Before the spores · soak, steam, cool
Substrate Prep
Soak 2-4 hours, steam 15-45 minutes, aim for 25-35% moisture. Both ranges are wide because they depend on things nobody writes down — how much you are cooking, how cold it went into the pot, what shape your lid is, how you cooled it. This works out where in them your batch actually lands, and tracks every gram of water on the way.
New to this? Start here
Koji is a mould, and moisture is the single thing it is fussiest about. Too dry and the starch never gelatinises, so there is nothing for the enzymes to work on. Too wet and the grains stick together, the mycelium sprawls across the surface instead of digging in, and Bacillus gets a warm nutritious bath. The surprise is where the water comes from. Soaking is the obvious step, but steaming adds around 100 g of water for every kilo of dry grain you started with, by condensing on the food, and cooling removes a chunk of it again — so how you cool matters as much as how long you soak.
- Wet-basis moisture —
- water as a percentage of total weight. 30% means 300 g of water in a 1 kg batch. Every figure here is on this basis.
- Gelatinisation —
- starch absorbing water and softening under heat. It needs both, which is why an unsoaked grain stays chalky inside no matter how long you steam it — and why amylase can barely touch it.
- Condensate —
- steam turning back into water on the cold food. It is how a steamer delivers heat at all, and it is water you did not measure out.
- Al dente —
- the texture target — resistant to the bite, never mushy, never raw. It is a moisture reading in disguise.
Your batch
The reference case behind the familiar numbers: a 2-4 h soak and a 15-45 min steam. Both ranges are load- and temperature-dependent, which is what this tool resolves.
Soak & drain
Steam
Condenses over the whole underside and drops it straight onto the food.
Cool down
The most underrated moisture lever here. Spores need the substrate below 40°C either way — but how you get there decides whether the batch sheds 46 g of water or almost none.
Finishing
Set it below where your process naturally lands and the tool works out how much water has to be driven off, and how long that takes.
- into the steamer
- 32.4%
- spores go on
- 36.3%
- harvest
- 32.1%
A steamer has no thermostat. It runs at whatever temperature water boils at where you are, delivers heat by condensing water onto the food, and loses some of that heat through the lid — which comes straight back down as drips.
- Steam temperature
- 100.0°C
- 101 kPa at 0 m
- Goes in at
- 21°C
- a 79 K climb
- Come-up energy
- 239 kJ
- c_p 2.32 kJ/(kg·K)
- Condensed on the food
- 106 g
- energy ÷ 2257 kJ/kg latent heat
- Dripped off the lid
- 21 g
- over 26 min
- Bed depth
- 4.1 cm
- in a 24 cm basket
- Cool it to
- 32°C
- spores go on below 40°C
- Evaporated on the way
- 46 g
- 45% of the heat, over ~35 min
- In the 35-50°C band
- 13 min
- no spores on it yet
- Water to drive off
- 49 g
- at 100°C
- Roughly
- 12 min
- on the same tray area
Every gram of water, step by step
Through every prep step the dry solids block stays exactly the same width — nothing you do to a grain adds or removes solids. Everything else is water moving in and out, and each step is a lever you control. Only the last row breaks it: the mould respires some of the substrate away as it grows.
- As bought1000 g · 12.0%
12% moisture, 880 g of dry solids that stay constant from here on
- Soak · 3.0 h at 20°C+463 g water · 1463 g · 39.8%
239 g absorbed into the grain — 93% of what it can hold, and the curve is nearly flat past here — plus 223 g clinging to the outside
- Drain · 30 min−161 g water · 1302 g · 32.4%
161 g shed, 62 g still clinging to the outside — this is the water that turns a steam into a boil
- Steam · 26 min at 100.0°C+127 g water · 1429 g · 38.4%
106 g condensed bringing it up to temperature, 21 g dripped off the lid
- Cool to 32°C · ~35 min−46 g water · 1382 g · 36.3%
45% of the heat released leaves as vapour, taking water with it
- Incubation · 48 h, ~10% mass lost−103 g water · 1244 g · 32.1%
Respiration and sweating over the cycle — 1000 g in, about 900 g out. Mostly water, so the substrate dries as it grows
Why soaking is not about softness
Starch needs water as much as it needs heat. Where the substrate sits on this axis when it goes into the steamer decides whether amylase has anything to eat at all — yours goes in at 32.4%.
Starch needs water as much as it needs heat. Short of about a quarter moisture it cannot fully gelatinise at any steaming temperature, and un-gelatinised starch is close to invisible to amylase. The koji still grows — it just has far less to work with, which is why an under-soaked batch can look perfect and taste of nothing.
The window called al dente: resistant to the bite, never mushy, never raw. Gelatinised on the inside where amylase needs it, dry enough on the outside that the grains stay separate and the mycelium is forced to go inward rather than sprawl across a wet surface.
Grains gelatinise into one another and free water sits on the surface. This produces "lazy" koji — flawless white fluff on the outside with no penetration and very low enzyme yield, because the mycelium never had a reason to dig. Wetter still and it is a Bacillus culture with a mould problem.
FAQ
Next: how much starter this batch needs is Spore Inoculation, the chamber it grows in is the Incubator Simulator, and steering the result toward sweetness or umami is Enzyme Balance.