Whisky made with backset added during the initial mash. Whiskies using backset include bourbon, corn whiskey and Tennessee whiskey. Backset, also known as backset stillage adds a lot of character to whisky and also promotes consistency across batches. Backset contains long chain fatty acids and alcohols that help produce esters. Done properly it can promote Lactic acid bacteria while reducing other bacteria.
Backset adds acidity to the wort which was historically beneficial for limiting unwanted bacteria. Sour mashing is also a throwback to the economic practices employed by early day moonshiners. Regulations stipulate that a sour mash contain at least 25% backset. See Title_27.