sour mash is spent stillage—mash that has had all the alcohol distilled out of it—that is added to a subsequent batch of mash before it is distilled. Many spirits incorporate souring processes in mashing and fermentation, but the “sour mash” process is most often associated with the American bourbons and Tennessee whiskies. Spirits consumers are generally familiar with the term because most large American whisky brands identify their products as “sour mash.” The largest-selling bottling of them all proclaims it right on the front label: “Jack Daniel’s Old No. 7 Brand Tennessee Sour Mash Whiskey.”
Credit for the modern sour mashing process using backset stillage is often given to Dr. James C. Crow, an early industry legend known for applying a scientific approach to whisky making, but the process certainly predates the 1830s, when Crow became active in American whisky distilling. By the 1750s, rum distillers in the British Caribbean were mixing “dunder”—spent stillage—in with the skimmings and molasses from which they made their rum. See dunder and rum. The process was being recommended by American distillers’ manuals by the 1810s but doesn’t appear to have become widespread until the middle of the century, when it became a selling point for bourbons and Tennessee whiskies (eastern ryes stayed with the sweet-mash process). See whisky, bourbon; whisky, rye; and whisky, Tennessee.
Today the most common practice in sour mashing is to return some portion of stillage to subsequent mash cooks. Whole stillage is what remains after distillation separates the new whisky from the fermented beer. Large, insoluble grain solids can be separated from the dissolved, thin stillage fraction by centrifugation or by running over gravity screens. This thin stillage, called backset, is added to the mash cooker along with water and fresh grain meal. The proportion of backset in the mash varies but typically ranges from 10 percent to 25 percent of the fresh mash by volume.
A second component of sour mashing that is a very traditional but less well known is the use of a lactic bacteria soured yeast mash. In this process, lactic bacteria are inoculated into a secondary mash stream used to condition and scale up the yeast at the distillery. These “lactics” produce lactic acid and lower the pH, acidifying the yeast mash. The two components of sour mashing, backset stillage and lactic soured yeast mash, work together to optimize pH balance throughout mashing and fermentation cycles. See lactobacillus.
Sour mashing practices vary across the industry, and the process is not strictly defined. The process has evolved over several centuries. It could be argued that the early practice of holding over fermented beer from a finished fermenter to provide active yeast to subsequent fermentation could provide some of the same nutritional and acidification benefits as backset stillage. However, that approach was not a best practice for yeast maintenance, and distillers learned to cultivate and inoculate a fresh, active yeast to start fermentation.
Bryson, Lew. Whiskey Master Class. Beverly, MA: Quarto, 2020.
Veach, Michael. “Early Sour Mash.” Bourbonveachdotcom, July 9, 2017. https://bourbonveach.com/2018/07/09/early-sour-mash/ (accessed March 29, 2021).
By: Kevin Smith