Oxygen in the Doburoku
A couple weeks ago I had a birthday party and I wanted to show one of my guests the doburoku-in-progress. I popped open the "fermentation vessel" (really just a shallow 14-quart plastic tub that happened to fit in my cooler) to show the rice/koji/yeast mixture when my guest, in surprise, asked, "you opened it?". That got me thinking...
In traditional brewing the fermentation occurs anaerobically, in the absence of oxygen, which is why we cap our fermentation vessels with an air-lock. That lets the build-up of carbon dioxide escape without letting oxygen in the outside air contaminate the brew. That I readily opened my fermentation vessel was apparently alarming because, in doing so, I flooded the vessel with oxygen-rich air. Oh noes! I didn't think much of it because the recipe I'd been trying to adhere to instructed me to mix the brew every day, which sort-of implies that it will be opened frequently. Consequently, it made me think about the relative merits of whether or not to add oxygen to the doburoku fermentation. Normally, it's to be avoided but this process is a little different: the koji mold is woking to convert starch (in the rice) into sugar (for the yeast) and perhaps it requires more oxygen than is readily present? I finally, today, found a short blog post that satisfied my concerns.
By placing all of the koji, rice and yeast in at the same time, the koji converts the starches to sugar pretty fast while the yeast are multiplying. As the oxygen runs out, the yeast completes their growth phase. This happens before enough yeast can be produced to handle the massive sugar buildup.
Stirring does two things, it mixes the ingredients as you would expect but it also introduces more oxygen. More oxygen means we will have more yeast growth.
So, there we go. Thanks, Will from homebrewsake.com.