“Sour mash is a process in the distilling industry that uses material from an older batch of mash to start fermentation in the batch currently being made, analogous to the making of sourdough bread. The term sour mash can also be used as the name of the type of mash used in that process, and a whiskey made using this process can be referred to as a sour mash whiskey. Sour mash does not refer to the flavor of the whiskey, as is sometimes thought.
In the sour mash process, the mash – a mixture of grain, malt and water – is conditioned with some amount of spent mash (previously fermented mash that still contains live yeast). Spent mash is also known as spent beer, distillers’ spent grain, stillage, and slop or feed mash, so named because it is used as animal feed. The acid introduced by using the sour mash controls the growth of bacteria that could taint the whiskey and creates a proper pH balance for the yeast to work. An established and active strain of live yeast is introduced into the mash that is to be fermented. By using an established and known fermented “sour”, this fermentation process controls the introduction and growth of foreign bacteria and yeasts that could damage the whiskey, and improves the consistency and quality of the liquor, so that every bottle tastes as close to the same as possible.
The sour mash process is used for the production of nearly all bourbon and Tennessee whiskey.” – Wikipedia