What is Koji and How Does It Work in Fermentation?

If you’re at all familiar with Japanese cuisine, you know that it uses a lot of fermented foods: natto, miso and sake, just to name a few. But did you know that koji is the building block of many traditional Japanese foods?

Let’s learn what exactly koji is and how it sparks the fermentation that makes all Japanese food so delicious.

Koji refers to any kind of steamed grains — most notably rice, barley and soybeans — that have been combined with the koji mold, Aspergillus oryzae. This mold is referred to as koji starter, and, left on the rice for a day or more, allows the rice to ferment enough to create other fermented foods, such as miso, sake, soy sauce and more.

Find out more: It’s Time to ‘Shoyu’ The Wonderful World of Japanese Soy Sauce

Koji has over 1,300 years of history in Japan, originally coming from China around 700 A.D. during the Asuka and Nara Periods. First appearing on the scene as a helpful ingredient in brewing sake, koji has now become ubiquitous in Japanese cuisine. 

There are three different types of koji: rice koji (either brown or white rice), barley koji and soybean koji.

Rice koji: The most popular type of koji, used to make sake, miso, soy sauce and other well-known Japanese ingredients. 

Barley miso: Mainly used for making barley miso and wheat soy sauce. Miso made with barley has a lighter taste along with a unique fragrant aroma from the barley koji itself.  

Soybean koji: Typically used to make soybean miso. 

Koji is an extremely versatile ingredient that can be used to make a wide variety of food. Here are just a few: 

If you’re looking for that perfect umami addition to your marinade, sauce or salad dressing, look no further than shio koji. With a balance between sweet and salty, this is a common ingredient to take your meal from so-so to outstanding. 

Sprinkle and rub some rice koji powder onto your steak to cut down on the time it takes to age it. As a bonus, the koji will add a hint of sweetness to your dish! 

If you’re unfamiliar, miso is another Japanese fermented paste most commonly used to make miso soup. Koji is what allows for the fermentation process that makes miso so delicious!  

Find out more: What is Miso? Ultimate Guide to Fermented Japanese Soybeans

Koji mold also plays a part in Japanese cuisine’s most popular condiment, soy sauce. The fermented ingredients are filtered into liquid to form soy sauce, usually after fermenting for two years or longer in the case of high-end soy sauce.

A popular drink during the New Year holidays, amazake is a sweet rice drink that can be served with or without the sake content hinted at in the name. If you’re making it from scratch, you can use either white or brown rice koji, each with a slightly different flavor. 

Although mirin was once consumed as a liquor during the Edo Period, nowadays, it’s used as a sweetener in many Japanese dishes. Mirin is made from shochu, another form of liquor, rice koji and glutinous rice. 

It may be surprising to hear that a type of fungus is good for you to consume, but have no doubt — koji is a Japanese superfood! 

For one, koji contains a myriad of nutrients, including vitamins B1, B2, B6, biotin, pantothenic acid and essential amino acids. 

Here’s a few other health benefits of eating foods with koji: 

Koji is rich in the enzymes amylase and protease, which help break down starch into glucose and protein into amino acids. 

Essentially, if you’re eating foods like vegetables or meat with, say, miso soup made from koji, that koji ingredient helps you ingest all the nutrients in the meal. 

Koji also feeds the growth of Blautia coccoides, or good bacteria, in your gut. A higher amount of B. coccoides in the intestines keeps everything running smoothly*, if you catch our drift.

*Source.

Want good skin, hair and nails? Try eating more foods containing koji. Because koji is packed full of Vitamin B, it promotes cell regeneration and recovery from fatigue with every bite. Additionally, it can keep the immune system and nervous system in balance.

To make koji at home, you’ll need a large amount of rice, koji starter, a cheesecloth and containers for storing the rice during the fermentation process. 

With multiple stages of fermentation, this recipe is best made at least the day before you need to use the koji. 

Here is a list of step-by-step instructions to make koji:

1. Steam rice in a pan or rice maker. 

2. Put the rice in a container lined with the cheesecloth. 

3. Sprinkle the koji starter on top of the rice while warm. 

4. Keep the rice warm in something like a crockpot or warmed oven. 

5. After a while, open the cloth and break up the clumps of rice before wrapping it once more and putting it back in the warming device. 

6. Wait 5-6 hours. 

7. For the second time, stir the rice and put it back in place. 

8. A few hours later, break up the koji rice a third time. 

9. Feel free to do another round or leave the koji as is, ready to use in the dish of your choice! 

Similar to sourdough, you can buy starters at most supermarkets in Japan. Alternatively, you could source koji from koji artisans, miso makers, rice stores and sake factories, as koji is a shared ingredient among them.

If you’re outside of Japan, going to an Asian grocery store is your best bet, but buying high-quality Japanese rice and fermenting it yourself will be the cheapest, albeit most time-consuming, route.

Want to learn more about fermented foods in Japan? Check out these blogs for more insight into Japanese miso, Japanese sake and Japanese soy sauce.

Koji is cooked rice and/or soybeans that have been mixed with Aspergillus oryzae, a culture that aids in the fermentation process. This culture is used widely across Japan, used in the production of miso, soy sauce, sake and so much more.

Koji is not the same as miso, but it is used to create miso. Rice is fermented with koji mold to produce koji, which is then used for the fermentation and maturation of soybeans. From here, a number of products can be produced, one of which is miso.

Koji mold is a filamentous fungus belonging to the genus Aspergillus, classified into three species: Aspergillus oryzae, Aspergillus sojae and Aspergillus tamarii. It is both and mold and a fungus.

Koji is linked to a number of health benefits, with proof of supporting digestion, a healthy gut and cell regeneration, along with high numbers of vitamins B1, B2, B6, biotin, pantothenic acid and essential amino acids.

Koji (麴) simply means malt or yeast in Japanese.