Free Koji Cultivation & Incubation Calculator
Master artisanal koji making. Calculate dry grain soaking hydration, tane-koji spore inoculation dosing, cedar tray surface area, and the critical 48-hour aeration schedule.
🌾 Grain Substrate & Batch Target
Steamed Grain Yield: ~1,350 g (35% moisture absorption)
• Hour 0 - 18 (Germination): Wrap in damp linen at 86°F - 90°F (30°C), 90% RH.
• Hour 18 - 24 (Te-ire / First Mix): Break clumps, spread into cedar trays.
• Hour 24 - 40 (Peak Metabolism): Intense fungal respiration! Air out to keep core < 104°F.
• Hour 42 - 48 (Harvest): Grains bound in snowy-white sweet-smelling mycelium.
Steam, Never Boil: Koji grain MUST be steam-cooked, never boiled like table rice. Boiling bursts the outer starch cells, creating sticky porridge that suffocates the aerobic mold. Steaming yields a dry, firm exterior with a fully gelatinized, tender interior.
Recommended Tools & Equipment
Tested hardware and components for high reliability
The Art & Science of Koji Cultivation
Koji is the biological engine behind East Asian fermentation. The filamentous fungus Aspergillus oryzae colonizes grains, secreting powerful digestive enzymes:
- Amylases ($alpha$-amylase, glucoamylase): Break starches into sweet glucose sugars (essential for sake, mirin, and amazake). Favored by higher incubation temperatures (98°F to 102°F / 37°C - 39°C).
- Proteases & Peptidases: Cleave complex proteins into savory glutamic acid and amino acids (essential for miso and soy sauce). Favored by cooler incubation temperatures (86°F to 91°F / 30°C - 33°C).
The Critical Steaming Hydration Target
Grain moisture control is everything:
Post-Soak Target: 128% to 132% Weight (soak until grain snaps cleanly between fingernails)
Post-Steam Target: 135% to 138% Weight (firm "rubber band" bite, never mushy)
Frequently Asked Questions
What does good, healthy koji smell and look like?
Healthy finished koji smells intoxicatingly sweet, like fresh chestnuts, passionfruit, and mushrooms. The grain kernels should be bound together in a solid, felt-like white cake of mycelium that penetrates deep into the heart of each grain.
What should I do if my koji turns green or black?
If your koji turns yellow-green or olive, the Aspergillus mold has completed its life cycle and produced mature spores. It is safe for making miso or sake starter, but has higher bitterness. If it turns pitch black, slimy, or smells of sour vinegar or rotten fruit, dangerous wild molds or bacteria have taken over; discard immediately.
Why are cedar morobuta trays traditionally used?
Cryptomeria (Japanese cedar) has natural antifungal tannins that suppress wild bacteria, and its porous grain acts as a natural humidity buffer, absorbing excess condensation when the mold sweats without drying out the substrate.