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Homemade shoyu koji in a glass jar with fermented rice koji and soy sauce.

Shoyu Koji (Japanese Fermented Soy Sauce Koji Recipe)

Yoko Segawa
Shoyu koji is a Japanese fermented seasoning made with soy sauce and rice koji. This easy recipe creates a rich, umami-packed condiment perfect for marinades, sauces, and everyday cooking.
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Prep Time 5 minutes
Fermentation Time 7 days
Total Time 7 days 5 minutes
Course Condiment
Cuisine Japanese
Servings 1 cup

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

Note 1: I like using a kitchen scale to measure the ingredients. When measuring soy sauce in weight, keep in mind that soy sauce is heavier than water. 130ml of soy sauce is about 150g.
Note 2: Signs your Shoyu Koji is ready:
  • 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.
Keyword fermentation, Japanese ingredients, Japanese pantry, koij, umami
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