Koji, Enzymes and the Long Ferments of East Asia

A mould that makes enzymes

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.

Growing koji

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.

From koji to 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.

Why koji is safe

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.