I grew up on my mother's homemade miso, but it wasn't until this year that I made my own. It's easier than it appears and incredibly delicious. You can find all the ingredients including domestic or Japanese dry soybeans, koji and sea salt (ara-jio), at Japanese markets, such as Mitsuwa, Nijiya, Marukai and Granada.
To start, rinse the soybeans and soak them overnight in double the amount of water. The next day, combine sea salt and koji. Set aside.
Drain the soybeans in a strainer. Cook the soybeans in a large pot for five or six hours until they can be squashed with your fingers. Set aside 1 cup of cooking liquid. Drain the soybeans in a strainer.
While the soybeans are still hot (about 100 degrees), mash them with a potato masher or a blender. Alternatively, you can put the cooked soybeans in a plastic bag and mash them with your feet. Combine the mashed soybeans with the salt and koji mixture and mix well, until the consistency becomes even. You can leave 10% to 15% of the soybeans whole if you like. If the miso mixture feels a little dry, take one-third cup of the reserved cooking liquid and pour it into the mixture. Be careful not to add too much water. Miso will soften naturally as it ferments.
Sprinkle 1 teaspoon of salt over the bottom of a crock. Put the miso mixture into the crock, mashing and pressing the miso with your hand, so there are no empty spaces or air bubbles. Sprinkle the surface of the miso mixture with a thin layer of salt. Cover the miso with plastic wrap. Cover the mouth of the crock with paper or a cotton cloth, and tie it with kitchen twine to keep out the dust. Place the crock in a cool place, like the pantry, and allow the miso to age for a minimum of six months, including one full summer.
During the fermentation, do the following:
After one month, take a big wooden paddle and turn the miso. Take what is at the bottom and bring it to the top and mix. Cover the crock as you did before and return it to a cool place. Repeat this step two more times, two and three months later. The color of the miso will gradually darken from yellow to caramel to brown to reddish brown. The miso will have a sweet, earthy smell. If you see any white mold on top, remove it with a spoon. Remove a month's supply and refrigerate. Re-cover the crock and return it to a cool place.
Miso tastes best after one year and keeps for 18 months in a cool place.