Shoyu koji is a Japanese fermented seasoning made with just two ingredients, soy sauce and rice koji. It's rich in savory umami with a hint of sweetness, and it also helps tenderize meat and fish. It's very simple to prepare and ferment at home. Use it like soy sauce to elevate your everyday meal with deep fermented flavor.
If you've already tried shio koji, shoyu koji is another simple fermentation recipe to try! Use it to marinate proteins and stir it into dressings, adding irresistible flavor to your everyday cooking.
If you're new to koji, make sure to check out my Koji Guide to discover why it's such an important ingredient in Japanese cooking!
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What Is Shoyu Koji?
Shoyu Koji is a Japanese fermented condiment made with koji rice and soy sauce. During fermentation, enzymes in the koji break down proteins and starches, creating a deep, complex umami and subtle sweetness.
You can use shoyu koji as a soy sauce substitute to elevate your everyday dish with a boost of umami. Not only for flavor, but the enzymes in shoyu koji also help tenderize meat and fish, making it especially useful for marinades.
It's a popular condiment in Japanese home cooking as a seasoning, sauces and marinade for meat, fish, and vegetables.
Why You'll Love Shoyu Koji
- Brings bold umami and complex flavor with a hint of sweetness.
- Easy fermentation recipe with 2 ingredients.
- Versatile seasoning for everyday cooking.
- Elevating soy sauce with a boost of umami.
- Helps enhance the natural flavor and tenderize ingredients.
Ingredients
You'll need the following ingredients to make Japanese soy sauce koji:
- 100g koji rice
- 130ml (150g) soy sauce, more as needed
Both regular white rice koji and brown rice koji work in this recipe.
Using high-quality soy sauce is key to the best flavor. I recommend using traditionally brewed soy sauce, which tends to have a rich flavor developed during fermentation (soy sauce is already a fermented food, so we're double fermenting!)
Those high-quality soy sauces are often sold in glass bottles, so it's worth checking the labels on those glass bottles in the soy sauce aisle to see how they're made.
How to Make Shoyu Koji (Step-by-Step)
- If the rice koji comes in chunks or blocks, break them apart into individual grains.
- In a clean jar, combine koji rice and soy sauce. Use a clean spoon to mix well.
- Loosely cover and let it ferment at room temperature for 5 days to 2 weeks, stirring once a day.
- Check the mixture daily. The rice absorbs soy sauce as it hydrates, so add more soy sauce to fully submerge the rice. When it starts to smell sweet and fruity, and the rice grains are soft, it's ready. Enjoy adding to your favorite recipes!
How to Tell Shoyu Koji Is Ready
Shoyu koji is ready when the flavor, texture, and aroma have bloomed into a fragrant mature version of soy sauce. Here are a few signs to look for:
- Texture: The mixture becomes slightly thicker and more cohesive. The rice koji are soft in the center and absorb the liquid. You can test the doneness by squeezing rice koji pieces between fingers. If they are soft and easily mash into a paste, it's ready.
- Aroma: It should have a slightly sweet and fruity aroma.
- Taste: Soy sauce flavor becomes more rounded and mellow, with a subtle sweetness that isn't present in soy sauce alone.
Shoyu koji usually takes about 5 days to 2 weeks to fully ferment at room temperature, depending on the environment of your home. It takes longer in winter (cold season) and shorter in summer (warm season), so make sure to check and stir daily!
How to Use Shoyu Koji
Shoyu koji is a very versatile condiment that easily boosts umami. Use it like soy sauce in your everyday cooking, and you'll be surprised how rich it tastes with minimal effort.
Marinade
Using shoyu koji as a marinade for meat and fish is one of the best ways to enrich flavor and tenderness. The enzymes help tenderize the protein while adding a rich, complex flavor.
Weigh your ingredients and use shoyu koji about 10% of the weight (e.g., 10g of shoyu koji for 100g of chicken). Or just enough to coat the surface of the ingredient.
You can use it in recipes like shio koji chicken for a tender, flavorful result.
- Pork: Brush shoyu koji on both sides of boneless, thin-cut pork chops and marinate for about 15-30 minutes. Then cook in a pan over medium to medium-low heat; shoyu koji can burn easily. Cover the pan if necessary to fully cook.
- Chicken: Cut the chicken thigh into bite-sized pieces and marinate in shoyu koji for about 15-30 minutes. Then cook in a pan until golden brown and cooked through. Cover the pan if necessary to cook through.
