Sour mash whiskey is a type of whiskey that is made using a process called sour mashing. The term "sour mash" refers to the use of material from an older batch of mash to start the fermentation of a new batch, similar to the way sourdough bread is made with a starter. The sour mash process is used to regulate bacterial growth and ensure continuity from batch to batch. The mash is a mixture of grain, malt, and water that is initially fermented to produce alcohol. Distillers regulate the pH of their mash to prevent unwanted bacterial growth. Adding a portion of spent mash, also called backset or stillage, to the current mash helps to ensure the pH remains within acceptable levels and ensures flavor continuity from batch to batch.

Sour mash whiskey is not sour in taste, and the term "sour mash" refers to the way it is produced, not how it tastes. Sour mash whiskey is a type of bourbon, which is only made in the United States and has to be made of at least 51% corn and aged in charred oak barrels to be labeled as such. The ingredients generally included in sour mash bourbon are grains such as barley, rye, or corn.

The sour mash process has been credited to Dr. James C. Crow, who was said to have invented or refined the process of using sour mash while he was working at Oscar Pepper as their master distiller in the 1830s. Crow used the sour mash process on a daily basis when making bourbon for consistency, and his bourbon, known as "Old Crow," is said to be the first "modern" bourbon.

In summary, sour mash whiskey is a type of bourbon made using a process called sour mashing, which involves using material from an older batch of mash to start the fermentation of a new batch. The sour mash process is used to regulate bacterial growth and ensure continuity from batch to batch. Sour mash whiskey is not sour in taste, and the term "sour mash" refers to the way it is produced, not how it tastes.