How is bourbon made?

Quick facts

• Federal rules set a number at four of the five steps; fermentation is the one they leave alone.
• Spirit leaves the still clear.
• Whisky Magazine puts Kentucky's angel's share at about 3 to 5% of a barrel a year; The Glenlivet puts Scotland's at about 2%.
• "Barrel proof" and "cask strength" are defined terms under 27 CFR 5.87.

Step one: the mash is at least 51% corn

Bourbon starts as a cooked mash, and corn must make up at least 51% of it. The rest is the distiller's choice, usually rye or wheat plus a little malted barley, there to break starch down into fermentable sugar. That recipe, the mash bill, is where two bourbons first go separate ways.

Step two: fermentation is the step with no federal number

Yeast then ferments the mash, turning its sugars into alcohol. The bourbon standard names no yeast, no fermentation time and no method; Four Roses, for one, works with five yeast strains. "Sour mash" belongs to this stage too, and is explained in the questions below.

Step three: distillation stops at 160 proof

The fermented mash is then distilled, and bourbon may not exceed 160 proof as it comes off the still. Proof is twice the alcohol by volume at 60 degrees Fahrenheit (27 CFR 5.1), so that ceiling is 80% alcohol. What runs off is clear: Buffalo Trace sells unaged spirit from its flagship bourbon mash bill as White Dog Mash #1, a clear spirit at 125 proof.

Bourbon can come off a column still, a pot still or both: Willett, for one, pairs a pot still with a column. Under 27 CFR 5.89, one run through a pot still or one run through a column counts as one distillation, and a label may claim "double distilled" or "triple distilled" only if that is true. Our directory of Kentucky's bourbon producers lists the distilleries we visit.

Step four: new charred oak, filled at no more than 125 proof

The spirit goes into oak at 125 proof or lower, and the barrel must be new, charred and oak, so one that has held bourbon cannot age another. Color is covered in how the barrel alone sets bourbon's color.

Age is time in oak between distilling and bottling (27 CFR 5.1); the rules behind the straight label, including when a label must state an age, have their own page.

The angel's share: what the barrel gives up

While bourbon rests in the barrel, some of it evaporates. Heaven Hill describes this angel's share as whiskey that slips away over time, most of all in hot climates.

Typical loss rates, by source

No single rate applies. Whisky Magazine has put Kentucky's annual loss at around 3 to 5% of a barrel, depending on its place in the warehouse. The Glenlivet gives about 2% a year for Scotland, and 3.5% to 4% a year early in maturation. How many bottles a barrel of bourbon yields works that loss through to a bottle count.

A Kentucky barrel often gains proof

Evaporation does not take water and alcohol in fixed proportions. In hot, drier air, relatively more water than alcohol escapes, so the proof inside a Kentucky barrel often climbs; in cool, humid Scottish warehouses more alcohol escapes and the strength drifts down, as the drinks site Alcohol Professor explains. A bourbon can leave the barrel stronger than it went in.

High floors lose more than low ones

Whisky Magazine gives the example of a nine-floor warehouse on a 90°F day: around 70°F at the bottom and around 120°F at the top. By the same source, barrels at the top can lose up to about 1% a year more than those at the bottom. Heaven Hill found that the lower floors of one of its rickhouses lost less, letting whiskey stay in wood longer. Wild Turkey, on a hilltop over the Kentucky River, ages whiskey in timber rickhouses; our guide to the Wild Turkey distillery describes the site.

Step five: bottling at 80 proof or more, with only water added

At bottling a distiller may add water and nothing else, and bottling strength may not fall below 80 proof. Some other American whiskeys may use a little coloring, flavoring or blending material; bourbon may not.

Frequently asked questions