Seriously, it’s just grain, water, and yeast – the same trio that make up a loaf. To be called “bourbon”, the initial distillate must come from at least 51% corn, be cut with water from a ceiling of 190-proof to at least 125-proof, and stored in new oak barrels that have been charred on the inside. There is no age requirement unless it’s “straight bourbon” -- in which case we’re talking a minimum of two years, with no added coloring or flavor.
It’s Also Sour Beer
As home brewers probably noticed, beer is just a whiskey precursor. The only difference? Heat. Beginning as "Distiller's Beer", the future whiskey is "distilled", meaning vaporizing and condensing multiple times, separating out the water and turning what was a very sour beer into something even more magical: whiskey.
The Barrels Are Filters
You probably know that whiskey gets its distinct color and flavor from the barrel. But that’s also where it’s naturally filtered. As weather gets warmer, the expanding whiskey seeps into the wood. As it cools, the whiskey contracts out, taking with it tannins, sugar, and other flavors -- but leaving behind congeners, i.e. bad stuff that’s made in the fermentation process. Ever see the inside of a pool filter? Same principle (minus the hilarious “-ool” signs), that charred carbon is incredibly porous and traps impurities.