The Umami Fungi: Koji Culture in Japan
The humble mold Koji is the secret to the rich umami flavors of iconic Japanese foods and beverages like miso, soy sauce, sake, mirin, and more.
In Japan, people mix it with a variety of foods to preserve, boost flavor, and reap the many health benefits that might just be the key behind Japanese longevity.
Koji is a powerful agent for fermentation with unique properties, and best of it all – it’s fairly easy to use at home. Let’s take a deeper look into what koji is, how it works, what famous Japanese products we have thanks to it, and how you can use koji in your own cooking to harness its umami power.
Fermenting miso
Multi-millennial Flavors
Civilizations throughout history cherished fermented foods. From beer, wine, sourdough bread, sauerkraut, cheese and yogurt, to miso and sake, fermentation usually happens thanks to fungus: Yeasts and molds have the power to unlock intricate flavors in ordinary ingredients.
Koji (麹 or 糀, Aspergillus oryzae) is a fungus that has been used for more than 2000 years in East Asia.
From rice-based drinks, booze, and seasoning, koji fungus is deeply integrated in the Chinese diet, supposedly going as far back as 7000 BCE.
However, koji-based products were brought to brand new heights in Japan.
Japanese people first brewed sake, rice wine, a bit after they started cultivating rice around 300 BCE. The flavorful rice wine is a national symbol today.
In the many centuries since then, sake, mirin, amazake, miso, soy sauce, and many other koji products got perfected and became the basis for much of Japanese cuisine.
In fact, this humble mold is so important for local food production that in 2006 it received the status of “National Fungus” of Japan.
In Japan, koji was traditionally used to ferment grains like soy and rice or marinate fish (koji salmon is delicious!), meat, and veggies.
More recently, chefs around the world embraced this powerful mold. Many are still finding new tasty possibilities with koji, adding it to meat, nuts, fruit, bread (yes – bread miso), and much more.
Health Benefits
Koji mold is not only edible – it’s beneficial for your health.
Many believe that the abundance of fermented food is the secret to Japanese longevity. While there’s probably more to it, eating fermented food is indeed good for you. It acts as a probiotic, boosting good bacteria and aiding the gut flora – especially if it’s living food like yogurt, miso, or dishes marinated in shio-koji.
How Koji Works
It’s a mold that eats and grows, so koji lives inside foods, making fermentation possible.
Small but powerful, koji spores are only between 3 and 10 micrometers large. As it eats, it releases enzymes that break down large substances into simpler forms.
The perfect condition for Aspergillus oryzae growth are 30°C and 75% humidity. It’s no wonder that koji thrives in the hot and humid summers in Japan!
As it grows, koji releases enzymes that break down nutrients: proteins, starches, and fats – this is a part of the mold’s eating and growing, and the secret behind its food processing powers.
These enzymes tenderize foods, give it umami flavor, and help with preservation.
Koji uses amylase to break down complex carbohydrates, which become sweeter in their simpler forms.
Proteins broken down by koji protease become flavorfully savory amino acids. That’s how koji turns the bland, protein-rich soybean into the aromatic miso.
Koji processes many kinds of nutrients, tenderizing them and imbuing them with umami – it’s the main reason why this mold is gaining popularity worldwide. Essentially, you can add koji to any kind of food and have something tasty come out of it.
It’s not only used to make miso, soy sauce, and sake – koji is also used to make pickles and marinate meat.
Home-Grown Koji
To this day, some Japanese families make koji-fermented foods like miso and pickles at home. With a bit of equipment to get the conditions right, you can make your own koji culture and ferments too.
You can grow your own koji culture on steamed and inoculated rice.
Koji rice is also a store-bought product – rice malt inoculated with koji, ready to ferment into shio-koji with water and salt
Koji Spores
To start the culture, you need koji spores. In Japanese, these are known as koji-kin, 麹菌 or tane-koji, 種麹. Add the spores to steamed grains like soybeans, barley, or rice (in a process called inoculation). Then, leave the grain and koji in warm, humid conditions – let it develop.
