What is Koji?


You might not have heard about it before, but if you have ever enjoyed Japanese cuisine, you have almost certainly reaped the benefits of koji. This humble little fungus, which is normally grown on rice or soy beans, has been used for thousands of years in Japan and other south-east Asian countries as a fermenting agent. It is thanks to koji that such iconic Japanese foods as soy sauce, miso, sake, and rice vinegar exist.

Koji was first used in China (where it is known as qu) in 300BC, and in Japan in 300AD. As a fermenting agent, koji was a vital component in food preservation. By 10th Century AD koji was being deliberately manufactured so the foods made with it could be produced in large enough quantities to meet demand. Since then the demand for koji has grown, and its use has diversified from being just a fermenting agent to also being a seasoning in its own right.

How is Koji Made?

Koji is made by inoculating steamed and cooled starches with the spores of a fermentation culture called Aspergillus oryzae. After the starch has been left in a warm place for a couple of days (long enough for different enzymes in the A.oryzae to feed on the starch and break down carbohydrates and proteins), it develops a white, fluffy outside layer of koji mould, that looks like blooming flowers when viewed under a microscope. It is this unique appearance that has earned koji a kanji (Japanese written character), that combines the characters for ‘rice’ (米) and ‘flower’ (花) to become 糀. The spores are separated and preserved for use at a later date, and the koji itself completes its destiny as either a fermenting agent or a seasoning.

Click through the tabs above to find out more about the role koji plays in making different foods, as well as the ways koji can improve the flavour and texture of just about any homecooked meal.

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Foods Made With Koji

Koji is instrumental in the making of some of Japanese cuisine’s most essential ingredients. Read below to discover how koji is used to make sake, soy sauce, and miso. Notice how, despite the manufacturing processes for all three foods being quite similar, the end products are all vastly different from each other. This signifies koji's versatility, and why it plays such an important role in Japanese cuisine.


The chemical process involved in making sake involves the use of two fermenting agents: rice koji and sake yeast. When the sake rice has been steamed and cooled, about 20% of it goes to a special room, called a koji mura, where it is laid out, inoculated with koji spores (known as koji kin in Japanese), and carefully monitored for about 48 hours to ensure even koji growth. This koji rice is then combined with the remaining rice, water, and sake yeast, and allowed to ferment in large, temperature-controlled barrels for 20-40 days (depending on the type of sake). During this time, the koji breaks down the complex starches of the rice into more simplified glucose, while the sake yeast converts the glucose to alcohol at the same time, in a process called ‘multiple parallel fermentation’.

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Soy Sauce

Before soy sauce can start to be brewed, the koji that will brew it needs to be grown. For this, koji spores are added to a mashed mixture of boiled water, wheat, and soy beans, that has been cooled to about 27 °C. The mix is given three days to develop its layer of koji before being combined with water, salt, lactic acid bacteria, and yeasts. This complete mixture ferments for six months, during which time the koji turns the grain proteins into amino acids and the starches into sugars, while the lactic acid bacteria turns some of the sugars into lactic acid and the yeast turns other sugars into ethanol. All of these processes are what gives soy sauce its complex flavour.

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There are many different varieties of miso, but one way to broadly segment all miso is by considering what starch was used to grow the koji. The vast majority of miso consumed in Japan is rice miso, meaning that the koji has been grown on rice. Soy bean miso and barley miso are also reasonably widespread. To make a relatively mild, sweet rice miso, rice is steamed, cooled, koji spores are added, and the rice is left to grow its koji for two days. After this, soy beans are steamed, cooled, and ground up into a powder. Batches of koji-covered rice, powdered soy bean, and salt are combined and mixed well before being sealed in airtight containers and allowed to ferment for 2-3 months. This gives the koji enough time to break the starches from the rice into sugars, and the proteins from the soy beans into amino acids.

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Koji in Cooking

It is the enzymes contained in koji that make it useful as a fermenting agent. These same enzymes have proven extremely beneficial when koji is added to dishes as a seasoning (normally after being combined with salt to make a mixture called shio koji). Read about the main benefits of using koji in your cooking below.

Flavour Enhancer

The main reason why koji is used so frequently as a fermentation agent is because koji is excellent at breaking down complex molecules, including converting starches into glucose and proteins into amino acids. Glucose and amino acids happen to be chemicals that make a lot of the foods we eat taste good to us, so adding koji to food causes more of these chemicals to be produced, which in turn enhances and improves the flavour.

Tenderiser and Odour Eliminator

When koji is used with meats and fish, the enzymes in the koji immediately get to work breaking down the tough proteins in these foods. Without as many of those tough proteins, the meat and fish immediately become more tender. As well as this, the koji also takes care of the unpleasant odours that can crop up with meats and fish. As such, koji is ideal for using as an ingredient in marinades.

Digestive Aid

Koji is frequently praised for its nutritional benefits as well as its flavour enhancing properties. As humans, we naturally produce enzymes in our guts that form part of our digestive system, but some of us do not produce enough of these enzymes, which can be problematic. This is what makes the enzymes in koji so beneficial, because they will help the enzymes we produce ourselves to break food down, making it easier for our bodies to absorb the nutrients we need.