Soak the soybeans in water overnight. Drain off the water the next day, and cook in fresh water slowly for about 4 to 5 hours. Skim off the scum that comes out of them carefully.
When the soybeans are soft enough that you can crush one between the thumb and forefinger of your opposite hand (your left hand if you're right-handed, and vice versa), put them in a container that you have sterilized with ethanol and mash them.
Reserve about 2 handfuls of the salt. Mix the remaining and salt and the rice koji together. (This mixture is called koji cut with salt.)
Mix the soybeans and the salt-koji together well. Add the cooking liquid from the soy beans until the mixture has a soft doughy texture, about the same as your earlobe feels.
Form the mixture from Step 4 into balls about the size of baseballs.
Spray a plastic storage container with ethanol, and sprinkle the bottom with 1/2 of the salt that was reserved in Step 3. Press the balls from Step 5 tightly into the container, leaving no gaps.
Cover the top with the remaining salt and spray lightly with ethanol. Cover with a piece of plastic wrap, spray with more ethanol and put on another layer of plastic wrap.
Wipe off any soy beans that are stuck to the sides of the container, spray with more ethanol and put the lid on. Wrap in newspaper and leave in a cool dark place.
Open it up in about 5 months to check on it. Mix it up from the bottom with a clean paddle.
Spray lightly with ethanol and cover with 2 sheets of plastic wrap.
It's ready to eat after about a year.
Story Behind this Recipe
My husband doesn't like sweet miso, so I make salty miso, too. It's left to mature for about a year, longer than for sweet miso. I use it along with sweet miso. See Recipe ID: 2131552 for my sweet miso recipe.
The amount of salt to add is calculated based on the following: The cooked soybeans weigh 2.5 times the dried soybeans, 450 ml of seed water, and 14% salt content. Shima maasu is about 92% sodium, so I re-calculate with that in mind. The ratio of soybeans to dried rice koji is 1000 : 500 to 800 or so. The more koji you add, the better tasting your miso will be.