Sour mash. It’s a common phrase on American whiskey bottles, but it definitely doesn’t signify that the whiskey inside is sour. Instead, it alludes to a production process that distilleries use to regulate bacterial growth and ensure continuity from batch to batch. Show A “mash” in the world of whiskey refers to the mixture of grain, water, and yeast that is initially fermented to produce alcohol. Distillers are very careful to regulate the pH of their mash; if it gets too high (basic), unwanted bacterial growth can occur.
 Fortunately, the distillery environment offers an abundant source of pH-lowering substance: spent mash, also called backset or stillage. This is the acidic, nutrient-rich substance that’s left behind in the still after a distillation run is complete. By adding a portion of that spent mash to their current mash (the remainder is often fed to livestock), distillers can ensure their mash pH remains within acceptable levels and ensure flavor continuity from batch to batch. Distilleries add more backset to their current mashes than you might expect—Chuck Cowdery reports that most distilleries use a 1:3 or 1:4 ratio of backset to new mash. (In the linked story, he also outlines another interesting way that distillers could sour their mash–by letting the mash sit for a few days before distillation and allowing microorganisms to do their work.) It’s also worth noting that virtually all major bourbon and Tennessee whiskey distillers use the sour mash technique, even though only a portion of them actively market their whiskeys as “sour mash.” Techniques similar to the sour mash technique are also used at many Jamaican rum distilleries, only there, distillers age their backset anaerobically (sometimes seeded with organic matter like fruit) before adding it to their mashes. This aged backset is called dunder, and its aroma can be truly shocking. However, distillation manages to transform those off-putting smells into the intense, fruity esters Jamaican rum is known for. Belle of Nelson poster for their sour mash whiskey Sour mash (or sourmash) is a process used in the distilling industry that uses material from an older batch of mash to start the fermentation of a new batch, analogous to the making of sourdough bread with a starter.[1] The term can also be used as the name of the type of mash used in such a process, and a bourbon made using this process can be referred to as a sour mash bourbon.[2] Sour beer may also be created with this process. Process[edit]In the sour mash process, the mash – a mixture of grain, malt and water – is conditioned with some amount of spent mash (previously fermented mash that still contains dead yeast, a good food source for live yeast). Spent mash is also known as spent beer, distillers' spent grain, and slop or feed mash because it is also used as animal feed. The acid introduced by the sour mash controls the growth of bacteria that could taint the Bourbon. An established and active strain of live yeast is introduced into the mash to be fermented. By using an established and known fermented "sour", this fermentation process controls the introduction and growth of foreign bacteria and yeasts that could damage the Bourbon and improves the consistency and quality of the liquor, ensuring every bottle tastes as similar as possible. Sour mashing is also a process sometimes used in brewing to make sour beers in a short time frame. In the brewing version of sour mashing, brewers mash in their grains to begin the brewing process, but instead of extracting the wort from the grains at the end of the mash (typically in less than 90 minutes), the brewer leaves the grains and wort together for as long as several days before draining the wort. During this time, the lactic acid bacteria naturally present on the grain ferment some of the sugars in the wort into lactic acid. This lactic acid adds sourness to the beer. In order to promote fast bacterial fermentation and reduce yeast activity, the mash is kept between 100–120 °F (38–49 °C). The brewer extracts the wort by sparging when they believe enough acid has been produced. Some beers employing sour mashing are fermented with the addition of brewing yeast but without a boil. Other brewers prefer to boil their sour mashed beer and then ferment it with brewing yeast. The resulting beers can range from mildly tart to noticeably sour. See also[edit]
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What makes it a sour mash whiskey?Sour mash refers to the grouping of grains used in the fermentation process. Typically, distilleries will use a one-to-three or one-to-four ratio with their sour mash. This means that for every sour mash bourbon batch, one-third or one-quarter of the mash is made up of an old batch, and the rest is new.
Do all whiskeys use sour mash?It's also worth noting that virtually all major bourbon and Tennessee whiskey distillers use the sour mash technique, even though only a portion of them actively market their whiskeys as “sour mash.”
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What makes a whiskey a sour mash?
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