Japanese With Ease: For a limited time free: Get cookbook

Koji: The Amazing Mold from Japan & It’s Uses

I love creating free content full of tips for my readers, you. I don't accept paid sponsorships, my opinion is my own, but if you find my recommendations helpful and you end up buying something you like through one of my links, I could earn a commission at no extra cost to you. Learn more

Check out our new cookbook

Bitemybun's family recipes with complete meal planner and recipe guide.

Try it out for free with Kindle Unlimited:

Read for free

Upon hearing the word ‘mold,’ we instantly think about our grandmother’s basement. In Japan, the next person instantly thinks of all the delicious things made from it. However, this mold we’re talking about is not any old mold but koji.

Koji is the mold formed on steamed rice, barley, or other base ingredients grains when inoculated with a strain of fungus called Aspergillus oryzae. The resulting product develops an intense umami flavor, which you can use to make other ingredients like miso, mirin, sake, etc. 

In this article, I’ll dive deep into what koji is all about, from its very terminology and nature to its significance in the realm of umami-filled Japanese culture and its health benefits.

Koji- Complete Guide on the Japanese “Longevity Secret”

17 Easy Recipes Anyone Can Make

All the tips you'll need to get started in Japanese cooking with, for a limited time, free as our first email: The Complete Japanese With Ease Cookbook.

We'll only use your email address for our newsletter and respect your privacy

What is koji?

One of the biggest confusions around the term “koji” is whether it refers to the Aspergillus spores or the product it makes when inoculated into a grain. 

Well, koji is used for both the spores and the product (mold) obtained from it when it is cultured into the grain.

To elaborate further, koji is classified into three categories:

  • Koji: These are steamed grains inoculated with koji-kin or koji mold. They are generally of three types: rice koji, barley koji, and soybean koji.
  • Koji starter: This term is generally used for koji spores with which steamed grains are inoculated. You will find this in powdered form, classified as white, yellow, or black Koji starters.
  • Aspergillus oryzae: This is the scientific name of the koji fungus.

While koji rice is surely the most common, there are other things you can inoculate with koji

Koji is made by introducing the spores of Aspergillus oryzae into steamed white rice, which then germinate and multiply over it. 

The whole phenomenon happens under controlled conditions, with just the right amount of humidity and warmth required for consistent growth of the fungus.

The fungus, as it grows, also produces several different enzymes, including:

  • Protease enzymes: To break down protein into simpler amino acids.
  • Amylase and Sacherase: To break down starches into simpler sugars.
  • Lipase enzymes: To break down fats into lipids, esters, and other aromatic compounds.

The process takes about 50 hours maximum.

The mold can then be introduced into larger quantities of rice, barley, or beans to trigger the same reaction.

Depending on the final product, you can use it with yeast to make amazake, mirin, and sake or umami-rich miso paste, along with numerous other products through fermentation.

Koji has been an integral part of Japanese cuisine for thousands of years and is deemed the “National Fungus” of the country.

Whether it’s yeast or not that’s a debate for another day.

Over the past millennia, the fame and use of koji have gigantically increased and have even transcended borders by now.

Apart from Japan, Now you will find many master koji artisans in China and Korea as well. Not to mention the experimental American chefs trying to explore koji to its limits. 

Completely defying the traditional use of koji, they are even inoculating cacao beans and curing pork loins with it to see if any exciting flavor comes out.

Anyways, back to Japan, the quote of expert Japanese cook Sanoko Sakai pretty much sums up the ingredient’s importance in a few words, saying:

“You can’t talk about Japanese cooking without talking about koji”

What does koji mean?

In Japanese, the word “koji” means “cultivate,” “heal,” and “peace.”

Generally a beautiful boy name in Japan, it’s also used for a mold made by combining Aspergillus spores with rice, soya, or barley.

The mold takes this name because of the fungus species (Aspergillus oryzae) used to make it, as it is also known as koji-kin in Japan.

Koji kin is a generic name used for those members of the group asexual ascomycetes and genus Aspergillus that are used to make fermented food.

In other words, koji derives from koji kin, while koji kin is named so because of its use to cultivate koji mold. 

What does koji taste like?

Koji has a rich, umami taste with, sometimes, a little sweetness. The smell of the mold, too, is sweet, with hints of chestnut and citrus.

When converted to shio koji, the taste of koji gets a little complex, having a dominant salt flavor and a much more refined umami flavor that illusively mimics soy sauce.

