Rice Koji (A. oryzae)

Fresh rice koji smells of sweet chestnut and mushroom, each grain bound in soft white mycelium. It is not eaten on its own — it is the enzyme engine you grow once and then spend across miso, shoyu, amino pastes, and quick garums. This recipe is the build; the Growing Koji method page covers the science and the heat-spike management in depth.

Ingredients

Koji takes no salt — it is an aerobic mold ferment, not a brined one. Quantities are dry rice plus a pinch of spore. A “Jar” here is one shallow tray’s worth (~0.5 kg dry rice); a “Crock” is a full proofer batch (~1 kg dry rice).

Component	1 Jar (small tray)	1 Crock (full batch)
Polished short-grain rice (dry)	500 g	1,000 g
Tane-koji spores (A. oryzae)	~0.5 g (1/8 tsp)	~1 g (1/4 tsp)
Soak / steam water	as needed	as needed

Yield

About 750 g fresh koji per Jar / 1,500 g per Crock (steamed rice gains weight from absorbed water). Use fresh within a few days, or dry/freeze to store.

Calculated values

Salt: none — koji is grown unsalted; salt comes later in the paste or brine.
Substrate moisture target: ~30-35% (steamed, distinct grains, no free water).
Incubation temperature: 28-32 °C / 82-90 °F ambient; keep the grain bed below 40 °C / 104 °F.
Duration: ~42-48 hours (about 2 days).
Finished pH: not an acid ferment — koji is mild, near-neutral; do not judge it by pH.

Readiness window

	Detail
Germination	Faint white haze, fruity aroma by ~24 h
Heat-spike window	Hours ~24-40; bed self-heats, must be cooled
Done	~42-48 h
Arrest	Refrigerate at 2-4 °C (a few days), or dry / freeze

Done when: 70-80% of the grains are bound in white-to-pale-yellow fuzz and the batch smells sweetly of chestnut and mushroom, with no green-yellow sporulation. Stop here — harvest before the koji goes to seed.

Method

Soak and steam the rice

Soak the rice overnight, drain well, and steam it (do not boil — boiled rice goes gummy and too wet). Steam until grains are cooked through but still distinct, targeting ~30-35% moisture. A squeezed grain crushes to paste with no hard core and leaves no water on your fingers.

Cool and inoculate

Spread the steamed rice and cool to about body temperature (~35 °C / 95 °F). Too hot kills the spores. Dust on the tane-koji and toss thoroughly so every grain is seeded — even distribution now means even growth later.

Bed and incubate at 28-32 °C

Mound the rice in a cloth-lined shallow tray, cover with a damp cloth, and hold the chamber at 28-32 °C / 82-90 °F with high humidity (90-95% early). A proofer, warmed cooler, or yogurt box works.

Manage the heat spike (hours 24-40)

Once mycelium establishes, the mold generates its own heat and the bed can climb past 40 °C even with correct ambient air. Furrow or break up the bed to shed heat, and probe the bed temperature every 6-8 hours through this window. Keep the bed below 40 °C.

Finish and arrest

By ~42-48 hours the grains are bound in white-to-pale-yellow mycelium and smell of chestnut and mushroom. Use immediately, or refrigerate at 2-4 °C for a few days, or dry/freeze to store. Do not let it sprout green-yellow spores — that means it ran too long or too hot.

Variations / mix-ins

Barley koji (mugi): swap pearled barley for rice for an earthier koji used in rustic misos.
Soy koji: grow on cooked soybeans (plus cracked roasted wheat for shoyu) — the starting point for Shoyu & Tamari.
Dried koji: dry fresh koji low and slow (below ~45 °C) to preserve enzyme activity for storage.