🥃 Sour mash

Sour mash is a method of making American whiskey in which part of the backset is added to fresh mash. Backset is the sour liquid left after the previous distillation. The acid lowers the pH, holds back unwanted bacteria and makes fermentation predictable. Almost all bourbon and Tennessee whiskey is made this way.

History. According to The Oxford Companion to Spirits & Cocktails, rum distilleries in the British Caribbean were adding backset («dunder») as early as the 1750s, and American manuals recommended it in the 1810s. The method became common in the mid-19th century. It is often credited to the Scottish doctor James Crow, who lived in Kentucky from 1823; according to scotchwhisky.com, however, Crow rather organised the process by measuring temperature, density and acidity.

How it is made. Backset is a sour liquid without live yeast, rich in organic acids. According to the Oxford Companion, it usually makes up 10–25% of the volume of fresh mash. Whisky Advocate gives an ideal fermentation pH of about 4.8–5.0. The word «sour» describes the process, not the taste: the whiskey does not taste sour.

Sweet mash. The opposite method is mash without backset. According to Whisky Advocate, small distilleries and those distilling in copper pot stills, such as Wilderness Trail or Castle & Key, mostly choose it. The big names — Jim Beam, Jack Daniel's, Evan Williams — work by the sour mash method. Some brands, such as Michter's or George Dickel, have put the term into the name of a product line.

Why it matters to you. The words «Sour Mash» on a bottle do not mean a separate type of whiskey or higher quality: it is a common technique that most bourbon uses without mentioning it on the label. The type is defined by the grain mix and ageing, so check whether it is a bourbon, a rye or a Tennessee whiskey. The WHO reminds us that there is no safe level of alcohol consumption.