What is koji? The mold that makes sake possible

Every sake label lists the same short set of ingredients: rice, water, koji, and yeast. Three of those are familiar. The fourth is the secret to the whole thing. Koji is what lets plain rice become sake, and once you know what it does, the rest of the brewing process makes a lot more sense.

Koji, in plain words

Koji is rice (or another grain) that has been inoculated with spores of a mold, then kept warm and humid until the mold spreads through it. In sake brewing the mold is Aspergillus oryzae, and the result looks like steamed rice with a light, frosty white coating. It smells sweet, a little like chestnuts.

The word "koji" can mean two things:

• Koji-kin: the mold itself, Aspergillus oryzae.
• Koji (or kome-koji): the steamed rice after the mold has grown on it. This is the ingredient you see on the label.

The spores brewers buy to start the process are called tane-koji, which means "seed koji."

Why sake needs koji

Rice is mostly starch. Yeast can't eat starch, only sugar. Grapes already contain sugar, which is why wine can ferment on its own. Beer brewers solve the starch problem by malting barley, letting the grain sprout so its own enzymes convert starch to sugar.

Rice doesn't malt the way barley does, so sake brewers use koji instead. As the mold grows, it produces enzymes that break rice starch down into sugar. It also produces enzymes that break down protein into amino acids, which add savory depth (umami) to the finished sake.

In the fermentation tank, something unusual happens: koji turns starch into sugar while the yeast turns that sugar into alcohol, both at the same time, in the same tank. This is called multiple parallel fermentation, and it's one of the things that makes sake different from both wine and beer. Read more in is sake a wine, a beer, or a spirit?

How brewers make koji

Koji making is often called the most important step in the brewery. A common saying among brewers puts koji first in importance, ahead of the starter mash and the main fermentation. It usually takes about two days, and it happens in a dedicated warm, humid room (the koji-muro).

1. Steam the rice. Polished rice is washed, soaked, and steamed until firm on the outside and soft inside.
2. Cool and spread it. The steamed rice is cooled to the right temperature and carried into the koji room.
3. Add the spores. The brewer sprinkles tane-koji over the rice and works it in by hand or machine.
4. Let it grow. Over the next day or two, the rice is mixed, spread out, and moved into smaller trays or boxes so the team can control temperature and moisture as the mold grows.
5. Finish and dry. When the mold has spread into the grains just the way the brewer wants, the koji is taken out of the room to stop its growth.

Brewers check it constantly, day and night. Small choices here, like how deeply the mold grows into each grain, shape how the finished sake tastes. For the full journey from rice to glass, see how sake is made.

How koji shapes the taste

Koji isn't just a tool. It's a flavor decision.

• Aroma and lightness. For Ginjo-style sake, brewers often grow koji that reaches deep into the grain while staying light on the surface. This feeds a slow, steady fermentation that helps bring out fruity and floral aromas.
• Body and umami. Koji that produces more protein-converting enzymes can give a fuller, more savory sake.
• Sweetness and dryness. How much sugar the koji releases, and how much the yeast consumes, affects whether a sake tastes dry or a touch sweet. The sake meter value on a label hints at the result.

The rice under the koji matters too. Highly polished rice, like the rice used for Junmai Ginjo, gives the mold a cleaner, starchy core to work on. Our polishing ratio guide explains why.

Koji beyond sake

Sake is only one of the things koji makes possible. The same mold, or close relatives of it, is behind many staples of Japanese cooking:

Food or drink	What koji does
Sake	Converts rice starch to sugar for fermentation
Miso	Ferments soybeans into a savory paste
Soy sauce	Ferments soybeans and wheat
Mirin	Helps create a sweet rice seasoning
Shochu	Converts starch before distillation (often with white or black koji)
Amazake	A sweet rice drink, often non-alcoholic
Shio koji	A salty seasoning used to marinate meat and fish

In recent years, chefs around the world have picked up koji too, using it to cure meats, deepen sauces, and build savory flavor in all kinds of cuisines.

Is koji the same as yeast?

No. They do different jobs and they're different organisms.

• Koji is a mold. Its job is to turn starch into sugar.
• Yeast is a single-celled fungus. Its job is to turn sugar into alcohol and aroma.

Sake needs both. Koji prepares the food, yeast does the fermenting. You can read how each ingredient fits in what is sake made from.

Koji in WESAKE

WESAKE Junmai Ginjo is made from four ingredients: rice, water, koji, and yeast. Nothing else added. It's brewed by Kaganoi Shuzo in Itoigawa, Niigata, from Gohyakumangoku rice polished to 60%. The result is dry and light, with soft umami and a crisp, clean finish.

See all four ingredients in what's in the can, or start from the beginning with what is sake.