According to scientific research, koji taste also highly depends on the type of Aspergillus species or “Aspergillus starter” used.

Different species of aspergillus have various capacities for breaking down proteins, carbohydrates, and lipids, which are the final determinants of koji flavor. 

Depending on the product you will make with koji, you must carefully select the right type of fungus.

As an example, let’s take miso

If you want super salty miso with more protein, you would like to go for a koji starter with high proteolytic activity.

Alternatively, if you like sweet rice miso with more starch content, you would like to go for a koji starter with high amylolytic activity. 

Koji can have several unique umami-ish tastes, depending on the conditions, the quality of raw materials used, and what you want to prepare with it!

How to make koji? Koji rice recipe

Making koji is a labor of love. You will have to be patient to make it perfect.

It takes about 45-50 hours from the initial step, with each step requiring extreme patience to be executed to perfection.

Here’s a brief overview of each step involved in making koji rice.

Step 1 – Washing and soaking

Just like every recipe related to rice, the preparation of koji begins with washing rice. You will need to rinse the rice 2-3 times and then soak it in clean water for a specified duration.

In spring and fall, the soaking duration should be between 6-12 hours, while in summer, the time should be between 3-5 hours. It can extend to 20 hours if you make koji in winter.

As the rice soak, don’t forget to cover the utensil (bowl) with plastic wrap. This will prevent any germs from entering the rice.

Step 2 – Draining the water from the rice

Once the soaking period is over, you will need to drain the water off the rice through a strainer. Here, it’s important not to shake the strainer or rigorously stir the rice.

This can break the rice, making its texture powdery, which isn’t suitable for making a perfect koji.

When left as is, the water will slowly drain from the rice, leaving grains that are dry enough and ready for steaming.

Step 3 – Steaming the rice

When the rice is dry enough, you will need to put them gently into a steamer cloth and steam them for about 10 minutes without closing the lid.

When the steam penetrates through the rice and you see the grains becoming somewhat transparent, it’s time to cover the steamer and let the rice cook for about 40 minutes.

It’s imperative to steam the rice very calculatingly when it comes to duration, as the texture of the rice is extremely important to how the resulting product will taste.

Step 4 – Seeding

In this step, you will need to spread the rice evenly over a dishcloth placed on a wooden tray and untangle the clumps.

After untangling the rice grains, you will need to sprinkle them with a koji starter.

Here it’s important to mention that you should not add the koji starter while the rice is too hot. The same stands true if the rice is super cold as well.

Hot rice will instantly kill the microorganism that starts fermentation, preventing the fermentation from beginning in the first place.

On the other hand, the fermentation won’t be as good in cold rice as well.

Only mix the koji starter when the rice temperature reaches 45 degrees, and then mix the rice carefully before wrapping it in a clean cloth while they’re still warm.

To ensure that the fungus grows to perfection, you will need to press the cloth as hard as possible so that the rice grains are packed close.

You will also need to tie the cloth with a rubber band, cover it with another cotton cloth for optimum humidity, and then put it in a koji fermenter, or a yogurtia, for the purpose.

Step 5 – Maintenance

Right after 18-20 hours of when you start warming the rice koji, it will need the first maintenance.

At this point, the rice should have a temperature somewhere between 38 to 40 degrees, the grains should be whitish, and there should be a sweet aroma.

If you see all of the signs mentioned above, It means your rice is on the way to turning into a delicious koji.

You will only need to look for any clumps of rice and untangle them, and then wrap the rice back and put it into the koji fermenter.

After three hours or so, when the rice temperature exceeds 40 degrees again, you will need to unpack the rice, put them in a koji lid, and untangle them. 

You will then need to put them again in the fermenter without a significant temperature drop.

Step 6 – Second maintenance

After the rice temperature rises above 40 degrees again, you will need to do a second maintenance, just like the first one.

At this point, the rice, or koji, should have a sweet, chestnut-like aroma.

Step 7 – Final touch

After several hours of the second maintenance and approximately 50 hours of the beginning point, the koji should be fully prepared. A few signs that might indicate this include the following:

  • Stretching of hyphae from the mold
  • Sticking of rice to each other
  • Completely white rice body

Best Koji spores to buy

If you intend to prepare koji at home, you might as well need some high-quality spores to ensure stellar final results.

That said, Hishiroku Koji Starter Spores might be what you are looking for. 

