Koji biology · which starter, how much
Spore Inoculation
"10 g of spore per kg" has meant three different things across this site — pure koji-kin, tane-koji, or a saved tomo-koji batch — and they are not the same product at the same dose. Pick what you actually have, and get the right number.
New to this? Start here
Every koji batch needs enough live mould to out-race Bacillus, which survives steaming as a dormant endospore in almost every batch (see the Incubator Simulator and Enzyme Balance tools). How much "enough" is depends entirely on which starter you have — kin is undiluted potency, so a little goes a long way; tane and tomo carry the same spores diluted onto grain or a saved batch, so they need more mass to deliver the same live-spore count.
- Koji-kin —
- pure, isolated mould spores, lab grade — no rice, no carrier. The cheapest and longest-keeping of the three, and the least forgiving to measure.
- Tane-koji —
- kin already dispersed onto grain — the starter most home cooks buy. Close to foolproof.
- Tomo-koji —
- a saved, spore-heavy batch used as a starter instead of a bought product. Not recommended here — see the safety section below.
- Competitive exclusion —
- the actual reason every dose here is dense rather than dainty — koji has to dominate the substrate before contaminating bacteria do.
The three products, side by side
Same mould, three ways to buy or make it — and three different doses because of how much carrier weight (or bacterial baggage) comes along with the spores.
- Shelf life —
- Lasts longest in the fridge of the three.
- Prep —
- Needs a scale that resolves 0.1 g, and, dosed undiluted, sterile technique to spread evenly. Diluting it into rice flour first trades a few minutes of prep for a dose you can actually see and spread.
Pure genetic potential with no carrier weight — the most professional, cost-effective route if you make koji often, and the least forgiving to measure.
- Shelf life —
- Shorter fridge life than kin, but still keeps for months.
- Prep —
- Already diluted onto grain — no flour dispersant to mix. Grind it and the pulverised rice carries the spores evenly through a fresh batch on its own.
Kin, pre-diluted for you. Takes more shelf space and expires sooner than kin, but is close to foolproof for a home setup.
- Shelf life —
- Must be dried and frozen immediately, or it does not keep at all.
- Prep —
- No prep beyond harvesting and dehydrating your own koji — the whole appeal, and the whole risk.
How koji was made for centuries before modern microbiology — romantic in theory, chaotic in a home kitchen. Aspergillus oryzae is a highly domesticated mould, and forcing it through generation after generation outside a sterile lab loses strength to wild, souring bacteria. By the third or fourth generation a batch usually smells funky or sour, or simply stops producing enzymes.
Tomo-koji safety, if you use it anyway
This tool does not recommend making or using tomo-koji — koji-kin or tane-koji is the safer choice whenever either is available. If you already keep a lineage going, these three rules are what keep Generation 1 defensible at all.
- 1Only ever Generation 1
Take tomo-koji only from a batch that itself started from koji-kin or tane-koji — never propagate tomo from tomo. By the third generation the bacterial load is usually too high to be safe.
- 2Harvest the “spore bomb” sections
Wait until the batch has gone fully to spore — usually 48 to 60+ hours, when it turns deep yellow-green or olive — and pluck out only the grains most heavily coated in spores. Don't grab a random handful.
- 3Dehydrate it immediately
Dry the harvested grains completely as soon as they're pulled, ideally in a dehydrator around 30°C (85°F). Left moist in a jar, the wild bacteria that came with it keep multiplying and ruin the seed. Once bone-dry, store it in the freezer.
FAQ
Growing the koji itself — chamber size, heat, mixing — is the Incubator Simulator's job; steering it toward sweetness or umami is Enzyme Balance.