What is koji?
In short, koji is one of the most important ingredients of soy sauce, miso, sake and other Japanese seasonings and drinks.
When my friends ask and I only have 5 seconds to answer, this is how I explain koji. But of course it is not only that, and I believe you are reading this eager to learn more. Feel free to just read the sections that interest you.
So, what exactly is koji?
Koji (pronounced 'koh-jee') is originally the name of the mould. Its scientific name is Aspergillus oryzae.
When you hear the word 'mould', you might picture the black spots that appear in bathrooms or the fuzzy growth that forms on forgotten fruit. Most moulds like these spoil food and can produce harmful toxins.
Koji mould, however, is completely different. Research has shown that Aspergillus oryzae does not produce any mycotoxins, making it perfectly safe to use in food production. Remarkably, this mould has been used in Japan since the Nara period (8th century) and is officially recognised as the country's "national mould" — a title that no other country has yet given to a microorganism.
Just as there are many varieties of flowers or vegetables, there are also countless strains of koji mould — some are better suited to making barley miso, while others are ideal for brewing flavourful sake. There may be thousands of unique varieties, including those not commercially available.
We grow koji on grains, most commonly rice, to produce rice koji or simply koji. This is the starting point for many of Japan’s most beloved seasonings and fermented foods.
Why do we grow koji on rice?
What we’re really interested in is not the mould itself, but the mould grown on rice — that is, rice koji. Because we use the power of the enzymes produced by the koji mould.
Humans break down food by chewing it and then digesting it in the stomach and intestines. We all have enzymes in our mouths, stomachs and intestines. These enzymes help break food down into forms that our bodies can absorb.
Koji mould, on the other hand, has neither a mouth nor digestive organs. Instead, it extends its mycelium into the rice and releases enzymes from the tips of these threads. These enzymes break down the surrounding rice, enabling the mould to absorb the nutrients — essentially performing digestion outside its own body.
Koji is known to produce more than a hundred different types of enzymes. These powerful enzymes break down soybeans, rice and other ingredients, transforming them into familiar fermented foods such as soy sauce and miso.
How do enzymes work?
Enzymes are protein molecules that act as catalysts — substances that trigger or speed up chemical reactions.
Their role is often compared to that of cutting tools. Just as we use scissors to cut paper and a saw to cut wood, each enzyme has its own speciality — it is designed to cut a specific molecular structure.
For example, amylase breaks down starch into sugars such as glucose and oligosaccharides (which fuel your gut biome!), while protease breaks down proteins into amino acids.
It is these enzymatic actions that create the gentle sweetness of koji amazake and the rich umami depth of soy sauce.
This might sound familiar if you have brewed beer before. We use amylase in malted barley to extract the sugar.
What is koji used for?
Now that we understand how the enzymes in koji break down and transform ingredients, let’s look at how it's actually used in food production.
Soy sauce, miso, mirin, rice vinegar, amazake, sake, shōchū…
Koji is behind many of the seasonings and drinks that form the foundation of Japanese cuisine.
Except for miso and amazake, koji is usually used as a starter ingredient — it carries out its enzymatic work during fermentation and is not present in the final product.
In recent years, however, new ways of enjoying koji itself have become popular. One example is shio(salt)-koji or soy sauce-koji, versatile fermented seasonings that use fresh koji as the main ingredient.
The appeal of koji is also spreading beyond traditional Japanese cooking. For instance, a three-Michelin-star restaurant in Denmark has created a roasted barley koji sauce to pair with potatoes, and a sake brewery taproom in London serves bar snacks seasoned with crushed koji and spices.
How rice koji is made
Here’s a brief overview of how rice koji is made.
① Soaking
The rice is washed and soaked in plenty of water until it is fully hydrated.
② Steaming
The soaked rice is steamed until firm — slightly harder than ordinary cooked rice. This texture helps the growth of the koji mould.
③ Inoculating
Once the rice has cooled to around 40 °C, the koji starter is sprinkled evenly over the grains and gently mixed in.
④ Cultivating
The inoculated rice is then wrapped in cloth and kept warm in a controlled chamber for around 48 hours.
During this time, the temperature naturally rises due to fermentation, so the rice is regularly checked and mixed to ensure the koji mould is happy and healthy.
⑤ Finishing
When the grains of rice covered opaque white mould and gives off a fragrant, chestnut-like aroma, it’s ready.
The temperature is lowered to stop the fermentation.
At this stage it’s called "fresh koji"; if it's dried at a low temperature to preserve the enzymes, it becomes “dried koji”.
From start to finish, making koji takes around three days.
Benefits of using koji
I think there are three main benefits of using koji — flavour, convenience, and health.
Flavour:
As mentioned in the previous section, koji breaks down proteins into amino acids, creating a deep umami flavour.
When you marinate meat or fish with koji-based seasonings such as shio koji, the enzymes tenderise the meat and enhance its natural flavour.
Convenience:
As koji seasonings already have a rich, complex flavour, you don’t need to combine multiple sauces or spices. A single spoonful of koji seasoning often does the job — simple yet delicious.
Health:
Since koji naturally adds umami and depth of flavour, it allows you to use less salt without compromising on taste.
Koji-tenderised foods are also easier to digest and absorb.
Additionally, the oligosaccharides and the koji mould itself support the growth of beneficial gut bacteria, and the mould produces B vitamins and minerals that can help reduce fatigue.
Personally, I have noticed better digestion and healthier skin since incorporating koji into my diet.
Although many health benefits have been proven by research, I cannot guarantee any health effects as koji is not a medicine. However, I truly believe it’s a powerful ally for living well and eating well.
Getting started with koji
Koji isn’t just for Japanese cooking; it also pairs well with British cuisine. And the best part? It’s surprisingly easy to get started.
Try swapping salt with shio koji in your everyday dishes.
Spread a thin layer of soy sauce koji on toast before adding your baked beans.
Take an onion koji steak marinade to your next family barbecue instead of a shop-bought one.
If you’re ready to invest a little more time and effort, miso making is a deeply rewarding project, whether on your own or with family and friends (Good news - we've got everything you need to get started here). Once you taste your own six-month-aged miso, you’ll never go back to the supermarket version.
Welcome to the world of koji!