Soy sauce is a fermented liquid condiment made from soybeans, roasted wheat, salt, and water, with origins in East Asia. This recipe guides you through the traditional two-stage fermentation process (koji and brine fermentation) to produce a rich, savory, and complex sauce at home. While it requires patience and precise conditions, the result is a deeply flavorful sauce that surpasses most commercial versions.
Ingredients
- 2 cups (400 g) dried soybeans
- 2 cups (250 g) whole wheat berries (or all-purpose flour)
- 1/2 tsp (2 g) koji starter (spores of Aspergillus oryzae)
- 3 cups (720 ml) filtered water (for brine)
- 1 cup (300 g) sea salt (non-iodized)
- 4 cups (960 ml) filtered water (for steaming/soaking)
Instructions
- Prepare the soybeans: Rinse the dried soybeans and soak them in 4 cups (960 ml) of water for 12–24 hours. Drain and steam them over boiling water for 2 hours, or until they are soft enough to mash easily between your fingers. Let them cool to room temperature.
- Prepare the wheat: Toast the wheat berries in a dry skillet over medium heat for 5–7 minutes, stirring frequently, until they are fragrant and lightly browned. Let them cool, then coarsely grind them in a food processor or blender (you want a cracked texture, not a powder).
- Make the koji: In a large, clean bowl, mix the cooled soybeans and cracked wheat. Sprinkle the koji starter over the mixture and toss thoroughly to distribute. Spread the mixture evenly in a shallow, perforated tray (like a bamboo steamer basket) to a depth of about 1 inch (2.5 cm). Cover with a clean, damp cloth. Incubate at 86°F (30°C) for 48–72 hours, stirring once or twice daily to keep the temperature even and prevent drying. The mixture should develop a white, powdery mold covering the grains and a pleasant, nutty aroma. This is your koji.
- Make the brine: In a large pot, dissolve the salt in 3 cups (720 ml) of water. Bring to a boil, then let it cool completely. The brine should be about 20% salt by weight (300 g salt to 1.2 kg water).
- Combine and ferment: Transfer the koji to a large glass jar or ceramic crock. Pour the cooled brine over it, stirring well. Cover the jar with a breathable cloth (like cheesecloth) and secure with a rubber band. Place in a cool, dark place (60–70°F / 15–21°C). This is the start of the brine fermentation (moromi).
- Stir and age: Stir the mixture once a day for the first month, then once a week for the next 5 months. The mixture will become a thick, dark paste. As it ferments, a liquid layer will form on top. Continue to stir to keep the solids submerged. After 6 months, the sauce should be dark brown, with a complex, savory aroma.
- Press and strain: Line a fine-mesh sieve with several layers of cheesecloth. Pour the moromi into the sieve and let it drain. Gather the cloth and squeeze firmly to extract as much liquid as possible. The liquid is your raw soy sauce.
- Clarify and pasteurize: Let the raw sauce settle in a jar for 24 hours in the refrigerator. Carefully pour off the clear liquid, leaving sediment behind. To stabilize and develop flavor, heat the sauce to 160°F (71°C) in a saucepan, then let it cool. This is optional but recommended for shelf stability.
- Bottle: Pour the finished soy sauce into sterilized bottles or jars and seal.
Tips
- Koji starter: You can buy koji starter (spores) online or at Asian grocery stores. If you can’t find it, you can use a piece of fresh koji (used for making miso) as a starter, but the process is less reliable.
- Temperature control: The koji incubation temperature is critical. If it gets too hot (above 95°F / 35°C), the mold may die. If it’s too cool, fermentation slows. A proofing oven or a warm spot with a thermometer works well.
- Patience is key: The flavor develops over months. Do not rush the fermentation. A longer fermentation (up to 12 months) yields a deeper, more complex sauce.
- Salt level: The high salt concentration prevents spoilage. Do not reduce the salt, or harmful bacteria may grow.
Serving Suggestions
Use your homemade soy sauce as a dipping sauce for dumplings, sushi, or spring rolls. It also works beautifully in stir-fries, marinades for meat or tofu, and as a seasoning for soups and noodle dishes. A splash adds umami to any savory dish.
Variations
- Tamari: Omit the wheat entirely and increase the soybeans to 3 cups (600 g). This produces a gluten-free soy sauce with a slightly thicker, richer flavor.
- Sweet soy sauce (Kecap Manis): After fermentation, stir in 1 cup (200 g) of palm sugar or brown sugar per 2 cups (480 ml) of sauce until dissolved. This is an Indonesian-style sweet soy sauce.
- Increased umami: Add a few dried shiitake mushrooms to the brine during fermentation for an extra layer of savory depth.
Storage and Reheating
Homemade soy sauce will keep in the refrigerator for up to 6 months, and at room temperature for up to 3 months if pasteurized. Always use a clean, dry spoon to avoid introducing bacteria. The sauce does not need reheating; it is used cold or at room temperature. Over time, a small amount of sediment may form – this is natural and harmless.
Frequently Asked Questions
Can I use regular flour instead of wheat berries? Yes, you can substitute all-purpose flour. Toast it lightly in a dry pan until pale brown, then cool. The texture will be finer, but the flavor will be similar.
Why is my koji not growing mold? This is usually due to temperature being too low or too high, or the mixture being too dry. Ensure the incubation area is consistently around 86°F (30°C) and that the cloth stays damp. If nothing happens after 72 hours, the koji starter may be inactive; try again with a fresh batch.
Is it safe to eat if I see black or green mold? No. Black or green mold indicates contamination, not the desirable white koji mold. Discard the batch and start over, ensuring all equipment is thoroughly sterilized.
Can I speed up the fermentation? You can increase the temperature to 80°F (27°C), but this may produce off-flavors. The traditional slow fermentation at cooler temperatures yields the best complexity.
How do I know when it’s done? The sauce should be dark, clear, and have a balanced salty-savory aroma. If it tastes too bitter, ferment longer. If it tastes flat, it may need more time or a touch more salt.
Can I reuse the leftover soybean mash? Yes, the spent solids (okara-like) can be used in veggie burgers, mixed into bread dough, or composted. It is not suitable for making another batch of soy sauce.
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