What is Koji? A Complete Guide to the Japanese Fermentation Culture (and How to Use It)

What is Koji? Definition of Japan's National Mold

Koji (麹) refers to two things: koji mold (the microorganism) and cooked grains with koji mold grown on (the ingredient).

What is Koji Mold (Koji Kin)?

Koji is a mold widely used in Japanse cuisine called Aspergillus oryzae.

Koji is actually a family of molds found in East and Southeast Asia that thrives in a high-humidity environment. If you've ever visited Japan, you know how humid the air can be, and that's actually the secret to why Koji thrives there! The type of koji that is key in Japanese cuisine is Aspergillus oryzae. But to Japanese home cooks, it's just called koji-kin (麹菌, meaning koji mold) or tane koji (種麹, meaning koji spores or seed koji).

It's the most foundational mold in Japanese cuisine and an ingredient of essential Japanese condiments like miso, soy sauce, mirin, rice vinegar, amazake, and sake. Koji is a secret ingredient behind the deep, complex umami base that defines the unique Japanese flavor profile! That's why, in 2006, the Brewing Society of Japan officially designated Aspergillus oryzae (Koji fungi, Kōji-kin) as the National Fungus of Japan. You can also read the official declaration here.

What is Rice Koji?

Koji, or rice koji, is an Japanese fermentation culture made by inoculating cooked rice with the fungus Aspergillus oryzae.

While different grains can serve as a home for the mold, the most common and widely used grain is rice. Koji-grown rice is called Kome Koji (米麹) in Japanese.

Once the koji spores develop a network, rice grains are covered with white fluffy koji mycelium. And they smell slightly sweet like chestnuts.

When making miso and sake, the process starts with making rice koji. Rice koji is not only for traditional condiments, but we can also make so many delicious sauces and dishes to enrich our fermentation life.

When home chefs refer to koji, it probably means rice koji. Because when we use koji in the kitchen, we don't start with growing koji mold (actually, you can make rice koji at home, but it's a 3-day process!). Instead, we use rice koji to kick-start the fermentation process. So in this post, I'll mainly talk about how to use rice koji in your kitchen to make your homemade meal rich in umami and

Is Koji Safe?

I know the word "mold" is a bit scary when we talk about food. But rest assured, Koji (Aspergillus oryzae) has a long history and has been enjoyed in Japanese cuisine for centuries.

The scientific debate about koji safety was heated in the 1960s, when scientists found Aspergillus oryzae looked almost identical to a wild, toxic mold called Aspergillus flavus. A. flavus is known to produce cancer-causing compounds called aflatoxin. So the global scientific community starts to see koji with deep suspicion.

But here's the very interesting part! The investigation found that A. oryzae is probably a domesticated version of wild mold, and it lost its ability to make toxins along the way. It also has a stronger starch-digesting ability than its wild ancestor.

Scientists think this change occurred because, in a safe environment, koji didn't need to produce toxins for defense. Instead, koji could focus its energy on digestion, breaking down starch and protein so it could grow, and making food delicious for us.

It's amazing to think that through history, people "tamed" the fungi to make it perfectly safe to consume and incredibly efficient at creating the delicious flavors we love today!

Also, the U.S. Food and Drug Administration (FDA) includes enzymes derived from Aspergillus oryzae on its list of substances Generally Recognized as Safe (GRAS).

Types of Koji used in Japan

So there are several types of koji used in traditional Japanese cuisine.

• Yellow Koji (Aspergillus oryzae, Ki-koji-kin) - The most important type of koji in Japanese cuisine, used for making sake, miso paste, and soy sauce.
• Black Koji (Aspergillus luchuensis, Kuro-koji-kin) - It's used to make shochu (traditional distilled spirits) and Awamori (Okinawa distilled spirits). Black koji produces a large amount of citric acid, preventing spoilage and making it possible to create a stable fermentation environment in warm and humid climates in the Southern Japan region, like Kyushu and Okinawa.
• White Koji (Aspergillus luchuensis mut. Kawachii, Shiro-koji- kin) - This variety originated as a natural mutation of Black Koji. It's widely used to make shochu.
• Red Koji (Beni koji kin) - Used to make Tofuyo (fermented tofu).
• Katsuobushi Mold (Eurotium herbariorum, Katsuobushi kin) - Traditional, high quality katuobushi is made by boiling, smoking, and fermenting bonito fillets. The long-aged (6 months+) Katsuobushi is rich in umami.

