Sour mash is a fermentation method where distillers add a portion of an earlier, already-fermented mash — called the backset or setback — into a fresh batch. The acidity keeps the pH stable, holds off unwanted bacteria, and keeps flavor consistent from batch to batch. Despite the name, sour mash doesn't make the whiskey taste sour.
Where the name comes from
Think of it like a sourdough starter. Each time a baker bakes, they hold back a little of the last batch to start the next one — passing along continuity and consistency. Sour mash works the same way: distillers save a portion of the leftover liquid from a previous distillation (the backset, also called stillage or spent beer) and stir it into the next batch of fresh grain mash.
Why distillers use it
That splash of already-fermented mash does real work:
- pH control — the acidity creates the ideal environment for yeast to thrive.
- Consistency — it carries character forward, so each batch tastes like the last.
- Protection — the lower pH discourages unwanted bacteria from spoiling the ferment.
It's so effective that nearly every Kentucky bourbon you've tasted was made with a sour mash process, even when the label doesn't say so.
Sour mash vs. sweet mash
The opposite approach is sweet mash, which uses only fresh grain, water, and new yeast with no backset. Sweet mash is rarer and trickier — without that acidity to protect it, the ferment is more vulnerable. A handful of distilleries make sweet mash whiskey as a specialty, but sour mash remains the backbone of Kentucky bourbon.
Taste what the process builds
The consistency sour mash creates is easiest to appreciate when you taste barrels side by side. That's exactly what our Premium Tasting is built for — pours straight from the barrel inside a government-bonded rickhouse. see all our experiences.
Keep reading: What is bourbon made from? · What proof is bourbon? · How long is bourbon aged?