Another exciting bit of information to come out of my visit to the George Dickel distillery was learning more about the sour mashing process.
To review (from a recent post about souring at Jack Daniel's):
"Souring" is not like sourdough starter in that the process' job is to remain consistency between batches. "Souring" means lowering the pH, making it more sour/acidic. "Sour mash"ing is adding backset (stillage) from the previous distillation run to the next one. The stillage contains lots of dead yeast that is food for the new yeast, but also it is acidic and lowers the pH.
At Jack Daniel's they use 30% backset/sour mash. That seems like a lot, so I was wondering: If the purpose of it is to simply lower the pH and provide nutrients, wouldn't a far simpler way to do that be to add some dried nutrients and acid like citric or hydrochloric acid?
Dickel's distiller Nicole Austin set me straight: the sour mash process is smart industrial waste reuse. Yes, the yeast get nutrients from the sour mash (dead yeast; they're cannibals like that). But the distillery also gets recycled water and heat out of it.
The sour mash is the hot liquid that comes off the still, separated from the grain solids but still containing dead yeast and other small particles. It has just been boiled during distillation, so obviously it's hot.
That stillage is added to the cooking as well as the fermentation process at Dickel. In preparing grains for distillation, they are ground up, cooked with hot water to break down carbohydrates into fermentable sugars, and then fermented in the next step. The cooking requires lots of super hot water, and a free source of hot water is the stillage!
At Dickel they also add some stillage/sour mash to the fermentation process as well as in cooking.
Austin says that in the ideal situation, you'd use all stillage water for cooking and fermentation - that would save fresh water. But the dead yeast solids are a limiting factor - too many of them and they stress the live yeast that needs to ferment the cooked grains. So the stillage water needs to be watered down, essentially. Additionally solids retain a lot of heat so it doesn't cool down as fast and this limits how much they can use.
I asked Austin if the sour mashing process then actually has much impact on the flavor of the finished whiskey. She said that it's basically "setting yourself up for success" with a good fermentation.
In conclusion, in addition to acidifying and providing dead yeast as nutrients for a healthy fermentation, the sour mash process is industrial recycling, using stillage both for water and for heat.
Hooray for new-to-me information!