Assume I know how to properly ferment grain and distill it into whisky, and that I have all the necessary equipment. If I want a "sour mash" whisky, how would I go about it?

From a bit of research, it seems that the term "sour mash" simply describes some sort of process where the distiller saves a bit of the old mash (maybe he/she "cultures" it somehow, to grow enough live yeast?) and adds it to a new batch to facilitate fermentation?

Is there anyone familiar with the process who could describe how to apply it to an existing hypothetical small-batch whisky making operation?


2 Answers 2


In the sour mash process, the distillers beer is distilled, and a portion of the spent beer is added to the next batch. Many organic acids are not very volatile, so they will remain in the spent beer. Adding this to the sweet mash will add free amino nitrogen and organic acids to the sweet mash. Yeast likes a slightly acidic environment, and many bacteria and molds do not. Adding the spent beer help keep these spoilage organisms away while also adding nutrients to the mash. The spent beer also helps with flavor consistency.

  • So when you say "spent beer" you are (are you?) talking about the wash product that is left in the still after distillation is complete? Or are you talking about using some of the fermented beer from prior to distillation? Jul 14, 2017 at 1:21
  • Rereading I think I understand: beer left after distilling. Less volatile acids would not boil off in the still and would remain in the leftover liquid, I save some (how much?) and add it to the wash from the next batch prior to adding yeast and fermenting? Jul 14, 2017 at 1:24
  • The term "wash" is not used in bourbon AFAIK. Some distilleries add the spent beer (also called backset, setback, or sour mash) to the mash cooker, and some add it to the fermentor after cooking. You can try to go with a percentage, or aim for a specific pH. A message board I found through google suggested 20%-25%. And, yes, spent beer is the leftovers from distillation with the solids removed. In bourbon, the solids remain in the beer and go through the still. Jul 14, 2017 at 1:38
  • OK, this is in alignment with my thinking. What I call "wash" is the cooked liquid (grain solids strained out) that is cooled, fermented, then distilled. I call it "mash" after fermentation. I have no idea if my terminology is correct. So if I want to end up with a sour-mash bourbon style product, I will leave the grain solids in and add some "setback" before fermenting. Thanks for all the info., much appreciated! Jul 14, 2017 at 12:47

This - kind of - answers your question. The breakdown of sour mashing in beer cannot be too different. The article linked explains the process with amazing detail. Here are some pertinent highlights:

Sour mashing requires only a small deviation from your normal routine and has three goals:

 1. Create an optimal environment for Lactobacillus.

 2. Prevent spoiling organisms from producing foul aromatics and flavors.

 3. Drop pH to produce desired amount of acidity/sourness.

The main difference between sour mashing and kettle souring is that sour mashing occurs before sparging with the mashed grains still present while kettle souring occurs after sparging with the full pre-boil volume (and frequently in the boil kettle). Additionally, sour mashing will have a moderately higher gravity because it is undiluted first runnings.

Sour Mash Differences I assume the sour mashing aspect of this is what happens when distilling whiskey.

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