lore of the Kitchen; Harold McGee
- Whisky Advocate: Guest Blog -- Chuck Cowdery on Those Confusing Bourbon Regulations
- The Washington Post: Spirits -- Understanding Alcohol Proof
- Photo Credit Martin Poole/Photodisc/Getty Images