Fermenting: Märzen Decoction (Primary Part 3)
Fermentation is just about complete, finalizing it with a diacetyl rest - popular followup questions include, what is diacetyl, and what does the “rest” do?
What is diacetyl? In short, diacetyl is a byproduct of fermentation that gives beer a buttery off-flavor. Causes include high fermentation temperature and poor initial wort aeration. (Check out all off-flavors here!)
What does the “rest” do? As diacetyl is produced by yeast, it is reduced. Once the yeast finishes production it enters a stationary period, then then consume the diacetyl, and reduce it - this happens faster at a higher temperature. Since ales ferment at a higher temperature, they need a far shorter rest than a lager. Since I’m brewing a Märzen, I must raise the fermentation chamber to ~70F, diacetyl levels will regulate within a few days.
I’ve got the temperature dialed in to 70F, my carboy says it’s moving on up and is about 68F now.
Stay tuned, moving to the secondary on Wednesday!
White Labs - Diacetyl Time Line
Homebrewing.com - Off Flavors/Off-Flavor Wheel
HomeBrewTalk - My First Lager - When to Start Diacetyl Rest?