The typical grain mixture for bourbon is 70% corn - with the remainder being wheat and/or rye, and malted barley. This mixture, called the mash, is fermented through a process called sour mash fermentation in which mash from a previous distillation is added to ensure a consistent pH across batches. The fermented mash is then distilled.
This clear spirit is placed in charred oak barrels for aging, during which it gains color and flavor from the wood. Bourbons generally appear darker the longer they age.
After aging, bourbon is withdrawn from the barrel, diluted with water and bottled to at least 80 US proof (40% abv). Most bourbon whiskey is sold at 80° proof. Other common proofs are 86, 90, 94, 100 and 107, and whiskeys of up to 151 proof have been sold. Some higher proof bottlings are "barrel proof," meaning that they have not been diluted after removal from the barrels.
Bourbon whiskey may be sold at less than 80 proof but must be labeled as "diluted bourbon."
On 4 May 1964, the U.S. Congress recognized Bourbon Whiskey as a “distinctive product of the United States," creating the Federal Standards of Identity for Bourbon. Federal regulations now stipulate that Bourbon must meet these requirements:
▪ Bourbon must be made of a grain mixture that is at least 51% corn.
▪ Bourbon must be distilled to no more than 160 (U.S.) proof (80% alcohol by volume).
▪ Bourbon must be 100% natural (nothing other than water added to the mixture).
▪ Bourbon must be aged in new, charred oak barrels.
▪ Bourbon may not be introduced to the barrel at higher than 125 proof (62.5% alcohol by volume).
▪ Bourbon which meets the above requirements and has been aged for a minimum of two years, may (but is not required to) be called Straight Bourbon.
▪ Bourbon aged for a period less than four years must be labeled with the duration of its aging.
In practice, almost all bourbons marketed today are made from more than two-thirds corn, have been aged at least four years, and do qualify as "straight bourbon"—with or without the "straight bourbon" label. The exceptions are inexpensive commodity brands of bourbon aged only three years and pre-mixed cocktails made with straight bourbon aged the minimum two years.
Bourbon may be produced anywhere in the United States where it is legal to distill spirits. Currently most brands are produced in Kentucky, where Bourbon has a strong association. Estimates are that 95% of the world's bourbon is distilled and aged in Kentucky. Bourbon has been made in Colorado, Kansas, Illinois, Indiana, Missouri, New Jersey, New York, Ohio, Pennsylvania, Tennessee, and Virginia.