There are a few variations on this simple fermented sauce, however, they usually involve the same three ingredients. It’s the proportions of salt/water/koji that vary. I’ve chosen to go with a happy medium, however, feel free to increase the water up to 3 cups for a less salty sauce or decrease it to 1 cup for a saltier sauce. If you’re looking to make a soy sauce replacement, you’ll want to use less water. I realize this seems backward, but soy sauce is an intensely rich and salty sauce. Using less water will provide a better substitute.
Combine the water, koji, and salt in a glass jar. Mix everything well to break up any clumps of koji rice.
Cover the jar with a cloth or a coffee filter held in place with a rubber band. Leave it to ferment at room temperature for 7 to 14 days. Stir once a day to aerate it (koji needs oxygen). After 7 days, lick the spoon after stirring to see how it tastes. The shio koji is ready when it starts to taste sweet and savory. (See the section above for details).
Once it has finished fermenting, you can either strain out the rice or purée the mixture in a blender. Even if you strain out the rice, I recommend keeping it for other culinary uses. See the section above for details.
Store the finished shio koji in an air-tight jar in the fridge. Shio koji will last in the fridge for up to 1 year.
I do a LOT of fermenting… at any given time I have milk kefir, sourdough, and a fermented beverage. (At the time of writing this, I have both tepache and kombucha going.) So my shio koji always catches a bit of wild yeast too. The wild yeast causes it to carbonate slightly. This is perfectly fine. The yeast will slow down once the shio koji is stored in the fridge.
It’s also fine to store shio koji at room temperature in a dark location (like the cupboard you store your vinegar and oils in). However, I recommend on using it up within 1 month when stored at room temperature.