Back outside the Grain Mill, the pavement is littered with kernels of corn. Fletcher tells me how Jack’s time-honored recipe for “Old No. 7” is 80 percent corn, 12 percent barley, and 8 percent rye. Just like the wood in its barrels, Jack Daniel’s is particular about its corn. Mixed with the magical waters from the cave, a little yeast, and a bit of mash from an old batch, the sour mash process kicks into gear. This process is bubbling inside Fermenter No. 4 like a pot on low boil as we step into the room. Fletcher slides its metal doors back. Eager as a kid I pop my head forward for a closer look and get blasted by fermented air. My nostrils light up like someone’s swabbed them with rubbing alcohol and for a few seconds it’s hard to breathe. Seasoned veteran that he is, Fletcher never even flinches from the fumes. Once my head rush subsides, he has me dip a finger in the mash for a taste. The corn swill is super sticky, still lightly sweet with a bite of straight alcohol. After six days this mash will be ready to be distilled in one of Jack Daniel’s legendary copper tanks, another component of the whiskey’s distinct character.