Yellow, black, and white koji are most widely used in Japan and are designated as the National Fungus of Japan.

This section is very niche! But this definitely deepens your understanding of Japanese fermentation, and there are so many kitchen items made with koji!

The Science of Koji: How it Creates Umami, Sweetness, and Tenderizes

So why are culinary experts obsessed with koji? How does koji convert food into a umami bomb? The secret lies in the enzymes that koji produce!

Enzymes are types of proteins in our bodies and living organisms that speed up chemical reactions, like breaking down foods into smaller molecules.

During the fermentation process, koji produces enzymes. The enzymes are what work wonders to add food complex umami flavor and tenderize protein! When koji grows on rice or beans, it releases the following three main types of enzymes that transform the ingredients into pure culinary magic.

Amylase: Digests Starch into Sugar

Amylase is the enzyme that turns starch into simple sugars such as glucose. This is why Amazake (a sweet rice drink) is naturally sugary even though no sugar is added! It's also why a long-fermented miso has a subtle, mellow sweetness. These sugars can be a base for further fermentation with yeast when brewing sake.

Protease: Digests Proteins into Amino Acids

This is a flavor powerhouse! Protease breaks down proteins into amino acids. One of the powerful amino acids produced during this process is glutamate, which is responsible for a deep, savory umami flavor we love! That's why miso and soy sauce are rich in umami. Koji creates glutamate from proteins in soybeans.

Also, when koji meets meat or fish, protease breaks down tough muscle fibers and makes them so tender even after they are cooked.

Lipase: Digests Fats into Fatty Acids

Lipase breaks down fats and oils into fatty acids. This process is involved in the aroma and flavor development of koji fermented goods. It creates the deep, complex flavor that miso paste, soy sauce, and sake have.

Note: These enzymes work not just to make foods delicious. They actually make it easier for us to digest, too, just like enzymes do naturally in our bodies. It's wellness and flavor working hand-in-hand.

How to Choose Koji

If you're ready to dive into the koji fermentation journey, the first step is get the rice koji (unless you grow your koji, which requires whole another expeties and experience). When I first started, I was a bit overwhelmed by the options, but it's actually very simple once you know what to look for.

Which is better, Dried or Fresh Koji?

There's fresh and dried koji on the market. Dried koji is literally the dried version of its fresh counterpart. So here's what you need to know to choose and use each.

• Fresh Koji (生麹, Nama Koji)
  • It has maximum enzyme activity, creating the most aromatic and delicious koji fermented foods in a shorter period compared to dried ones.
  • Use this for: Authentic amazake and homemade miso paste.
  • Storage: Up to 2-3 weeks in refrigerator or 3 months in freezer.
  • Moisture Content: 25-30%
  • Availability: Hard to find in stores and online.
• Dried Koji (乾燥麹, Kanso Koji)
  • This is a dried variety of fresh koji, making it convenient and stable. The enzyme power is weaker than that of fresh ones. There are grain types and block types.
  • Use this for: Everyday cooking, like making shio koji and shoyu koji. Very beginner-friendly.
  • Storage: Store in a dry, cool, and dark place such as a cupboard. It's shelf-stable for months, so I always keep it in my pantry to make umami-rich meals.
  • Moisture Content: <10%
  • Availability: You can easily find one online. My favorite one is this dried rice koji!

If you are a beginner, I highy recommnd to start with dried koji! It's much easier to find and store, and the results are still absolutely delicious! I always keep a bag in my pantry so I can whip up Shio Koji whenever I almost run out.

How to Rehydrate Dried Rice Koji

While there's a little difference in fermentation power, the fresh and dried koji can be used interchangeably.

One thing you need to keep in mind is water content. When you use dried koji instead of fresh, the rule of thumb is you'll need to add about 25% of lukewarm water in weight: Swapping 100g of fresh koji with 100g of dried koji + 25g of water.

But you always need to adjust the amount of water. I usually need more water, up to 40g of water per 100g of dried koji.

