As any whiskey maker will tell you, all bourbon is whiskey but not all whiskey is bourbon. And all sour mash is bourbon whiskey but not all bourbon is sour mash. Both bourbon and sour mash whiskeys are uniquely American spirits, descended from moonshine and produced primarily in Kentucky and Tennessee. Connoisseurs argue over which is superior, but all agree that both are fine additions to the distiller's craft.
Bourbon whiskey is made in the United States, nearly all of it in Kentucky and Tennessee. Traditional methods use limestone-spring water and oak barrels, both of which are abundant in these states.
Bourbon whiskey had its beginnings in territorial Kentucky in a county named for the Bourbons, the ruling family of France at the time. Farmers used leftover silage corn instead of wheat or rye, the customary grains used for mash.
When the Whiskey Rebellion of 1791 took a toll on distillers in other western states, the distillers in the Blue Ridge Mountains of Kentucky and Tennessee were able to hide from the revenue agents successfully enough to build a thriving, unregulated industry. The moonshine whiskey that these mountain folk produced evolved into the refined whiskeys called bourbon and sour mash.
Bourbon's unique character comes from the 51 to 79 percent corn in its recipe. The fermentation process begins with the addition of water to crushed or rolled grain, and the fermented mash is distilled to produce a spirit that is no more than 80 percent alcohol by volume. Bourbon may be double-distilled and aged at least two years in charred oak barrels. The result is a mellow, woody blend of flavors that may be bottled straight out of a single barrel or blended from a number of barrels in a small batch.
Sour mash whiskey uses the bourbon recipe but starts the mash with leftovers from a previous batch, much like the starter in sourdough bread. The sour mash process gives a sweeter, deeper flavor to the final product.
The alcohol-by-volume content of bourbon and sour mash is adjusted to between 40 and 50 percent (80 to 100 proof) at bottling time.
The name bourbon has been legally controlled only since 1964, so makers exist in other counties. However, only bourbon made in Kentucky may use the name of the state on the label.
Tennessee whiskey uses the bourbon recipe, but the distilled spirits are filtered through maple charcoal, adding a different overtone to the flavors. Makers specify "sour mash" on the label if that process is used.
Several societies are devoted to the study of these American spirits, and tours of distilleries are popular in the hill country of Tennessee and Kentucky.