Hishiroku Koji Starter Spores - Powdered Kairyou Chouhaku-kin 20g

(view more images)

It has great glycation and proteolysis power and will maintain the white color of koji. However, make sure not to use it on grains other than rice and wheat.

You’ll love how your first koji experiment goes with this on hand!

What is the origin of koji?

According to some sources, koji is said to have originated in Japan about 2000 years ago.

Other sources say that it became a part of the Japanese diet about 1300 years ago, based on the fact that it was mentioned in the Japanese literature in 713, in the Nara period.

The literary piece is named “Harima no Kuno Fudoki,” which describes the sake-making method using koji.

However, the actual development in the use of koji took place in the preceding periods like Muromachi (1336-1573), Edo (1603-1868), and Meiji period (1868-1912).

It was when the use and production of koji moved from household to industrial scale, and the spore was effectively marketed for its role in fermentation.

In the Meiji era, the production saw its peak, and the time that followed would make the mold a fermentation ingredient that would account for creating almost all the popular foods eaten in Japan today, either directly or indirectly. 

At the beginning of the 21st century, The Japanese government recognized koji’s status as a staple ingredient and its role in Japanese cuisine.

And just like that, the Brewing Society of Japan named koji the “National Mold” of the country.

Right now, koji is more famous than ever before, both in and outside Japan, and is loved and used in any part of the world where Japanese food lovers exist.

What is the difference between koji and miso?

Koji is the term used for the fungus aspergillus oryzae or the mold it makes when inoculated in rice, soybeans, or barley.

On the other hand, miso is the product obtained from the fermentation of this koji in salt water.

Where koji is used as a base ingredient in miso, amazake, and soy sauce, miso is used as a base ingredient in the Japanese staple dish: miso soup.

Miso has a peanut-butter-like texture and a very salty but umami-rich taste.

You can also use miso in dishes other than miso soup, including batters like okonomiyaki, different dressings, and sauces. 

If miso is the soul of Japanese cuisine, koji is its creator.

Types of koji

As we mentioned at the start of the article, the term “koji” is used for both the koji starter (the spores, Aspergillus oryzae) and the culture created after we add the spores to grains.

Let’s have a look at both:

Types of Aspergillus (koji starters)

The type of koji starter will significantly affect the final taste of the koji. Following are some common koji starters used for different purposes:

  • Yellow koji mold: Also known as Aspergillus sojae, yellow koji mold is used to prepare koji mold for the production of miso, soy sauce, and sake. The spores of this mold are light green or, sometimes, yellowish brown.
  • White koji mold: Also known as Aspergillus Kawachi, white koji mold is commonly used to produce shochu. It has brownish spores.
  • Black koji mold: Also known as Aspergillus iuchuensis, black koji mold is used to produce Okinawa liquor.
  • Red koji mold: Also known as the Monascus genus, red koji mold produces Chinese red wine, Shaoxing wine, and tofu-yo.
  • Bonito mold: Also known as Aspergillus glaucus, bonito mold is used to produce katsuobushi.

Types of koji (with grains)

Now that you know the different strains of aspergillus and their specialties, let’s have a look at the different types of koji you can make with respect to the type of grain the fungus grows on:

  • Rice koji: When the aspergillus fungus is inoculated in rice grains, and it breaks down the proteins and carbohydrates of rice, the final product we get is rice koji. You can use it to make rice miso, sake, mirin, amazake, and vinegar. It is also the most common type of koji.
  • Barley koji: When barley is inoculated with a koji starter, the final product obtained from the fungus activity is barley koji. Barley koji is primarily used for making barley miso and shochu.
  • Soybean koji: When soybeans are inoculated with aspergillus spores, the final product obtained is called soybean koji. It is mainly used for making soybean miso.

Learn more about what makes rice koji different from barley koji

How can you use koji?

Though you can’t eat koji directly on its own, there are a bunch of other tasteful foods/foods you can make with it.

Those ingredients or foods are often the reason for the unique umami taste Japanese cuisine is known for!

Let’s have a look at some of them:

Miso

Miso is a Japanese staple ingredient in many dishes, including miso soup, okonomiyaki, ramen, stir-fries, and countless other recipes.

Miso has a very salty flavor with strong hints of umami.

It is prepared by fermenting koji in salt water for a specified duration—generally, the more the fermentation period, the better the taste.

Here at Bite My Bun, we have shared some fantastic miso soup recipes you must try at least once!

