Bourbon must be produced in the United States from a fermented mash containing at least 51% corn. It is distilled to no more than 160 proof, entered into new charred oak containers at no more than 125 proof, and bottled at 80 proof or higher.
It does not have to come from Kentucky
Kentucky is bourbon’s best-known centre and a major source of production, but the federal category is not confined to it. Bourbon can be made in any US state if the whiskey meets the applicable rules.
Why new charred oak matters
A new container offers a strong reservoir of extractable oak compounds. Charring changes the wood surface and helps create flavours commonly perceived as vanilla, caramel, spice and toast. The requirement also means used bourbon barrels become available to Scotch, Irish and other producers.
Does bourbon need an age statement?
Bourbon has no general minimum ageing period, but it must spend time in new charred oak to qualify. “Straight bourbon” must be aged for at least two years, and straight bourbon younger than four years requires an age statement under US labelling rules.
The rest of the mash bill
After the minimum 51% corn, distillers can use rye, wheat, malted barley and other permitted grains. Rye often contributes a firm, spicy impression; wheat may read softer; malted barley can support conversion as well as flavour. These are tendencies, not tasting guarantees.
Primary reference: US Alcohol and Tobacco Tax and Trade Bureau.