What is sour mash in whiskey?

With sour mash, part of the spent mash liquid from the still, called backset, goes back into the next mash before the yeast is added.

That liquid stays behind in the still after the alcohol has been taken out. In the trade it is called backset, setback or stillage. It is a sour, thin liquid without alcohol.

The distiller mixes that with the fresh, cooked grain and with water. Only then does the yeast go in. Fermentation therefore starts in a more acidic mash than when only clean water is used.

The word sour refers to that acidity in the vat. It is a process term from the distillery.

Backset lowers the pH of the fermentation, so bacteria get less of a chance and successive batches come to resemble each other more.

Research into the bacterial community in mash for distilled spirits describes how adding backset or stillage lowers the pH of the fermentation. That slows the growth of bacteria and keeps the batches more consistent.

Lactic acid bacteria are the biggest concern here. They can displace the yeast and spoil a fermentation. In the same study they make up the biggest shift in the bacterial community during a whiskey fermentation.

For a distillery that wants to make the same whiskey year after year, that stability is the heart of the matter.

Sweet mash is a fermentation in which no backset from an earlier batch is used, so the mash consists only of grain, water and yeast.

The difference therefore lies in one ingredient. With sweet mash the fermentation starts at a higher pH. There is no acidic liquid from the previous round keeping the bacteria in check.

That demands more of the hygiene in the distillery and of the pace at which the yeast gets going. In the trade literature, sour mash and sweet mash are set side by side as the two ways of starting a mash for distillation.

27 CFR part 5, subpart I sets out the standards of identity for distilled spirits, and the term sour mash does not appear there.

That subpart runs from paragraph 5,141 through 5,166 and regulates, among other things, what whisky is. The whisky class is distilled from a fermented mash of grain, distilled at less than 95 percent alcohol.

Within that are the types. Bourbon requires a mash of at least 51 percent corn, distillation to at most 80 percent alcohol, casks of new charred oak and a cask fill of at most 62.5 percent alcohol.

Sour mash is not in any of those tables. So there is no legal standard that lays down how much backset must go in or how acidic the mash must be.

Sour mash on a label describes the working method in the distillery and says nothing about the class or type of whiskey in the bottle.

The designation does appear in official documents. A ruling of the American tax authority for alcohol and tobacco from 1954, for example, mentions Sour Mash Bourbon Whisky as a name on a label.

The mandatory designation on such a bottle, however, remains the class and the type. Those two determine what you are buying. Sour mash at most sits alongside them, as an additional statement about the making.

Two bottles that both state sour mash can therefore belong to different types. The term does not distinguish them from each other.

Sour mash refers to the acidity of the mash in the fermenter and not to sourness that you taste back in the finished glass.

The backset does not go into the cask. What remains in the still after distilling is used to start the next fermentation. The distillate itself then goes on to maturation.

Anyone who visits us in Bathmen will find 163 whiskies from the United States from 59 brands, out of a total of 3,883 whiskies. We are happy to walk you through the designations on those labels.

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