Japanese fermentation: What is Koji?
Let’s talk about koji — the quiet powerhouse behind so many of Japan’s umami flavors. At its core, koji is simply rice (or sometimes barley or soybeans) that’s been inoculated with a mold called Aspergillus Oryzae — the national mold of Japan! (Yes, Japan has its own mold) It’s the invisible hand behind miso, soy sauce, amazake, shio koji, soy koji, sweet koji and more, unlocking deep, layered flavor through fermentation.
Today, more Michelin-starred chefs around the world are experimenting with koji — aging meats with shio koji to boost tenderness and savoriness, making koji marinades that bring out the sweetness in vegetables, or even crafting their own koji butter and fermented sauces for unexpected complexity. If you’ve worked with koji sauces like shio koji, you already know it can add roundness and a quiet depth that lingers.
For koji beginners, let’s briefly go over the basic mechanism of koji, why it brings so much umami to your dishes, and the 3 foundational types of koji sauces. To get a deeper insight, we talked to Jinbo-san a fifth-generation miso maker in Tochigi — the koji master who gave us a glimpse into what it takes to craft exceptional koji. Read on to find out.
Why is Koji rich in umami?
Here’s where the magic of koji gets scientific. As it grows, koji produces about 100 different powerful enzymes — most notably, Amylase and Protease. Amylase breaks starches into sugars, giving koji-based ingredients like amazake their gentle, natural sweetness. Protease breaks proteins into amino acids, including glutamic acid, the compound behind that unmistakable umami flavor.
These enzymes are the reason why koji doesn’t just add flavor, it transforms. That’s also why koji works so well as a marinade. It tenderizes meats and fish from the inside out while amping up depth, making it one of the most versatile tools for professional kitchens.
3 types of Koji sauces for chefs: Shio Koji, Soy Koji, Sweet Koji
There’s more than one way to harness the power of koji in professional kitchens, and koji sauces are where things get especially chef friendly. Let’s take a look at the features and application examples of the 3 basic types of koji sauces.
1.Shio Koji
Shio koji is probably the most versatile of the bunch. It’s a fermented mix of rice koji, salt and water. Think of it as a marinade, seasoning, and umami booster all in one. It tenderizes proteins, draws out natural sweetness in vegetables, and adds subtle complexity without overpowering. Perfect for flavor layering. Brush it on fish before grilling, or use it as a base for vinaigrettes, glazes or compound butter. Try using shio koji in place of where you’d use salt and you’ll taste the difference.
2.Soy Koji
Soy koji is made by fermenting soybeans with rice koji and salt. It’s richer, darker, and carries a deeper savory profile compared to shio koji. Why? It has over 10 times more glutamic acid. Think somewhere between soy sauce and miso but with its own personality. While shio koji is suited for layering flavor, soy koji has a powerful flavor on its own.
3.Sweet Koji
Sweet koji, also known as the base for making amazake, is where things take a gentler turn. Made by fermenting rice with koji and water, it’s naturally sweet with no added sugar. Use it to sweeten sauces, drinks, or desserts where you want both body and umami. For starters, try replacing sugar or mirin for a nuanced sweetness.
If you’re thinking to yourself, “I make my own koji sauces”, hold on for one sec. We still recommend you give these koji sauces a try. You’ll immediately notice the clean, refined flavor just as many of the Michelin-starred chefs we visited have given their praises. What sets them apart? The quality of the koji itself.
The Process of Koji fermentation: How to craft high-quality Koji
In Japan, brewers and miso makers pass down koji-making techniques through generations. Each maker has their unique method and philosophy of making koji and it’s almost impossible to replicate. Chefs who’ve tasted the results know why it’s worth the effort.
Crafting high-quality koji isn’t just about having the right ingredients. It’s about mastering every step of a delicate, time-sensitive process. Jinbo san, a fifth-generation miso maker in Tochigi walked us through how he makes koji the traditional way.
It starts with hydrating rice until the moisture reaches just the right level. Then it’s steamed until the grains are fluffy on the surface but still springy in the center. Once cooled to about body temperature, the rice is evenly inoculated with koji spores.
From there, things move into the muro, the fermentation room. He transfers about 300 – 400 lbs of rice onto moveable trays and moves it into the muro for 48 hours of carefully controlled fermentation. Inside, temperature and humidity must be monitored constantly. But machines can only do so much.
As the koji mold starts to grow, it generates heat. If that heat isn’t released, temperatures can spike to 140°F, killing the culture. That’s where the most physically demanding part comes in. It’s called momi-hogushi or hand-breaking the clumps of rice. Jinbo san says it’s like doing construction work in a sauna. Twice during fermentation, he loosens every single clump by hand to ensure airflow and healthy mold growth — something no machine can do properly. He swears this is the most important part of the entire koji-making process. No exaggeration.
This is easier said than done in such an extreme environment. Most people will be tempted to call it a day after most of the grains are broken apart. But koji masters like Jinbo san who care deeply about the final outcome know very well, small details make a big difference in quality.
What’s fascinating is that even when using the same strain of koji mold, the final character of the koji depends heavily on how it’s grown. Shio Koji, Soy Koji, and Sweet Koji all use the same koji mold, but their flavor shifts based on how the mold is nurtured. Adjusting temperature during key phases encourages certain enzymes to flourish. The higher temperatures bring out amylase, for the sweeter products like amazake or sake, while lower ranges support more balanced, savory koji sauces. According to Jinbo san, temperature control can be automated, but the touch, timing, and tenacity needed to keep koji alive? That still rests in human hands.
Protecting the future of rice koji and small farmers in Japan
During our interview with Jinbo san, we also talked about the importance of cultivating high-quality rice, currently facing a crisis due to multiple challenges. Koshihikari rice is prized for its ideal texture and starch structure for koji making, but it’s becoming harder to grow in parts of southwestern Japan. Rising water temperatures due to the climate crisis is preventing farmers from cultivating high-quality grain.
On top of that, Japanese farmers are aging with the number of young people stepping in to take over? Devastatingly low. At the same time, Japan’s domestic demand for rice has dropped year after year. This is directly affecting the future of koji making.
Chefs, by choosing the koji products on The House of Umami, you’re not only getting unparalleled ingredients, you’re a part of the system that’s preserving a tradition and craft – essential to today’s culinary world. You can support the rice farmers and koji makers by purchasing our koji sauce collection here.