To be legally considered bourbon, a spirit must be made in the United States, contain at least 51% corn in the mash of grains used to distill it, be aged in new charred oak containers, and lack any additives other than water. However, spent mash from a previous batch of bourbon, called "sour mash," is fair game to add to a new batch.

One concern when fermenting bourbon is bacterial growth due to pH that's too high, and adding acidic sour mash drops the pH to the ideal levels for efficient fermentation, while also minimizing the risk of unwanted bacterial growth. Using sour mash also has the added benefit of ensuring consistency between different batches of bourbon.

The process of adding sour mash to bourbon has been largely credited to Dr. James C. Crow, who was said to have invented or refined the process himself in the 1830s. Nearly all major Tennessee whiskey and bourbon distilleries use the sour mash process, even if they don't list sour mash as an ingredient on their bottles.