It’s right there on the front label of the best-selling American whiskey in the world. “Jack Daniel’s Old Time Old No. 7 Quality Tennessee Sour Mash Whiskey.” If it weren’t there, probably no one would ever ask: “what is sour mash?” While there are several things that make Jack Daniel’s unique, sour mash is not one of them.Sour mash is as ubiquitous in the production of Bourbon whiskey as corn. Every major Bourbon brand, and straight rye too, could put the words “sour mash” on its label, even though only a few do. So what exactly is “sour mash” and what does it have to do with making whiskey?The meaning could not be more literal. “Sour Mash Whiskey” is simply whiskey made using the sour mash process. A mash, of course, is a solution of ground up grain and water, prepared to make grain starch available for conversion to sugar and then, through fermentation, into alcohol.“Souring” refers to the addition of acid to adjust the mixture’s pH, which renders the mash more receptive to yeast and hostile to other micro-organisms that might interfere with fermentation.There are a couple of ways to sour a mash but the only technique generally practised is to mix into each new batch some volume of mash from a previous distillation. Known as “spent mash” or, more colloquially, “slop” due to its use as livestock feed, it contains no alcohol, no sugar, and no living yeast, but it does contain yeast nutrients as well as the all important acid.“Spent mash” is euphemistically called a by-product, even though distillers think of it as waste. It makes a very nutritious livestock feed, but the cost to dry it pretty much eats up all the profits.Therefore, as a source of acid for the producer it is convenient and essentially free.Dr. James C. Crow is credited with introducing sour mash to the American whiskey industry. The problem it addressed was inconsistency. In Crow’s day, which was the first half of the 19th century, whiskey makers in America had trouble producing the exact same whiskey from batch to batch.Sour mash was the answer and the practice was quickly adopted throughout the industry.Sour mash became universal so fast because it became branded as “the good stuff.” Along with sour mash, Crow introduced better hygiene, routine aging and other techniques, so it probably would have been more accurate to credit his whole package of reforms for the improved quality, but the man on the street attributed it to sour mash. Even after every distiller had adopted sour mash, it remained a byword for whisky quality, as it is to this day.The alternative, known as “sweet mash” is not so much a technique as the absence of one. The mash is simply prepared without slop or any other souring agent. Sweet mash works, but it produces a very different whiskey, as Woodford Reserve proved in 2008 with its limited Master’s Collection release of 1838 Sweet Mash Bourbon. The whiskey was made in every way like standard Woodford Reserve, except no “sour” was used.This stressed the yeast, causing it to produce a very different flavour profile.Dr. Crow first became employed at the distillery now known as Woodford Reserve in1838.In another experiment, Buffalo Trace demonstrated an alternative way to sour a whiskey mash, by allowing lactobacillus bacteria to develop and produce lactic acid. As with the Woodford Reserve experiment, everything except the souring method was standard Buffalo Trace practice, yet the whiskey was very different. That experiment was released in early 2011 as Colonel E. H. Taylor Jr. Old Fashioned Sour Mash Bourbon.Although every major whiskey distillery in the United States uses sour mash, they each use it a little differently in terms of how much spent mash they add and when they add it. The average is about 25 per cent, which is a lot of the tub to fill with something that is not fermentable. Some use spent mash in the yeast mash as well as the whiskey mash. Some introduce it at the cooker stage, while others just add it to the fermenters.Sour mash whiskey isn’t really a type, even though many people still think it is.Although souring a mash will cause yeast to create different flavours than they would otherwise, there is no consistency in how they differ. A soured mash does not produce a sour taste in the whiskey. You cannot determine by taste if a whiskey is sour mash or not.Ironically, very few micro-distillers use sour mash, even those who make whiskey every day and therefore have spent mash available. Those are two words you won’t see on their labels.