In American whisky making, it’s common for distillers to use some of the fermented grain in the mash—before it goes into the still—to mix in with the next batch to be fermented. That portion is called the “backset,” and the whole process is called “sour mash,” just like it says on the bottles. The practice began in the 1800s, probably as a way to keep the same yeast strain consistent.
Adam Rogers in Proof