- Karaage (Japanese fried chicken): This is one of my favorite ways to use shoyu koji! Marinate cut chicken thigh pieces in shoyu koji (and some grated garlic too). Then coat them in potato starch and deep fry until crispy and fully cooked.
- Ramen Eggs (Ajitsuke Tamago): Making homemade ramen eggs is even easier with shoyu koji. Add soft-boiled eggs and shoyu koji to a resealable bag, coating the eggs thoroughly. Refrigerate overnight.
Olive Oil-Based Recipes
Shoyu koji and olive oil are a surprisingly delicious flavor combo! Richness from oil and umami from shoyu koji complement each other. Just a drizzle can enrich oil-based recipes with rich, fermented flavor.
- Gambas al Ajillo (Spanish Garlic Shrimp Dish): Add just a spoonful to Gambas al Ajillo to bring a deeper umami flavor, making you want to dip your bread endlessly.
- Pasta: Stir in a spoonful of shoyu koji to olive oil-based pasta, like garlic pasta or mushroom pasta. Savory umami makes them so irresistible.
- Roasted Vegetables: Toss vegetables with olive oil and a little shoyu koji before roasting. It helps bring out their natural sweetness while adding a rich umami flavor.
Sauces and Dressings
Add shoyu koji to sauces, stir-fries, or salad dressings for an extra umami boost. Dipping sashimi in shoyu koji or making crudo with it is also delicious.
You can also use koji in soups and sauces. For example, it adds depth to dishes like creamy roasted tomato soup.
In place of soy sauce
Use it as a substitute for soy sauce to enhance the flavor of vegetables, rice dishes, and simple meals. I love using it in fried rice!
Shoyu Koji vs Shio Koji
Shoyu koji and shio koji are both fermented condiments made with rice koji. They are packed with umami and work as natural flavor enhancers and tenderizers for fish and meat.
Shoyu koji is made with soy sauce, so it's naturally rich in amino acids from soybeans. During fermentation, enzymes work on proteins and develop a deeper, more complex umami flavor.
Shio koji, on the other hand, has a more neutral and mild flavor. It seasons the food beautifully while the enzymes in shio koji help enhance the natural flavor.
Both are versatile seasonings in Japanese cooking. Use the one depending on the dish and flavor you're going for.
When to Use, Shoyu Koji or Shio Koji?
- Shoyu Koji: Bold umami flavor. Best for sauces, dressings, and stir-fries.
- Shio Koji: Versatile flavor enhancer. Best for everyday use and salt substitute.
They both work great as marinades for meat and fish.
Storage and Shelf Life
Once fermentation is complete, store shoyu koji in an airtight container in the refrigerator and use within 3 months.
It will continue to develop flavor slowly over time, even when refrigerated.
Always use a clean spoon to both stir and use, keeping it clean and helping prevent contamination.
Tips for the Best Shoyu Koji
- Stir daily: It's important to mix once a day during fermentation! It helps even fermentation and prevents mold from growing.
- Submerge: Check daily and make sure the koji rice is fully covered in soy sauce. Add more soy sauce as needed.
- Keep it clean: Use a clean jar and a spoon.
- Use good ingredients: Using quality soy sauce makes a huge difference in flavor.
- Refrigerate after fermentation: It's important to put it in the fridge after the fermentation stage is done to keep it fresh longer.
FAQ
Yes, both fresh and dried rice koji work for making shoyu koji. I always keep dried rice koji in my pantry and make koji condiment.
It usually takes about 5 days to 2 weeks at room temperature, depending on the environment. It takes longer in the cold season and shorter in the warm season.
Shoyu Koji (Japanese Fermented Soy Sauce Koji Recipe)
Ingredients
- 100 g koji rice
- 130 ml soy sauce (150g) *see note more as needed
Instructions
- If the rice koji comes in chunks or blocks, break them apart into individual grains.
- In a clean jar, combine koji rice and soy sauce. Use a clean spoon to mix well.
- Loosely cover and let it ferment at room temperature for 5 days to 2 weeks, stirring once a day.
- Check the mixture daily. The rice absorbs soy sauce as it hydrates, so add more soy sauce to fully submerge the rice. When it starts to smell sweet and fruity, and the rice grains are soft, it's ready (see note 2). Enjoy by adding to your favorite recipes!
Notes
- The rice koji are soft in the center.
- Smells fruity, slightly sweet.
- The texture is slightly thick, creamy, and porridge-like.
- Taste a slight sweetness that wasn't present in soy sauce.
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