By Chris Middleton

The lost history of sour mash

How acidity came to create more complex flavours
The sour mash method made Tennessee whiskey and Kentucky Bourbon famous. It’s also known as yeast-backing and back-setting in using a similar method practised for thousands of years to make sourdough bread and beer by preserving the yeast culture from batch to batch.

In Bourbon production, a residual of the distilled spent wash from the still is added to a fresh batch of wash to be fermented. Bourbon distillers usually add up to 35 per cent of the spent wash (also called backset, slops or stillage). In other distilling industries, it’s called acidification, lees or dunder. The carbon dioxide bubbling through the wash forms carbonic acid during fermentation, causing the beer to taste mildly sour.

Peter Shaw advocated that 17th century English distillers acidify or sour their washes by adding the juice of lemons, oranges or tamarinds to the still, thus lowering the pH levels and improving the spirit taste.

Jamaica in the tropical West Indies around 1700 started back-setting their rum fermentation. They acidified the fresh wash using a third or more of the volume of the distiller’s spent wash calling it dunder. By the 19th century, the recycling of distiller’s spent wash had evolved from adding fresh dunder to making more complex biochemical flavours where dunder was left for many months to be infected with bacteria, wild yeasts and microflora creating new congeners, amino acids and esters. Jamaican plantation owners had extensive trade relations and property in the North American colonies, so the transmission of dunder was likely adopted by numerous rum distilleries operating along America’s Atlantic coast. While the tropical conditions did not plague the early Virginian and Pennsylvanian whisky distillers, knowledge of yeast-backing was evident in the first American manuals on distillation.

Yeast-backing or the sour mash method as it became known in the Bourbon industry, delivers the distiller several significant benefits. Firstly, by reducing the mash’s pH, it inhibits bacterial growth creating a more hospitable environment for the yeast to propagate. In the olden days, hygiene and cleanliness were not understood. Add the region’s hot, humid climate, the risk of infection and spoilage multiplied; hence, why distillation was conducted in the cooler eight months and suspended during summer.

Sour mash also produced greater flavour consistency batch-to-batch by limiting bacterial contamination. Distilleries west of the Appalachian Mountains also gained the benefits of hard water filtered through a karst limestone platform running from Canada to Alabama. Yeast cells thrive in the calcium and magnesium carbonates in hard water furnishing the budding yeast with a soup of nutrients from the previous setback’s yeast’s dead, autolysed cells supply extra nitrogen, vitamins and other nutrients for metabolisation. Hard water is also high in alkalinity, by adding spent wash it reduced the new unfermented wash to an ideal reproductive environment for the yeast.

The region's acclaimed hard water and climate were major factors to why sour mash stabilised the fermenting conditions allowing yeast strains to populate unencumbered giving the fermented mash desirable flavour compounds. Sour mash even performed better in the new steam stills entering the industry during the early 19th century.

Fresh yeast from dona jugs or pressed yeast supplies were not always available. Distillers could harvest yeast spores from orchards and grain; others found sour mash a workable alternative as a yeast-starter. Also by leaving the cooked mash tubs exposed to the elements for a week, the mash could be soured before distilling. As late as 1912, the US Government recognised three yeasting methods. Yeast-back or sour mash, commercial yeast replacing sweet mash, and ‘wild yeast, the old time method, few use’.

While Tennessee distillers converted to the sour mash method before the Civil War, Kentucky continued this method into the 1870s.

Today, most Bourbon distillers use the sour mash method, although it is an option not a legal requirement for Bourbon or Tennessee whiskey.