A bourbon whiskey mash bill must contain at least 51 percent corn.
United States regulations define bourbon as a whiskey made from a grain mixture containing at least 51 percent corn. The remaining grains commonly include rye, wheat and malted barley. Rye can add peppery spice, while wheat often produces a softer profile; malted barley supplies enzymes that help convert starches into fermentable sugars.
Bourbon must be distilled to no more than 80 percent alcohol by volume and entered into new, charred oak containers at no more than 62.5 percent alcohol. It must be made in the United States, although it can be produced outside Kentucky. Kentucky is strongly associated with bourbon because of its long production history, limestone water and established distilleries.
The 51-percent threshold distinguishes bourbon’s legal grain composition from other American whiskey categories. A bourbon can use substantially more corn, but it cannot fall below the minimum.