Koji is a commonly used ingredient in Japanese cooking. I first came across the concept of fermented rice when I was very young, and my mom made mi jiu, which means rice wine. But it’s actually mushy sweet rice with extremely low alcohol content. She regularly put it in dessert soups, and sometimes I ate it with a spoon out of the bowl. There are no laws against underaged consumption of fermented rice, right?
The Chinese version, which requires super clean hands, has been an elusive. I keep growing mysterious pink and black mold. But the Japanese version, where the proper kind of mold has already been grown onto the rice and dried, is much easier to manage.
You might be able to find Koji already made, or in a tub as dry rice like this at your local Asian food store.
I made 3 kinds. Sweet koji (which apparently people don’t really do), Shio-koji or salt koji, and Shoyu-koji or soy sauce koji.
For the salt koji, I used 3:1 koji to salt ratio.
For the soysauce koji, I used just enough soysauce to come to the top of the rice, and the covered the rice with a little water.
Each batch is mixed with a clean utensil every day for 7-10 days until the rice is a little mushy. The salt koji will go from unbearably salty to slightly sweet. The sweet koji will most likely be done first, since without the high salt content, the mold grows uninhibited.
Once done, put the sauces in the refrigerator.
These sauces can be used as a marinade for fish, or just as salt. Instead of using a 1 tsp of salt, I would use 2 tsp of the salt or shoyu-koji.
I marinated some fish with 2 tsp of salt koji and 4 tsp of sweet koji. It added so much umami to the fish that I didn’t need anything else! I think because of the alcohol content, the flavor permeated the fish well. This photo is of some kind of a thai fish. There was also cod, but I forgot to take a picture before it got eaten.
This is definitely the kind of sauce that you want to keep in the back of the fridge. It tastes super fancy, and stays for a long time.
If you’re more adventurous than I am, you can even turn it into sake.