Recipes like amazake work best with fresh koji. If your recipe calls for fresh koji, try the method below. But most of the time, you don't need to rehydrate the koji; you just need to add extra water. Use the golden ratio above to measure your extra water.

The Simple Method to Rehydrate Dried Koji:

1. Measure koji and water. Break up the koji if it comes in a block.
2. In a bowl, mix the koji with lukewarm water(40-60°C, 104-140°F).
3. Let sit for 1-2 hours, mixing every 15 minutes, until soft to the center.

Rehydrated koji darkens fast, so use it immediately.

Where to buy Koji

• Japanese or Asian Grocery Stores: Look in the refrigerated section (for fresh) or the dry pantry aisle (for dried).
• Health Food Stores: Many natural food stores now carry Koji in the "fermented foods" or "macrobiotic" section.
• Online: I always get my dried rice koji at Amazon!

How to Use Koji in Your Home Kitchen

Once you get your koji, it's time to activate its superpower! Try these simple Japanese koji recipes to elevate your everyday dish!

Shio Koji (Salt koji, 塩麹)

If you're a beginner, I highly recommend making shio koji as a first step! Shio means salt in Japanese, and shio koji is a simple fermented seasoning made with rice koji, salt, and water.

• What it is: Shio koji can be used as a sauce, a seasoning, and a marinade! Use this instead of salt to add umami and reduce salt (you don't need much shio koji to season food, as umami deepens the flavor). It also tenderizes chicken, pork, or fish when it's used as a rub or marinade.
• How to use: Coat your protein in Shio Koji and let it marinate in the fridge for the koji magic to happen! Let it sit for at least 30 minutes, or overnight for the best results, before cooking. Also, add it to soups, stir fries, and salad dressings to add umami.

You can easily make shio koji at home. But if you want to start cooking with it right away, you can also get premade Shio Koji online. Click here for the traditional Shio Koji on Amazon.

Put it into Practice: Want to see the power of Shio Koji in action? Try my Creamy Roasted Tomato Soup with Shio Koji for a dairy free umami boost. The koji enzymes transform simple roasted tomatoes into a umami-rich, complex bowl of comfort! And blended shio koji becomes like creamy milk without dairy.

Shoyu Koji (Soy Sauce Koji, 醤油麹)

Shoyu means soy sauce in Japanese, and shoyu koji is an upgraded soy sauce condiment packed with umami and sweetness with irregistable aroma.

• What it is: The Koji enzyme reacts with soy sauce to create umami and sweetness (more umami than shio koji). It also tenderizes meat and fish.
• How to use: I love using this over grilled salmon or marinating chicken to make karaage (fried chicken). It also makes great dressings and is an umami-rich sauce over dumplings.

Amazake (甘酒)

Amazake means sweet sake in Japanese, but amazake made with koji is not an alcoholic drink (Amazake made with sake lees has a little bit of alcohol). Authentic amazake is very sweet without any added sugar, making it a healthy sweetener. It's often called "drinkable IV drip" in Japan because it's so full of vitamins!

• What it is: By fermenting rice koji and rice, the mixture turns thick, creamy, naturally sweet drink.
• How to use: You can drink it warm, blend it into your morning smoothie, or even use it as a natural sweetener in desserts and cooking.

You can make at home or get premade amazake at Amazon!

Miso Paste (味噌)

Miso paste is just made with rice koji, soybeans, and salt. Making miso at home is surprisingly simple and easy! The complex flavor is developed during the long fermentation process.

• What it is: Traditional Japanese seasoning paste.
• How to use: Make miso soup, dressings, marinades, glazes, and baking.

Put it into Practice: Simple miso soup is an ultimate comfort food. Try my Kabocha Squash Miso Soup to enjoy the rich flavor of miso paste.

Quick Pickle with Shio Koji

Shio koji instantly boosts the flavor of vegetables! Make simple Tsukemono (Japanese quick pickles) for the best gut-healthy side dish.

• How to use: Slice up cucumbers, radishes, carrots, or avocado, toss them in a bag with a little Shio Koji, and massage gently. Let them sit for 1-2 hours. You'll have a crunchy, savory, probiotic-rich side dish ready for your lunch bowl!