The typical grain mixture for bourbon, known as the mash bill, is a minimum of 51% corn, with the remainder being wheat, rye, and/or malted barley. A mash bill that contains wheat instead of rye produces what is known as a wheated bourbon. The grain is ground and mixed with water. Usually, though not always, mash from a previous distillation is added to ensure a consistent pH across batches, and a mash produced in that manner is referred to as a sour mash. Finally, yeast is added and the mash is fermented. The fermented mash, referred to as the wash, is then distilled to (typically) between 65% and 80% alcohol. Distillation was historically performed using an alembic or pot still, although in modern production, the use of a continuous still is much more common.
The resulting clear spirit is placed in newly charred American oak barrels for aging, during which it gains color and flavor from the caramelized sugars in the charred wood. Changes to the spirit also occur due to evaporation and chemical processes such as oxidation. Bourbons gain more color and flavor the longer they mature. Maturity, not a particular age, is the goal. Bourbon can age too long and become woody and unbalanced.
After maturing, bourbon is withdrawn from the barrel, usually diluted with water, and bottled to at least 80 US proof (40% abv). Most bourbon whiskey is sold at 80 US proof. Other common proofs are 86, 90, 94, 100, and 107, and whiskeys of up to 125 proof can be sold. Some higher-proof bottlings are marketed as “barrel proof“, meaning that they have not been diluted or have been only lightly diluted after removal from the barrels. Bourbon whiskey may be sold at less than 80 proof but must be labeled as “diluted bourbon”.
Barrels still contain 20 pounds of bourbon within the wood. They cannot be re-used for bourbon, and are sometimes sold to the Scotch whiskey industry.