Making bourbon involves fermenting a mixture of corn, rye, barley and sometimes wheat, then distilling the resulting alcohol. In the United States, the mixture must be at least 51 percent corn to be defined as bourbon.
Internationally, bourbon may simply refer to American whisky, however in the United States bourbon is distinct from Tennessee whisky in that whisky is filtered through sugar-maple charcoal while bourbon is not. The term "mash" refers to the grain mixture in the bourbon. The distiller mixes the grain with fresh spring water, then cooks it to extract the starch. Each type of grain requires a different cooking temperature.
After the cooked grains cool, the distiller adds yeast and allows the mixture to ferment in large tanks. During fermentation, the yeast converts the sugars from the grains into alcohol. The result of this fermentation process is a beer that is around 8 or 9 percent alcohol by volume.
Once the fermentation process is complete, the beer is distilled into raw bourbon by heating the liquid so that the alcohol becomes vapor and rises to the top of a specially designed column. Upon accumulation of the alcohol, the distiller runs the bourbon through a copper pot called a doubler. This improves the flavor of the raw bourbon. The bourbon then ages in wood barrels for at least two years before bottling.