Sour mash or as it is often called in Scotland, ‘Back Set’ is where the pot ale or clarified spent wash is used as part of the mashing liquid. It is often incorrectly stated that this is where the yeast comes from, no, the yeast is all dead by that stage. A similar process is sometimes used in rum production.
The practice of sour mash was apparently developed in the first half of the 19th century by James C Crow, who studied but did not graduate from University of Edinburgh. He is often referred to as Dr Crow, but he may never have earned that title in his life time. It is more likely that he simply perfected the process by taking a more scientific approach and understanding to what was already a common practice. As a distiller the good Doctor was faced with a number of problems, namely the water was at the wrong pH, the fermentations were poor, high levels of bacterial infection, and the product was inconsistent.
He used his scientific training and experimented with a number of things, some of which resulted in the sour mash process used today. The pot ale has a low pH while his mash had too high a pH, so by putting some of the low pH pot ale into the mash, the pH moved into the correct zone for the enzymes in the malted grain and the yeast to work better. Also, the pot ale has a lot of yeast nutrients present so the yeast worked even better. As a result of these modification the spirit yield at the distilleries he worked at increased significantly.
The word sour is not a reflection on the taste of the spirit but the sour taste of the low pH pot ale. The water supply to most Scottish malt distilleries is already at the correct pH and for that reason the practice is not normally carried out here.
Dr Crow is only remembered for his sour mash process, but he also introduced good distilling practice, process hygiene and brought scientific rigor to the whisky production process. Process hygiene is even more important in the warm climate of Kentucky. All of which improved the process and helped to maintain a consistent product.
Changing the mash pH changes how the yeast works, which results in a different flavour profile of the spirit. The esterification works better in a more acid environment, so we get more esters.
Here at InchDairnie we have made some spirit using the sour mash process. We have replaced some of our mashing in water with some fresh pot ale drawn off direct from the wash still into our mash conversion vessel. The bench top experiments carried out with our good Fife friends at Tatlock and Thomson showed the extent the pH would reduce while the wash ferments. Armed with this information we, along with our yeast supplier AB Biotek, selected a yeast variety that would tolerate this low pH and still keep fermenting. The resulting fermented was distilled in our wash and spirit stills. The spirit is now maturing in ex bourbon casks.
The resulting spirit flavour was more floral and fruitier but some of the background complexity was lost as compared to our Strathenry new make spirit.
For a more detailed history of Doctor Jim Crow look out for the writings of Chris Middleton, www.thewhiskeywash.com