Sake · Production · Study guide
Kōji
A study guide to kōji — the mould that makes sake possible, the enzymes it secretes, sō-haze versus tsuki-haze, and how the kōji room shapes style.
There is a Japanese brewing proverb: ichi kōji, ni moto, san tsukuri — "first kōji, second the starter, third the main mash". It ranks the craft of sake in order of difficulty, and kōji comes first because without it there is no sake at all. Kōji is steamed rice that has been deliberately colonised by a mould, Aspergillus oryzae, whose enzymes do the one thing yeast cannot: turn the rice's starch into sugar. Every other step assumes kōji has already done its work.
Two things are worth fixing first: what kōji actually produces (a set of enzymes, chiefly starch-splitting amylases), and the fact that brewers can steer those enzymes by how they grow the mould — a fragrant ginjō and a rich junmai start from different kinds of kōji.
The one thing to fix first: kōji is an enzyme factory
Dust steamed rice with the spores of Aspergillus oryzae (the kōji-kin), keep it warm and humid, and the mould grows into each grain, secreting enzymes as it goes. Two families of enzyme matter:
- Amylases — break the rice starch into glucose. This is the sugar that yeast will ferment, and it is why kōji is indispensable: it feeds the parallel fermentation drip by drip. See how sake is made.
- Proteases — break the rice protein into amino acids and peptides. These carry umami and savour, and in excess, heaviness.
By controlling how much and what kind of enzyme the kōji carries, the brewer sets how fast sugar is released and how much savoury weight the sake will have. Kōji is not a flavour you taste directly; it is the tap that controls everything downstream.
The two kōji styles
The distribution of the mould across and into the grain gives two working styles, matched to two ends of the sake spectrum.
| Style | How the mould grows | Enzyme profile | Suits |
|---|---|---|---|
| Sō-haze (総破精) | Mould spreads over most of the grain's surface and inward | High enzyme content, plenty of protease | Rich, full-bodied junmai; warmer fermentations |
| Tsuki-haze (突破精) | Mould grows in patches that drive deep into the grain | Lower surface enzyme, less protease | Delicate, fragrant ginjō and daiginjō |
The shorthand: sō-haze for body and savour, tsuki-haze for fragrance and finesse. A ginjō brewer wants just enough enzyme to sustain a slow, cool fermentation without building too much amino-acid weight — so a restrained, penetrating tsuki-haze kōji.
Key facts
| Organism | Aspergillus oryzae — the kōji mould (kōji-kin) |
| Grown on | Steamed, cooled white rice |
| Key enzymes | Amylases (starch→glucose) and proteases (protein→amino acids) |
| Where | The kōji room (kōji-muro) — warm (~30–40 °C) and humid |
| Time | About 45–48 hours of tending |
| Two styles | Sō-haze (rich) and tsuki-haze (fragrant) |
The kōji room, briefly
Kōji is made in a small, cedar-lined, insulated kōji room kept warm and humid, and the work is round-the-clock. Cooled steamed rice is carried in, spores are scattered over it, and for the next two days brewers repeatedly break up, spread and re-mound the rice to control its temperature and moisture as the mould takes hold and generates its own heat. Get the temperature curve wrong and the enzyme balance — and with it the sake's style — is wrong. This is why kōji is considered the most skilled job in the brewery, and traditionally the preserve of the master brewer.
In this guide
The full guide below goes deeper into the making and the choices:
- The steps of kōji-making, hour by hour
- Handmade koji-buta trays versus mechanised production
- Yellow, white and black kōji — and what each brings
- How kōji choices show up in the finished sake
- Classic exam questions
Making kōji, step by step
The process runs to a tight schedule over roughly 48 hours:
- Bringing in (hikikomi) — steamed rice is cooled to the right temperature and moved into the kōji room.
- Inoculation (tanekiri) — spores of tane-kōji (seed mould) are sprinkled evenly over the rice and mixed in, then the rice is bundled to hold warmth.
- First mixing and mounding — as the mould establishes and starts to warm the rice, it is broken up and heaped to even out temperature and oxygen.
- Spreading out (mori) — the rice is divided into smaller lots — traditionally into shallow wooden kōji trays (koji-buta) — so its temperature can be managed grain-lot by grain-lot as the mould peaks.
- Final work and sending out (dekōji) — at the target enzyme development the kōji is removed and cooled to stop the mould before it grows too far.
The whole art is the temperature curve: warmer growth favours protease and a richer, sō-haze kōji; a cooler, more controlled curve favours the restrained tsuki-haze kōji that fragrant ginjō needs.
Handmade versus machine
Top ginjō and daiginjō kōji is often still made by hand, in trays, because the close control over each small lot is hard to beat and the stakes are highest at the top of the range. For larger volumes, breweries use automatic kōji-making machines that regulate temperature and humidity mechanically — reliable and consistent, if less able to chase the last degree of finesse. Many breweries mix the two: machine-made kōji for the base, handmade for the flagship.
Yellow, white and black kōji
Sake is overwhelmingly made with yellow kōji (ki-kōji), the classic Aspergillus oryzae, which produces clean sake and little acid. Two cousins, borrowed from the distilling world, are now used by adventurous brewers:
- White kōji (Aspergillus luchuensis mut. kawachii) produces citric acid, giving fresh, tart, lively sake.
- Black kōji (Aspergillus luchuensis), the mould of Okinawan awamori, produces even more citric acid and a bolder, sharper profile.
White and black kōji are the exception rather than the rule, but they explain the crisp, citrussy modern sakes that don't taste like the traditional style — a fault to rule out before you call such acidity a flaw (faults & food pairing).
Classic exam questions
- What organism is kōji? — the mould Aspergillus oryzae (kōji-kin), grown on steamed rice.
- What do kōji's amylase enzymes do? — convert rice starch into glucose for the yeast to ferment.
- What do proteases contribute? — amino acids and peptides, i.e. umami and savoury weight.
- Which kōji style suits fragrant ginjō — sō-haze or tsuki-haze? — tsuki-haze (patchy, penetrating, lower enzyme).
- Roughly how long does kōji-making take? — about 48 hours.
- What does white or black kōji add? — citric acid, and so a fresher, tarter sake.
Kōji is the quiet first mover: fix how the mould is grown and you have set how fast the sugar flows and how much savour the sake carries, long before the yeast gets to work.