Bourbon whiskey is made from maize, unpeated malted barley and another cereal, usually rye. As little as 51% maize may be used, but 60-7O% is more common. Traditionally made malt is used at a level of ca. 15%, but malt made using gibberellic acid, which has twice the a-amylase content, is permitted and used at lower levels. In this case glucoamylase from Aspergillus niger may also be added The choice of third cereal, the small grains' depends on the desired flavour of the matured whiskey. Although rye is most common, barley and wheat are also used and each impart a unique flavour to the distillate.