In a few days, your rice will change texture and become fluffy, grains covered with the filamentous “fur” of koji. This is how you get a koji culture that you can use to ferment anything else.
Cooks, remember: in many of its forms, koji is a living food. The fungus culture continues to live in your fridge, and activates to eat and grow when you use it with another food. It doesn’t go bad easily, unless contaminated. Give it some time, always use clean utensils, and don’t toss it out right away if it smells a bit funky – it is mold, after all, it should be a bit funky. But if something doesn’t look right, Reddit is a good forum to ask.
Shio-koji
However, growing a koji culture and using it for fermentation requires a good set-up and extra time.
If you’re just trying koji out, you can get Shio-koji (塩麹, literally “salt koji”) – a sweet-smelling paste of fermented koji rice and salt that you can use as a marinade for just about anything.
Later along your journey, you can make shio-koji at home, using your self-grown koji culture. It’s a simple ferment of water, salt, and rice koji.
Sake Kasu
Along with shio-koji, sake kasu (酒粕) or sake lees is a popular marinade in Japanese cuisine. In essence, sake lees are a byproduct of sake production. It’s a fruity white paste that can imbue umami into many kinds of fruit, veggies, and fish.
Mirin also has its own lees product – mirin kasu, which tastes sweeter.
Types of Koji Mold
The most commonly used and mentioned type of koji is Aspergillus oryzae. However, it’s not the only one, by far. The Aspergillus fungus family is large, and many of its types are used in Japanese cuisine:
Yellow Koji – Aspergillus oryzae and Aspergillus sojae
Used in the production of miso, soy sauce, sake, and amazake.
As their names suggest, the oryzae strain works great with rice – it produces the right ratio of enzymes proteases for proteins and amylases for starches in the rice.
Soybeans have more protein, and the koji strain Aspergillus sojae makes more proteases to answer to the task.
Miso paste can be made of varying ratios of grain: soy, rice, and barley are most often used. Depending on the mix, different types of yellow koji might be used in miso production.
Mixing grain, salt, and koji to make miso at home
Black Koji – Aspergillus iuchuensis
The black koji creates a lot of citric acid, which makes it a beloved part of the Okinawan awamori production. It’s also sometimes used to make other alcoholic beverages like some kinds of shochu and sake.
There are also
· White Koji (Aspergillus kawachii) is a mutant form of black koji, and it’s used to make shochu.
· Bonito mold (Aspergillus glaucus) is used in the traditional way of making katsuobushi, smoked and fermented skipjack tuna shavings. This type of koji helps with the process of drying the fish, breaking down fats, and imparting it with umami flavor.
Koji Charcuterie: Vegetables and Pickles
Koji works wonders on fish and meat, but why not think bigger – you can make koji charcuterie out of almost anything, including many types of veggies and fruits.
As one Redditor pungently described: “imagine a beet that carries the funkiness of a Genoa salami with an almost fatty richness that punches you right in the mouth.”
Koji can also be used on veggies – often root vegetables like carrots and beets, but you can try it on almost anything that’s not dry, fruit included. As a result, you get almost meaty, umami, flavorful, a bit funky veggie, its original flavor amplified so you only need a little piece for an explosion of flavor.
Making full-on koji vegetable charcuterie takes patience, but it’s an amazing vegan finger food option. You can also marinate or quick-pickle vegetables with shio-koji for tasty and healthy results. Recently, people discover more and more new uses of koji, like fermenting fruit like apples or nuts like pistachio.
Koji – Unlocking Umami
Koji fungus is much more than a mold; it is the very essence of Japanese culinary tradition and its deep umami flavors.
From crafting the foundational flavors of miso, soy sauce, and sake to creating modern twists on vegetables and meats, koji’s enzymatic powers transform ingredients into umami-rich delights.
Its ability to enhance flavor, preserve food, and support health makes it a staple in Japanese kitchens, and soon, among chefs and home cooks worldwide.
