How to Make Soy Sauce at Home: A Fermentation Guide

Q: What’s the difference between soy sauce, tamari, and shoyu?

Shoyu is the Japanese word for soy sauce — it’s made with roughly equal parts soybeans and wheat. Tamari is a Japanese soy sauce made with mostly soybeans and little to no wheat, resulting in a thicker, less sweet sauce. Soy sauce is the broad English term covering all varieties. Chinese soy sauce tends to use a higher ratio of soybeans to wheat than Japanese shoyu.

Bottles of homemade fermented soy sauce in various stages of aging, from light amber to dark brown

Yes, you can make real fermented soy sauce at home — but it takes 6–12 months. This isn’t a quick weeknight project. Traditional soy sauce (called shoyu in Japanese) requires growing koji mold on soybeans and wheat, fermenting the mixture in brine for months, then pressing and pasteurizing the liquid.

The result is completely different from store-bought soy sauce. Homemade shoyu has a deeper, more complex umami flavor with sweet, fruity, and almost wine-like notes that commercial soy sauce can’t match. If you’re serious about fermentation, this is one of the most rewarding projects you can take on.

What You Need

Ingredients

2 cups dried soybeans — organic, non-GMO preferred
2 cups whole wheat berries (or all-purpose flour as a substitute)
Koji starter (Aspergillus oryzae spores) — available online from fermentation suppliers like Cultures for Health or GEM Cultures. About ½ teaspoon for this batch size.
1 cup sea salt — non-iodized (iodine can inhibit fermentation)
4 cups water — filtered or dechlorinated

Equipment

Large pot for cooking soybeans
Sheet pan or shallow baking tray (for koji incubation)
Clean kitchen towels or cheesecloth
Large glass jar or ceramic crock (half-gallon to 1-gallon)
Thermometer
Fine mesh strainer or cheesecloth for pressing
Sterilized glass bottles for storage

Step-by-Step Recipe

Step 1: Prepare the Soybeans

Soak 2 cups of dried soybeans in water overnight (12–16 hours). They’ll roughly double in size.
Drain, then cook the soybeans in fresh water. Boil for 4–5 hours until they’re soft enough to crush easily between your fingers. (A pressure cooker cuts this to about 45 minutes.)
Drain and let the cooked soybeans cool to about 95°F (35°C) — warm but not hot.

Step 2: Prepare the Wheat

Roast the wheat berries in a dry pan over medium heat until golden brown and fragrant, about 10–15 minutes. Stir frequently to prevent burning.
Let cool, then roughly crack the roasted wheat. You can use a blender on pulse, a grain mill, or put the wheat in a bag and crush with a rolling pin. You want coarse pieces, not flour.

Step 3: Grow the Koji (2–3 Days)

This is the most critical step. Koji (Aspergillus oryzae) is the mold that breaks down proteins and starches into the amino acids and sugars that create soy sauce’s flavor.

Combine the cooled soybeans and cracked wheat in a large bowl. Mix thoroughly.
Inoculate by sprinkling ½ teaspoon of koji spores over the mixture. Toss to distribute evenly.
Spread the mixture in a thin layer (about 1–1.5 inches deep) on a sheet pan or shallow tray lined with a clean kitchen towel.
Incubate at 80–86°F (27–30°C) with high humidity for 48 hours. Cover loosely with a damp towel.
- If you don’t have an incubation chamber, your oven with just the light on (no heat) often works. Place a pan of warm water on the bottom rack for humidity.
- Stir gently every 12 hours to ensure even mold growth.
The koji is ready when it’s covered in white or pale green mold with a sweet, mushroomy smell. If you see black, orange, or pink mold, discard and start over — those are contamination.

Step 4: Make the Moromi (Brine Mash)

Make a brine: Dissolve 1 cup of sea salt in 4 cups of water. Stir until fully dissolved.
Combine: Transfer the finished koji to a large glass jar or ceramic crock. Pour the brine over it and stir well.
The salt concentration should be about 17–18% by weight. This is high enough to prevent harmful bacteria while allowing the beneficial fermentation organisms (lactic acid bacteria and yeasts) to work slowly.
Cover the jar with cheesecloth secured by a rubber band. Don’t seal it airtight — the moromi needs some air exchange.

Step 5: Ferment the Moromi (6–12 Months)

This is the long stage. Place the jar in a spot with temperature fluctuations — traditionally, soy sauce is fermented outdoors where seasonal temperature changes drive different stages of microbial activity.

Stir the moromi every 1–2 days for the first month, then weekly after that. Stirring introduces oxygen and distributes the microorganisms evenly.
Color changes: The moromi will slowly darken from tan to deep brown over the months as Maillard reactions and enzymatic browning develop.
Aroma: The smell will evolve from grain-like to increasingly complex — malty, fruity, and deeply savory.
Minimum fermentation: 6 months. Traditional Japanese shoyu ferments for 12–18 months. Longer fermentation = deeper, more complex flavor.
Taste periodically starting at 6 months. When the flavor is rich, complex, and intensely umami, it’s ready to press.

Step 6: Press and Strain

Line a fine mesh strainer with several layers of cheesecloth. Place it over a large bowl.
Pour the moromi into the cheesecloth and let gravity do the work for several hours.
For maximum yield, gather the cheesecloth into a bundle and press firmly — or place a heavy plate on top as a weight.
The liquid that drains out is raw soy sauce. The solids (called shoyu kasu) can be used as a marinade paste or composted.

Step 7: Pasteurize and Bottle

Heat the raw soy sauce to 176°F (80°C) in a saucepan. Hold at this temperature for 15–20 minutes. This stops fermentation, kills any remaining organisms, and helps clarify the liquid.
Skim any foam that rises to the surface.
Let cool, then pour into sterilized glass bottles. Seal tightly.
Optional aging: Let the bottled soy sauce rest for 2–4 weeks before using. The flavor mellows and integrates during this time.

