Whisky Magazine Issue 101
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Charles K. Cowdery looks at the sour mash process
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.