Rinse the soy beans, then soak them in plenty of water overnight (for a 24 hour period). The soy beans will triple in size from rehydrating, so be sure to use a large pot.
Here is what they should look like after soaking. Depending on the soy bean, scum may appear, but without removing the scum, set the pot to boil (do not refresh the water).
Bring to a boil over high heat, then reduce to the lowest setting. Skim the scum and simmer until the soy beans can be slightly crushed between your fingers (this should take anywhere from 3 to 8 hours).
This is how it should look after skimming off the scum from the surface. Do not bother to remove it too carefully; just skim enough to remove the majority of it.
Test by crushing a soy bean between your fingers, and if it can be slightly crushed, turn off the heat, cover with a lid, then let sit overnight.
Return the pot to heat and simmer for about 30 minutes over low heat. They are now ready to be turned into miso.
Prepare the necessary ingredients and equipment: 2 bowls or pots, a 10-liter pickling tub, 2 large food-grade plastic bags, and a potato masher.
While simmering the soy beans, break up the koji into small pieces, then combine with the salt.
Line the pickling tub with a double layer of the food-grade plastic bags.
Mash the soy beans (to your desired consistency) with a potato masher while still hot . Do not discard the boiling water from the soy beans.
This is what the soy beans look like after they are mashed (I mashed them to a miso-like consistency). For easy mashing, use a meat grinder, if available.
Once the soy beans have cooled to the touch, mix in the koji. Then, mix in the boiling water from the soy beans until the consistency resembles miso paste.
Pack the miso into patties, as though making hamburger patties, while pressing out any air pockets.
Slam the miso patties into the tub as hard as you can, and after several handfuls, press out the air and level the surface, then repeat.
Level the surface while pressing out the air, and very lightly sprinkle with salt.
To the extent possible, press the excess air from the inner bag and tie it closed with string.
Place a drop lid on top, then weigh the miso down with a weight equivalent to the weight of the beans. If you don't have a weight, use a bag of salt. The key is to press down on the miso with substantial weight.
Press out any air, and secure the outer plastic bag with a string. If you have red chili peppers, place them inside the outer bag to inhibit mold growth. You could also use shichimi (7 season chili spice mix) spice.
Cover the outside of the bucket to prevent dust from entering. Store it in a cool, unheated environment. Every 2 months, stir it up, and taste until it reaches the desired degree of fermentation.
Story Behind this Recipe
These days, they put additives and who knows what in store-bought miso. If you're making careful choices about what you eat, you also need to watch what's in your condiments. As long as you know the trick, it's easy to make, so give it a shot. This is the miso-making process that got me hooked.