Growing koji on brown rice is a little trickier than on white rice.
If I treat brown rice exactly like white rice, I will get a batch that is poorly grown, one that is not effective. Poor koji growth, poor enzyme action. Bad miso that doesn’t mature well, amazake that’s not sweet, shio-koji that doesn’t do it’s job.
The problem is the “brown” part, the outer bran layer. Koji can’t take hold on the bran layer. So to make brown rice koji, something needs to be done to the bran to let the koji sneak into the inner, starchier parts of the rice.
Luckily for us, rice bran is not so sturdy. it can be scratched and broken only with a bit of scraping. This can be done when the rice is still dry (maybe a few whirls in the food processor) or after the rice is soaked (again, maybe a few whirls in the food processor) or after the rice has been fully steamed before or after sprinkling the spores (vigorous rubbing or mixing). I have had better luck after steaming, but it depends on your equipment and preference. If you happen to own a rice mill, then you can set it to polish just a tiny little bit. Just enough to lightly damage the outer layer.
So this is an example of a pretty good batch. You see there’s not much growth on the outside, but look closely where the koji is actually growing.
Koji is in the cracks. Cool, yes?
Easiest way to see how good a batch is… cut the grain open. if you see extensive koji penetration inside every grain, you are OK. Not sure if you can see in this photo clearly, but the second pair from far left is the worst. But look at the others. Pretty good penetration.
If you are still not sure after visual inspection, the next thing to do is to make amazake. The flavors will tell you what you grew. After that, as always, the ultimate test is using it to make what you wanted to make in the first place, then tasting it when you think it should be done. Did the koji deliver?
It took me more than a hundred batches of growing koji and playing with them to be able to tell with confidence. Experience should never be underrated!
This batch of brown rice koji went straight into brown rice miso. I am going to wait at least a full year for it to be ready. Maybe 18 months. Miso with brown rice koji takes longer than miso made with white rice koji because of the bran, but the flavors are complex and deep, also because of the bran. Totally worth the extra trouble, I tell myself every year.