I spoke about the creation of the mash in an earlier post. That mash has to go through some more key steps before meeting the barrel and beginning the aging process to become bourbon. Those key steps include fermentation and distillation. Generally speaking, fermentation is the process of alcohol creation and distillation is the process of alcohol collection.
After the sour mash, or backset, is added to a fresh mash mixture, yeast is introduced. Yeast is a living microorganism that has a specific role in bourbon (and, well, all alcohol) production: it feeds on the sugars created when, during mashing, the enzymes in the malted barley reacted with the grain starches. The by-products of the yeast's feast is carbon dioxide, the release of which makes the liquid bubble and froth during fermentation, and alcohol. After a few days of alcohol and CO2 releasing, the alcohol content rises to between 8 and 11%. Depending on the yeast strain used, the rising alcohol percentage eventually kills off the yeast and effectively completes fermentation. Yeast, by the way, should not be underestimated or thought of being merely a fungible commodity. Each yeast strain is different and is capable of influencing the flavor of the bourbon - some yeasts make bad-tasting whiskies. Distillers guard their yeast strains very cautiously; for example, Beam still uses yeast from the same mother batch used when distillation reopened after Prohibition.
The end of fermentation marks the beginning of the (usually) two step distillation process. The now-alcoholic mash is pumped into the first still, the column still, so named for it columnar shape. It can be several stories high, and is about a yard across. The fermented goop is pumped into the top of the still, where it slowly trickles down through a series of pierced horizontal plates spaced about 18 inches apart and attached to the inside of the column. As gravity slowly pulls the mash down through the grated plates, steam enters and rises from the still's bottom. Since alcohol has a lower boiling point than water, the hot steam vaporizes the alcohol and carries it up and out of the still, leaving behind all the non-alcoholic mash components.
The vaporized alcohol is then condensed into what is a clear, potent liquid before it starts its second distillation. Distillation number two occurs in another still, called a "doubler." The doubler removes even more water from the condensed alcohol, therby increasing the liquid's proof. And as you'll recall, law says "bourbon" can't be distilled to an alcohol content greater than 160 (80 proof). From the doubler, it's on to the barrel and aging.
Distillation allows distillers to regulate the alcohol content of what will eventually become bourbon. This is important as proof and flavor have a direct relationship, and in order for bourbons to meet the standards of their master distillers, control over the alcohol content at this early stage of production is vital.