Jim Beam starts with water filtered naturally by the limestone shelf found in Central Kentucky. A strain of yeast used since the end of Prohibition is added to a tank with the grains to create what is known as "dona yeast", used later in the fermentation process. Hammermills grind the mix of corn, rye and barley malt to break it down for easier cooking. The mix is then moved into a large mash cooker where water and set back are added. The "set back" is a portion of the old mash from the previous distillation—the key step of the sour mash process, ensuring consistency from batch to batch.