KO EN Japanese Side dish⏱ 300 minEdit history

Miso (Fermented Soybean Paste)

Name: Miso (Fermented Soybean Paste)
Prep time: 1 h
Cook time: 4 h
Total time: 5 h
Yield: 20 servings
Cuisine: Japanese
Ingredients: 600 g dried soybeans; 600 g rice koji; 240 g sea salt; 50 g seed miso; 20 g salt; 1 weight

The Japanese paste made by growing koji mould on rice, mixing it with cooked soybeans and salt, and leaving it for months. This site's fermentation guide teaches you how to keep mould out — lactic bacteria do not need oxygen but moulds do, so hold everything under the brine. Miso starts by growing a mould on purpose, in air, first. And the mould does not ferment anything: it manufactures enzymes and then stands aside, and the enzymes work for months.

Ingredients

20servings

Makes about 2kg
dried soybeans · soaked overnight600 g
rice koji · buy it ready made; do not grow your own600 g
sea salt · about 12% of the finished weight240 g
seed miso · optional; any live shop-bought miso50 g
Packing
salt · to cover the surface20 g
weight · 10-20% of the weight of the mixture1

Steps

Buy the rice koji. Aspergillus oryzae is grown from a pure spore culture at around 30°C for two days, and it is not something to hand to whatever mould is in your kitchen. Shop-bought koji should be white with a yellow cast and smell sweet, like chestnuts. Anything black, green or pink is not koji and is not used.
Boil the soaked beans in fresh water until one crushes between thumb and forefinger: 30 minutes in a pressure cooker, 3–4 hours in a pot. Undercooked beans keep the enzymes out, and no amount of waiting fixes it later.
⏲ 240 min
Reserve about 400ml of the cooking water and drain. Cool the beans below 40°C. Mixed in hot, the koji's enzymes denature on contact and die, and then nothing at all happens over the following year.
⏲ 30 min
Crush the beans with a pestle or by hand; some texture left is fine. In a large bowl rub the koji and the salt together thoroughly first (shiokiri-koji), then work in the crushed beans and the seed miso.
⏲ 15 min
Add the reserved cooking water a little at a time until it is slightly firmer than an earlobe. Too dry and the enzymes have no water to move in; too wet and liquid pools on the surface and kahm yeast settles there.
⏲ 5 min
Form fist-sized balls and throw them hard into the bottom of the crock — the point is to drive the air out. Press each layer down with your knuckles and finish by flattening the surface with your palm.
⏲ 10 min
Scatter a thin layer of salt over the surface, press film flat onto it and set the weight on top. Do not seal the lid — gas is produced. Keep it somewhere cool and dark.
⏲ 5 min
Six months to a year, through one summer. At about three months, open it, turn it top to bottom (tengaeshi) and pack it down again. Skim any white film off the surface; if black or green fuzzy mould appears, dig that part out generously and discard it.
It is ready when the colour has gone from pale beige to red-brown and it smells of miso. Moved to the fridge it holds roughly the flavour it had that day. For miso soup, take the broth off the heat first and then whisk it in.

Tips & Variations

Variations

Shiro (white) miso: twice as much koji as beans, 5–6% salt, and only one or two months. Sweet and mild; Kyoto's saikyo miso is the archetype.
Aka (red) miso: the beans steamed rather than boiled, 13% salt, one to three years. The colour deepens by the Maillard reaction, so time is literally the colour (browning and Maillard).
Hatcho miso: no rice at all — the mould is grown directly on the beans, and it takes two to three years. Astringent and very dark; Okazaki in Aichi is where it comes from.
Mugi miso: barley in place of rice as the substrate. Common in Kyushu and Shikoku.
How doenjang differs: Korean meju is a block of cooked beans left to catch whatever wild moulds and bacteria are in the air, then submerged in brine to give soy sauce and doenjang at the same time. Miso uses a pure cultured rice koji and separates no liquid off. Same beans, same salt; a different cast doing the work.

Tips

Fermentation basics tells you to keep this out, and miso grows it first. That guide's third lever reads: "Lactic bacteria do not need oxygen; moulds do. So pressing everything under the liquid is the whole anti-mould strategy." Miso's opening move is the exact opposite — a mould grown deliberately, in air, on a tray, at 30°C for two days. The one thing that changes is that a person decides which mould, which is why step 1 says to buy the koji.
And the mould does not ferment anything. What Aspergillus oryzae does is manufacture amylase and protease; once the salt goes in the mould itself all but stops. The months that follow are those leftover enzymes cutting starch into sugar and protein into amino acids — glutamate among them (building umami).
Which gives this site's enzyme thread a fourth entry. Yeot sprouts barley to import an enzyme from outside. Pumpernickel fights the amylase rye already has. Haring simply leaves the fish's own enzymes in. Miso hires an organism to manufacture them.
What separates it from cheonggukjang is the organism, not the time. That page says "same beans, same salt, different time", which is true and is half of it. Cheonggukjang is Bacillus subtilis off rice straw doing its work in three days; miso is a mould's enzymes doing theirs over a year. Whether it pulls into threads is that difference made visible.
Never guess at a mould colour. The guide's rule — black, green or pink fuzzy mould is discarded — holds here exactly as written. Buy the koji, skim the white surface film (kahm yeast), and if anything fuzzy appears, dig that area out generously. 12% salt and packing it airless are the other half of the safety.
Cutting the salt cuts the safety, not the flavour. That is why a 5% shiro miso is given two months and then refrigerated, while a paste meant to sit for a year needs 12%.
Do not boil it. As that guide already says, boiling drives off both the aroma and the live cultures. Off the heat, then whisk it in.