Simply put, whiskey is a distillate of a fermented malt beverage. Bourbon whiskey, as defined by law in the United States, is whiskey that has clearly defined characteristics. First, it must be made from a grain mixture that is at least 51% corn, though in practice most bourbon is two thirds or more corn. Second, proof upon distillation must not be over 160; when barreled it cannot be above 125. In addition, bourbon must be aged in charred new oak barrels. This is not all as complicated as it may sound at first, at least in theory, though in practice it is not easy to get or make charred oak barrels except on a commercial level. However, the steps in the process of making bourbon whiskey are fairly straightforward.
The first step in making bourbon whiskey is to become a licensed distiller. This is very important, since distilling alcohol at home is illegal in the United States. Compared to the headache and paperwork this step entails, the rest is a breeze. Basically, it all boils down to four steps: fermenting, distilling, aging, and bottling.
Generally, bourbon is made through a sour mash fermentation. This means that part of the mash used in a previous fermentation is added to the new one. The use of a sourdough starter in breadmaking is somewhat analogous. The addition of spent mash helps ensure a certain level of continuity of flavor from one batch to the next. The fermentation step is basically like making beer, but without the addition of hops. Yeast is added to the mash, converting sugars from the malted corn and other grains into alcohol and carbon dioxide.
Once fermentation is complete, it is time to distill the resulting liquid. While commercial distilling is exacting and complex, the basic idea is a simple one. The fermented brew is heated, and alcohol, which boils at a temperature lower than that of water, vaporizes. The alcohol vapor escapes through a tube or hose, which is then cooled in order to turn the alcohol back into liquid form, at which point it is collected in another vessel. The resulting liquid is clear and has a very high alcohol content.
After it is distilled, the resulting liquor is put into new oak charred barrels. In order to be labeled "straight bourbon," it must age in the barrel a minimum of two years. During the aging process, the bourbon takes on color and flavors contributed by the wood of the barrel.
Once ready for bottling, the bourbon is diluted with water to the desired proof. Most bourbon is bottled at 80 proof, though this is by no means universal.