Homemade Miso

Rinse soybeans under cold water, sorting out any shriveled or discolored beans. Cover with at least three times their volume in cold water and soak for 18–24 hours at room temperature. The beans will roughly double in size and the skins will become taut. Drain and discard the soaking water — it contains flatulence-causing oligosaccharides. The beans are ready when a fingernail presses through one easily with minimal resistance.

Cover soaked soybeans with fresh cold water by 3 inches and bring to a boil over high heat. Skim and discard the grey foam that rises in the first 10 minutes — this is bitterness. Reduce to a vigorous simmer and cook for 3–5 hours until the beans are very soft: they should crush completely between two fingers with no resistance. Reserve 2 cups of the cooking liquid (tamari-like, deep amber, savory). The beans must be soft enough to mash — under-cooked beans create a lumpy, gritty miso that ferments poorly.

Drain the cooked soybeans (reserving cooking liquid) and spread on a sheet pan to cool to below 40°C (104°F) — hot beans will kill the koji mold spores. Once cool enough to hold comfortably, mash using a potato masher, food mill, or by pressing in a zip-lock bag. The goal is a smooth, uniform paste with minimal bean chunks. Some texture is acceptable in country-style miso; total smoothness is the goal for refined miso.

In a large bowl, combine rice koji and salt. Toss with your hands until fully mixed — the salt coats each grain of koji. Add this koji-salt mixture to the cooled bean paste. Mix thoroughly with your hands, kneading and squeezing until the koji is fully incorporated and no white koji clumps remain. If the mixture feels too stiff and crumbly, add reserved cooking liquid tablespoon by tablespoon until the paste holds together when squeezed into a ball. The mixture should hold its shape like wet clay.

Sterilize your fermentation crock or container with a clean cloth dampened with shochu or vodka. Form the miso mixture into baseball-sized balls and throw them firmly into the vessel — this packing motion eliminates air pockets, which cause mold growth. Press each layer firmly flat with a flat spatula or your fist. When all the miso is packed, smooth the surface completely flat. Sprinkle a thin layer of salt (about 2 tbsp) over the entire top surface as a protective barrier. Press plastic wrap directly onto the surface, then add a weight (a sealed bag of salt works well).

Store at cool room temperature (60–75°F / 15–24°C), away from direct light. White or tan surface mold (tamari) is normal and expected — scrape and discard at the 30-day mark when you inspect. Shoyu-like liquid that pools on the surface is tamari: stir it back in. Check monthly: at 45 days the miso is a young shinshu style — golden, mildly sweet, and light-bodied. At 90 days it becomes a rounded, complex pale yellow miso. At 6 months: full-bodied aka miso. At 1 year: robust, dark, concentrated hatcho-style depth.