Bourbon is made from a grain mixture that’s at least 51% corn, combined with smaller amounts of rye, wheat, or malted barley. Beyond the grain, bourbon gets its character from water, yeast, and the new charred oak barrels it ages in. Federal law dictates every one of these requirements, and the spirit must be distilled and aged in the United States.
The Grain Mix: Corn Comes First
The recipe of grains used to make bourbon is called a mash bill. By law, corn must make up at least 51% of that recipe, but most bourbon producers use between 60% and 75% corn. Corn provides the sweetness bourbon is known for, giving it a rounder, fuller body compared to other whiskeys.
The remaining portion of the mash bill typically includes two other grains. Rye is the most common secondary grain and adds spice, pepper, and a bit of bite. Malted barley usually rounds out the mix at around 10 to 12%, and its primary job is practical: barley contains enzymes that help convert the starches in all the grains into fermentable sugars. Some bourbon makers swap out rye for wheat, which produces a softer, gentler flavor profile. These “wheated bourbons” have a loyal following, though they’re less common than the traditional corn-rye-barley combination.
Water and Why It Matters
Water goes into bourbon at nearly every stage: cooking the grains, fermenting the mash, and diluting the spirit before bottling. The quality of that water has a direct effect on the final product. Kentucky became the historical center of bourbon production in large part because its limestone bedrock naturally filters out iron and sulfur from groundwater. Iron makes whiskey taste bitter and can turn it black, so iron-free water is essential.
Limestone-filtered water also carries dissolved calcium carbonate, which may help fermentation by keeping acidity levels in a range that supports the bacteria and yeast doing the work. That said, any clean, iron-free water source works. Bourbon is legally produced in states well beyond Kentucky, from New York to Texas, as long as the water meets the same basic quality standard.
Yeast and the Sour Mash Process
Once the grains are cooked in water to release their sugars, yeast is added to ferment that sugary liquid into a low-alcohol “beer.” Different yeast strains produce different flavor compounds during fermentation, so distilleries guard their proprietary yeast cultures carefully. The specific strain a distillery uses is one of the things that makes its bourbon taste distinct from a competitor using a nearly identical grain recipe.
Most bourbon is made using the sour mash process. This means a portion of spent mash from a previous fermentation batch, still containing dead yeast and acids, gets mixed into the new batch. The acidity from this spent mash controls the pH of the fresh batch, which prevents unwanted bacteria from growing and tainting the bourbon. It also helps keep flavor consistent from one batch to the next, so every bottle from a distillery tastes the way you’d expect it to.
New Charred Oak Barrels
Bourbon must be aged in new, charred oak barrels. This is one of the rules that most clearly separates bourbon from other whiskeys. Scotch and Irish whiskey, for example, typically age in used barrels. The “new barrel” requirement means bourbon gets the full intensity of the wood’s flavor compounds on first contact.
Charring the inside of the barrel is where much of bourbon’s signature flavor originates. When the interior wood is exposed to high heat, a compound called lignin breaks down and produces vanillin, which is the same molecule responsible for vanilla flavor. The longer and more intensely a barrel is charred, the more vanillin it releases. Another wood component, hemicellulose, breaks down into simple sugars under that same heat, caramelizing on the barrel’s interior surface. As bourbon rests inside the barrel over months and years, it absorbs those caramelized sugars, picking up the toffee and caramel notes that define the spirit’s taste. The charred layer also acts as a natural filter, stripping out harsher flavors from the raw distillate.
Proof Limits at Every Stage
Federal regulations control how strong bourbon can be at three key points. During distillation, bourbon cannot exceed 160 proof (80% alcohol). This cap preserves grain flavor in the spirit; distilling to a higher proof would strip away many of the taste compounds that make bourbon taste like bourbon rather than a neutral spirit like vodka.
When the distillate goes into the barrel, it must be no higher than 125 proof. This limit was set in 1962 after distillers experimented with various entry proofs in the late 1950s. Before Prohibition, barrel entry proof was typically much lower, around 107. The proof at which bourbon enters the barrel affects how it interacts with the wood over time, influencing the final flavor.
At bottling, bourbon must be at least 80 proof (40% alcohol). There’s no maximum at this stage, which is why you’ll find bottles ranging from 80 proof all the way up to barrel-strength offerings that can exceed 130 proof.
Straight Bourbon vs. Regular Bourbon
All bourbon must spend some time aging in those charred new oak barrels, but there’s no specific minimum for standard bourbon. “Straight bourbon” has stricter rules: it must age for at least two years, and if it’s been aged less than four years, the exact age must be stated on the label. Straight bourbon also cannot contain any added coloring, flavoring, or blending materials. Regular bourbon technically allows those additions, though most reputable brands skip them anyway.
Most straight bourbons age well beyond the two-year minimum. Four to eight years is a common range, and some premium bottles sit in barrels for a decade or more. During that time, seasonal temperature swings cause the wood to expand and contract, pushing the bourbon deeper into the charred oak and pulling it back out, which intensifies the flavor extraction.
It Doesn’t Have to Come From Kentucky
Bourbon can be made anywhere in the United States. Federal law only requires that it be “distilled and aged in the United States,” with no state-specific restriction. Kentucky produces the vast majority of bourbon and has deep historical ties to the spirit, but distilleries in Indiana, Tennessee, Colorado, and dozens of other states produce bourbon that’s every bit as legally legitimate. The one hard geographic rule: bourbon cannot be made outside the U.S. and still be called bourbon.