Koji, Enzymes and the Long Ferments of East Asia

What is koji?

Unlike lactic fermentation, the East Asian long ferments begin with a filamentous mould. Aspergillus oryzae — domesticated over more than a thousand years and selected to be non-toxigenic — grows through steamed grain, secreting a dense enzyme package as it goes.

Two enzyme families dominate. Alpha-amylase and glucoamylase cleave starch into fermentable glucose, supplying sweetness and food for later yeast and bacteria. Acid and neutral proteases cleave proteins into peptides and free amino acids, above all glutamate — the molecule responsible for savoury depth.

How is koji made?

Rice is soaked, drained, and steamed rather than boiled, producing grains that are cooked through but discrete. Cooled to around 35 °C, it is inoculated with tane-koji spores and incubated at 28–32 °C at high humidity.

After 18–24 hours the grains bind into a mat and generate their own heat; the batch must be broken up and furrowed to prevent temperatures exceeding 40 °C, which kills the culture. Finished koji at 42–48 hours is white, fluffy and smells of chestnuts and citrus. Green or brown sporulation means it was left too long — still usable for miso, but bitter in amazake.

How does koji become miso?

Miso is cooked soybeans, koji and salt, mashed together and packed anaerobically. The koji enzymes hydrolyse soy protein into amino acids over months, while salt-tolerant organisms — Tetragenococcus halophilus and the yeast Zygosaccharomyces rouxii — contribute acidity and aromatic esters.

Salt controls the clock. Sweet white miso at roughly 5% salt is ready in 3–8 weeks and tastes of rice and light sweetness. Red miso at 12–13% may age for two or three years, developing dark colour through Maillard reactions between free amino acids and reducing sugars.

Which bacteria and moulds actually ferment miso?

Three organisms do most of the work, and only one of them is a mould. Aspergillus oryzae — the koji mould — is grown on rice or barley before the miso is mixed, and supplies the enzymes, chiefly proteases and amylases, that break soy protein and starch into amino acids and sugars. It does not survive the salted mash itself; its contribution is the enzyme load it leaves behind.

The rest is bacterial and yeast work carried out under high salt. Tetragenococcus halophilus, a halotolerant lactic acid bacterium, produces the lactic acid that lowers the pH and gives miso its background sourness. Zygosaccharomyces rouxii, a salt-tolerant yeast, follows it and produces the alcohols and furanones — HEMF above all — that account for most of what people recognise as the smell of aged miso.

No single starter contains all three. A. oryzae is inoculated deliberately from purchased spores; T. halophilus and Z. rouxii arrive from the ingredients, the equipment and previous batches, which is why a miso house that has been running for decades produces a recognisable character that a first batch at home will not.

Is koji mould safe to eat?

Aspergillus flavus, a close relative, produces aflatoxins. Domesticated A. oryzae strains have lost the functional gene cluster for aflatoxin biosynthesis and are recognised as safe for food use. This is precisely why koji should be started from a commercial spore supplier rather than from wild mould.