Shoyu Koji

If you've made shio koji and fallen for its tenderising, umami-rich magic, let me introduce you to its equally brilliant sibling: shoyu koji. Instead of fermenting rice koji with salt and water, you ferment it with soy sauce. The result is a thick, dark, slightly sweet, intensely savory paste that tastes like the best parts of soy sauce and miso had a baby.

If you're familiar with Indonesian kecap manis—that thick, sweet, syrupy soy sauce—shoyu koji lives in similar territory. It's not as overtly sweet (no palm sugar here), but it has that same viscous, almost syrupy quality and a natural sweetness that develops from the koji's enzymes breaking down starches into sugars. The umami is profound. The texture is luscious. And unlike kecap manis, it's a living ferment, full of beneficial enzymes and bacteria.

Shoyu koji works anywhere you'd use soy sauce, but with more depth and complexity and sweetness.  And here's the killer application: fresh tomatoes. Slice ripe tomatoes, drizzle with shoyu koji, with the rice still there, before blending. Hey I don't always blend mine anyway.  The tomato's acidity, the shoyu koji's umami and sweetness, the way it clings to each slice—so good. 

Makes approximately 400ml

Ingredients:

• 200g rice koji (fresh or dried)
• 200ml soy sauce (Japanese shoyu is traditional, but any good-quality soy sauce works) I love to use smoked soy sauce for this one too! So good. 

That's it. Two ingredients.

Equipment:

• Clean glass jar (500-750ml capacity)
• Spoon for stirring
• Breathable cover (cloth, paper towel, or loose lid)

Method:

1. Combine koji and soy sauce: In your jar, mix the rice koji and soy sauce together thoroughly. The koji should be completely covered by the soy sauce. It will look like rice swimming in soy sauce initially.
2. Cover loosely: You want airflow, but you also want to keep dust and insects out. Use a cloth secured with a rubber band, a paper towel, or a jar lid rested on top (not screwed on tight).
3. Ferment at room temperature: Place the jar somewhere at room temperature, out of direct sunlight. Ideal temperature is 18-25°C.
4. Stir often: This is the only active work required. Once a day, if you can, give your shoyu koji a good stir. You'll notice changes:
   • Days 1-3: The mixture looks grainy, smells strongly of soy sauce
   • Days 4-7: The koji grains soften, the mixture becomes creamier, and a subtle sweetness develops
   • Days 7-10: The grains are very soft (you can crush them easily between your fingers), the paste is thick and glossy
   • Days 10-14: The flavor mellows, becomes more complex, and the sweetness rounds out
5. Taste and decide: After 7 days, taste it. If the koji grains are soft and the flavor is rich, sweet, and deeply umami, it's ready. If it's still quite sharp or the grains are firm, let it go a few more days. Some people blend theirs into a smoothe sauce. I like the grains. 
6. Refrigerate: Once it's ready, transfer to the fridge. The cold slows fermentation dramatically. Shoyu koji will keep for 6 months or longer refrigerated.

Cultural Context: Japan Meets Indonesia (Sort Of)

Shoyu koji is thoroughly Japanese—part of the koji fermentation tradition that includes sake, miso, soy sauce, mirin, and amazake. It experienced a resurgence in Japan around the same time as shio koji (early 2010s), when home cooks rediscovered these traditional techniques and started experimenting.

Kecap manis, on the other hand, is Indonesian—a thick, sweet soy sauce made with palm sugar, star anise, and garlic. It's essential in Indonesian cooking: nasi goreng, satay, gado-gado. The two condiments come from completely different traditions and aren't interchangeable in recipes.

This comparison only matters: if you've tasted kecap manis and loved that thick, sweet, umami intensity, then shoyu koji will make sense to you immediately. It occupies similar flavor territory—dark, glossy, sweet-savory, coating—even though it gets there through entirely different means. Kecap manis uses palm sugar for sweetness; shoyu koji creates sweetness through enzymatic breakdown of the starche in rice. Kecap manis is cooked; shoyu koji is raw and alive and full of digestive enzymes. 

Think of them as cousins twice removed: both from soy, both umami-rich, both sweet, but shaped by different culinary histories.

How to Use Shoyu Koji:

The tomato revelation:

• Slice ripe, room-temperature tomatoes thickly
• Drizzle with shoyu koji (be generous)
• Optional: add a touch of good olive oil, flaky salt, torn basil
• Serve immediately
• This is especially stunning with heirloom tomatoes in summer

As a marinade:

• Chicken thighs: coat and marinate for 2-4 hours, then grill or roast
• Salmon or white fish: brush on before pan-frying or baking
• Eggplant slices: coat and grill until caramelised
• Tofu steaks: marinate for 30 minutes, then pan-fry

As a finishing sauce:

• Drizzle over roasted vegetables (especially mushrooms, broccolini, or brussels sprouts)
• Stir through fried rice or noodles at the end of cooking
• Spoon over grilled steak or lamb chops
• Mix into grain bowls for instant umami

As a seasoning:

• Stir into soups or stews instead of soy sauce (add at the end to preserve enzymes)
• Mix into salad dressings (shoyu koji + rice vinegar + sesame oil = magic)
• Use as a dipping sauce for dumplings or spring rolls
• Brush onto grilled corn or sweet potato

The ratio: Use shoyu koji anywhere you'd use soy sauce, but remember it's thicker and slightly sweeter, so start with less and adjust to taste.

Notes:

• Soy sauce choice: Japanese shoyu (especially koikuchi/dark soy sauce) is traditional and gives the most authentic flavor. Tamari works beautifully too, especially if you want gluten-free. Chinese soy sauce works but will give a slightly different flavor profile.
• Texture: Shoyu koji should be thick, glossy, and almost paste-like when ready. If it's too thick, you can thin it with a tiny bit of water or more soy sauce.
• Colour: It will be very dark brown, almost black. This is normal—the soy sauce is already dark, and fermentation deepens the color.
• Sweetness development: The sweetness comes from amylase enzymes breaking down starches in the rice koji into simple sugars. It's not cloying or candy-sweet—it's a balanced, subtle sweetness that enhances savory dishes.
• Fermentation time: I've fermented mine for months and months - it just got richer. 
• Raw vs. cooked: Shoyu koji is alive—full of enzymes and bacteria. If you add it to hot dishes, those enzymes will be destroyed, which is fine (it still tastes delicious), but you lose the tenderising and digestive benefits. For maximum benefit, use it raw as a finishing sauce or marinade.

Why This Works

Shoyu koji is fermentation at its most elegant: two ingredients, time, and enzymatic transformation. The koji's enzymes break down the proteins and starches in the rice, creating amino acids (umami) and sugars (sweetness). The soy sauce provides salt, which controls fermentation and adds its own deep umami. The result is a condiment that's greater than the sum of its parts.

And that tomato trick? It works because tomatoes are naturally high in glutamates (umami compounds), and shoyu koji amplifies that while adding sweetness and salt. It's a perfect umami explosion. Try it once in summer with a ripe tomato from the market, and you'll make shoyu koji every year for the rest of your life.

Rice koji available at here online
Also look at making shio koji and our Shio Koji Marinated Mushrooms - see recipes here.