Homemade UMAMI-Rich, Japanese Fermented Seasoning: Soy Sauce Koji (Shoyu Koji)
- Aug 6, 2025
- 4 min read
Updated: Feb 23
Two Easy Methods for Rich, Umami-Packed Flavor
Soy Sauce Koji is a delicious, fermented seasoning made from soy sauce and rice koji. It's incredibly easy to make at home with just two ingredients, and it adds deep umami to everything from stir-fries and dressings to marinades and soups. In this post, I’ll show you how to make it in just 8 hours using a fermentor — or the traditional method over 2 weeks at room temperature.
CONTENTS
WHAT IS SOY SAUCE KOJI? |
Soy Sauce (Shoyu) Koji is a flavorful Japanese condiment made by fermenting rice koji with soy sauce. This enzyme-rich blend breaks down proteins and starches in food, unlocking savory umami and natural sweetness. It’s gut-friendly, full of amino acids, and a versatile substitute for salt or soy sauce in cooking.
GUT HEALTH BENEFITS OF SHOYU KOJI |
Like other koji-based ferments, Shoyu Koji is rich in enzymes and beneficial microorganisms. These support digestion by helping break down food more efficiently, easing the load on your digestive system. Fermented seasonings like this may also help balance the gut microbiome — an important factor in immune health and mood regulation.
INGREDIENTS | (Makes about 300ml)
100g dried rice koji
200ml soy sauce
Method 1: Quick Fermentation Using a Fermentor in 6-8 Hours
Sterilize your tools: Place all utensils and containers in boiling water for at least 8 minutes. Use only heat-resistant, BPA-free glass or plastic. Let air dry completely.
In a sterilized container, combine Ingredients: Mix rice koji and soy sauce in a heat-safe container. Stir until evenly coated.
Ferment: Place in your fermentor and set to 60°C (140°F) for 6-8 hours. Alternatively, you can use sous vide (my recommendation: Anova Sous Vide Precision Cooker 3.0), or rice cooker with a warm setting. The key is the temperature control!
Stir Occasionally: Stir every 3–4 hours for even fermentation.
Cool and Store: Let it cool, then transfer to a clean jar. Store in the fridge.
Method 2: Traditional Room Temperature Fermentation (2 Weeks)
If you don’t have a fermentor, no problem! Here's how to make it the traditional way:
INSTRUCTIONS |
Sterilize your tools (same as above).
Mix Ingredients: In a clean jar or container, combine rice koji and soy sauce.
Stir Daily: Stir once a day with a clean spoon.
Ferment for 10–14 Days: Keep the jar at room temperature, ideally between 68–72°F / 20–22°C.
Check for Readiness: After about 10 - 14 days...
✔ It’s ready when:
The koji is softened
The aroma is rich and slightly sweet
The flavor is mellow and umami-rich
👉 Once ready, store it in the refrigerator and use within 3 months.
HOW TO USE SOY SAUCE KOJI |
As a marinade for tofu, chicken, or fish
Mixed into salad dressings or dipping sauces
Stirred into soups, rice, or noodles
As a soy sauce alternative with added depth
SHOPPING TIPS |
Miyako RICE KOJI:
UPDATED on 2/19/2026:
I recently found this dried koji (made in Japan) on Amazon, and it has become my new go-to for homemade miso and all of my koji-based seasonings.
This brand is more cost-effective while still maintaining excellent quality. The grains are consistent, the fermentation is stable, and the flavor develops beautifully — especially for miso, salt koji, soy sauce koji, and amazake. Good koji is the foundation of everything. If your koji is high quality, your final product will naturally taste more balanced and deeply umami-rich.
This is the one I personally use now in my kitchen.
Oryzae RICE KOJI:
UPDATED on 2/19/2026:
This is another excellent dried koji option available on Amazon, also made in Japan.
It is slightly more expensive than my current go-to brand, but the quality is very good. If my primary choice isn’t available, this is the one I purchase.
One thing I always tell my students: try a few different koji brands. Just like wine or miso, there are subtle differences in aroma, sweetness, and enzymatic strength. Over time, you’ll find the one that fits your preference and cooking style. Quality koji truly makes a difference.
Other RICE KOJI:
Look for dried rice koji at Japanese or Asian grocery stores in the refrigerated section or online. Brands like Cold Mountain, Marukome, or Hikari are reliable. Japanese supermarkets often carry a few different kinds of KOJI. Online retailers like Nijiya, Mitsuwa, Marukai, Amazon, or Japanese Pantry shops are good options!
SOY SAUCE [Shoyu]:
Look for soy sauce that only contains water, soybeans, wheat, and salt—nothing else! And of course, it has to "NO MSG"!
My recommendation is "Kikkoman Organic Soy Sauce", which is made with whole soybeans (a.k.a, Marudaizu) instead of defatted, processed soybeans. This is a great quality to look for when choosing a good soy sauce for making Soy Sauce Koji!
GLUTEN-FREE SOY SAUCE [Shoyu]:
Many soy sauces — and even some tamari — contain wheat. If you need gluten-free, always check the ingredient list carefully.
It should contain only:
Soybeans
Salt
Water
Koji culture
This gluten-free version from Kikkoman is simple and reliable, and I often recommend it to students who are avoiding gluten but still want proper fermentation quality.
GLUTEN-FREE TAMARI SOY SAUCE [Tamari Shoyu]:
This is a premium tamari soy sauce aged for 2 years in wooden barrels.
It is:
Gluten-free
Made in Japan
Non-GMO soybeans
Rich, intense umami
Full-bodied and complex
The depth of flavor is beautiful.
FINAL THOUGHTS |
Whether you're using a fermentor or sticking to the traditional method, making Shoyu Koji at home is simple and rewarding. It’s a powerful, gut-friendly seasoning that enhances both flavor and health. Try it in your favorite dishes and feel the difference — your gut will thank you!
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