Shoyu

Shoyu is made with a fermented paste of soybeans, roasted grain, koji, and brine. It takes about two years to prepare perfectly and has a robust umami taste.

Shoyu is popularly used as a condiment, often sided with dishes like sushi, used as a soup base for ramen, and as a marinade and seasoning for different dishes.

Though shoyu is of Chinese origin, it is now equally popular in Japan and is loved for its unique taste.

Mirin

Whether it’s your favorite noodles, wholesome tonkatsu or perhaps delicious tempura, none can be complete without mirin at its side.

Mirin used to be a high-end sweet liquor in the Meiji period. It has a distinctly sweet flavor and syrupy consistency, with 14% alcohol content.

Today, it is enjoyed both as a seasoning and as a condiment.

Though some people also like to consume it as a beverage, it isn’t as popular now due to its high sugar content.

Amazake

Amazake, or sweet sake, is a Japanese version of an organic energy drink prepared with rice koji, known for its amazing health benefits.

It is also a sweet liquid and used as a sweetener, apart from its use as a beverage. You will find it in two varieties: white rice amazake and brown rice amazake.

Unlike mirin, it generally has 0% alcohol content and is an everyday consumable among individuals of all ages in Japan.

Other uses

If you aren’t about going traditional and stuff, you can also use koji as a meat tenderizer. Though the trend comes from western chefs, rice koji is quite an excellent tenderizer for a juicy steak.

Another advantage of tenderizing meat with koji is the subtle flavors it adds to the meat. Along with flavors from your favorite marinade, koji gives the meat a sweet flavor that really refines the final taste of the meat.

Is koji healthy?

If you delve deep into why the Japanese have such a long life expectancy and good health, you will see the use of koji!

Yup, it is known as the “secret to longevity” of Japanese people. How true is it? We’re yet to find out.

But there’s no denying that koji does have some notable effects on overall body health.

Following are some health benefits you can expect by making koji a part of your diet:

Anti-fatigue effects

When made into amazake, koji has proven effects on fatigue recovery. It contains vitamin B and Branch-Chained amino acids (BCAAs).

Both compounds have functional roles in maintaining energy levels and reducing the duration of muscle recovery after intense exercise.

This is one of the reasons why amazake is such a favorite post-workout drink among fitness savvies in general and long-distance runners in specific.

Role in maintaining intestinal health

According to a research study, Aspergillus oryzae, or koji, has a significantly positive impact on promoting intestinal health.

Koji produces several metabolites, which improve the fecal weight of healthy adults and defecation frequency.

Also, Aspergillus oryzae has probiotic effects, including a massive role in pancreatic tumor suppression.

Role in maintaining skin and hair

Koji also plays a role in maintaining skin complexion and glossiness of hair.

According to different studies, the effects mentioned above can be attributed to the ample amino acids in koji.

Moreover, koji also contains a sufficient amount of thiamin, niacin, pantothenic acid, and Vitamin B6. 

These all have a combined effect on skin, hair, and brain cell regeneration. Moreover, they also play a massive role in improving the immune and nervous systems.

Also read: Is teriyaki healthy? It depends on how you make it!

Are there any side effects of koji?

There are no known side effects of koji yet.

However, if you experience any of the following symptoms after consuming koji, you should immediately see a doctor for an allergic reaction:

  • Muscle pain/weakness
  • A tingly feeling in the fingers and toes
  • Fever, flu symptoms, body aches, chills
  • Pale or gray appearance of lips/tongue
  • Nausea, vomiting, stomach pain
  • Diarrhea
  • Unusual bleeding from the nose

Conclusion

Koji has been used in Japan for over 2,000 years and has remained an essential part of Japanese cuisine.

Nearly all the primary foods, drinks, and sauces, whether miso, amazake, or liquor, need koji for their preparation.

In other words, it wouldn’t be wrong to say that more than half of Japanese cuisine is practically non-existent if we exclude koji.

Not to mention all the health benefits that have earned it a status as one of the healthiest food materials in Japan and the world.

Check out our new cookbook

Bitemybun's family recipes with complete meal planner and recipe guide.

Try it out for free with Kindle Unlimited:

Read for free

Joost Nusselder, the founder of Bite My Bun is a content marketer, dad and loves trying out new food with Japanese food at the heart of his passion, and together with his team he's been creating in-depth blog articles since 2016 to help loyal readers with recipes and cooking tips.