Timeline at a Glance

Stage	Time	What Happens
Soak soybeans	12–16 hours	Beans hydrate and soften
Cook soybeans	4–5 hours (45 min pressure cooker)	Proteins become accessible to koji
Grow koji	48 hours	Aspergillus oryzae mold colonizes soybean/wheat mixture
Ferment moromi	6–12 months	Enzymes, bacteria, and yeast develop umami flavor
Press and pasteurize	1 day	Liquid separated, stabilized, bottled
Optional aging	2–4 weeks	Flavors mellow and integrate

Troubleshooting

Problem	Cause	Fix
Koji won’t grow	Temperature too low or spores old/dead	Ensure 80–86°F and high humidity; use fresh koji starter
Black/green/pink mold on koji	Contamination	Discard and start over. Sanitize all equipment before next attempt
Moromi smells rotten	Insufficient salt or contamination	Check salt concentration (should be ~18%). If severely off, discard
Fermentation seems stalled	Too cold	Move to a warmer location (70–80°F). Stir more frequently
Too salty	High salt-to-water ratio	Dilute finished sauce slightly with water; blend with a less salty batch
Cloudy sauce	Incomplete pressing or residual particles	Re-strain through finer cheesecloth or a coffee filter
Weak flavor at 6 months	Normal — needs more time	Continue fermenting. Most soy sauce improves significantly between 6–12 months

Homemade vs. Store-Bought Soy Sauce

Factor	Homemade (Naturally Brewed)	Store-Bought (Mass Produced)
Fermentation time	6–18 months	2–3 days (chemical hydrolysis) to 6 months (naturally brewed)
Ingredients	Soybeans, wheat, salt, water, koji	Often includes caramel color, corn syrup, preservatives
Flavor complexity	Deep, layered umami with fruity and malty notes	Simpler, saltier, one-dimensional
Cost per bottle	~$2–5 in ingredients (makes ~2–3 cups)	$3–15 depending on quality
Effort	High — requires months of attention	None

Most cheap soy sauce is made using chemical hydrolysis — soybeans are broken down with hydrochloric acid in 2–3 days, then neutralized, colored, and flavored. It’s technically soy sauce, but it skips the entire fermentation process that creates real flavor complexity. Check the label: if it says “naturally brewed,” it was fermented. If it lists “hydrolyzed soy protein,” it wasn’t.

Gluten-Free and Alternative Versions

Traditional soy sauce contains wheat and is not gluten-free. For alternatives:

Tamari — a Japanese soy sauce traditionally made with little or no wheat. Many commercial tamari brands are certified gluten-free. You can make tamari at home by using soybeans only (no wheat) and following the same process above.
Coconut aminos — made from coconut sap, not soybeans. Lower sodium, slightly sweeter, but not a true fermented soy sauce.
Rice-based substitution — replace wheat with roasted rice for a gluten-free version. The flavor profile will be slightly different (less sweet, more straightforward) but still develops good umami.

Storage

Pasteurized homemade soy sauce keeps for 1–2 years in a sealed glass bottle at room temperature. Refrigeration extends its shelf life further and slows any ongoing flavor changes. Once opened, refrigerate and use within 6 months for best flavor.

Signs of spoilage: off smells (rotten rather than fermented), visible mold on the surface, or an uncharacteristic slimy texture. If the sauce just tastes stronger or more intense over time, that’s normal aging — not spoilage.

Frequently Asked Questions

Is soy sauce fermented?

Real soy sauce is, yes. Traditional soy sauce (Japanese shoyu, Chinese jiangyou) is made by fermenting soybeans and wheat with koji mold for 6–18 months. However, much of the cheap soy sauce sold in supermarkets is made through chemical hydrolysis — an industrial shortcut that breaks down soybeans with acid in days rather than months. Check the label: “naturally brewed” means fermented; “hydrolyzed soy protein” means it wasn’t.

How long does it take to make soy sauce at home?

The active preparation takes about 3 days (soaking, cooking, growing koji). After that, the moromi (brine mash) ferments for 6–12 months with minimal hands-on work — just stirring once or twice a week. Most people find 8–12 months produces the best flavor. You can taste it at 6 months, but the flavor improves significantly with more time.

Where do I buy koji starter?

Koji starter (Aspergillus oryzae spores) is available from online fermentation suppliers like Cultures for Health, GEM Cultures, and Amazon. Make sure you’re buying spores specifically for soy sauce or general-purpose koji — not koji meant exclusively for sake or miso, as different strains have different enzyme profiles. A small packet (enough for many batches) costs $10–15.

Can I make soy sauce without wheat?

Yes — this is essentially tamari. Use soybeans only and follow the same process. The result will have a slightly different flavor profile (less sweet, more purely savory) but still develops excellent umami. Some people substitute rice for the wheat component to provide the carbohydrates that koji needs.

What’s the difference between soy sauce, tamari, and shoyu?

Shoyu is the Japanese word for soy sauce — it’s made with roughly equal parts soybeans and wheat. Tamari is a Japanese soy sauce made with mostly soybeans and little to no wheat, resulting in a thicker, less sweet sauce. Soy sauce is the broad English term covering all varieties. Chinese soy sauce tends to use a higher ratio of soybeans to wheat than Japanese shoyu.

Is homemade soy sauce safe?

Yes, when made properly. The high salt concentration (17–18%) prevents the growth of harmful bacteria — the same preservation principle used in vegetable fermentation. The key safety rules: use the correct amount of salt (don’t reduce it), discard any batch that develops colored mold during koji growth, and pasteurize the final product before bottling.

How to Make Soy Sauce at Home: A Fermentation Guide