How Traditional Japanese Soy Sauce Is Made: Koji, Moromi & Slow Fermentation

By Maile Bohlmann

Soy sauce is easy to underestimate because it is so familiar. It sits on the table. It gets splashed into marinades. It disappears into stir-fries, dressings, dipping sauces, soups, rice bowls, noodles, and late-night eggs. We use it almost casually, as if it were simply “salty brown liquid.”

But good Japanese soy sauce—carefully brewed real shoyu—is one of the great fermented seasonings of the world. Sure, it's salty. But it can also be sweet, roasted, malty, earthy, fruity, smoky, round, sharp, delicate, deep, or almost caramel-like depending on how it is made.

The fascinating part is that traditional soy sauce starts with a very small list of ingredients: soybeans, wheat, salt, water, and koji. Then time does the strange and beautiful work.

At ChefShop, we carry several traditional soy sauces, and Yugeta is one of the makers we especially love to feature because their soy sauces show just how much personality can come from method, ingredients, aging, and restraint. But this article is not only about one maker. It is about the traditional process itself: how soybeans and wheat become koji, how koji becomes moromi, how moromi slowly transforms, and why a good shoyu tastes like so much more than salt.

The Short Version: How Traditional Japanese Soy Sauce Is Made

Traditional Japanese soy sauce, or shoyu, is made through a fermentation process that usually follows this path:

1. Soybeans are steamed to soften them and make their proteins available for fermentation.
2. Wheat is roasted and crushed to develop aroma and provide carbohydrates that contribute sweetness and fragrance.
3. Soybeans and wheat are inoculated with koji mold to create shoyu koji.
4. The koji is mixed with salt water to form a mash called moromi.
5. The moromi ferments and ages, often for many months, as enzymes and microorganisms build flavor, aroma, color, acidity, and umami.
6. The aged mash is pressed to separate the liquid soy sauce from the solids.
7. The raw soy sauce is clarified and usually heated to stabilize flavor, aroma, and color.
8. The finished shoyu is bottled and ready to use in cooking, dipping, seasoning, and finishing.

That sounds orderly, but in reality, soy sauce is not assembled so much as grown, coaxed, watched, and waited for—sometimes for as long as two years or more

What Is Shoyu?

Shoyu is the Japanese word for soy sauce. In English, “soy sauce” can refer to many different sauces from many different countries, but Japanese shoyu usually refers to a fermented seasoning made from soybeans, wheat, salt, water, and koji.

The wheat matters. In many Japanese soy sauces, wheat is not a filler or afterthought. It contributes aroma, sweetness, and balance. That is one of the reasons Japanese shoyu often tastes rounder and more fragrant than people expect if they are used to thinking of soy sauce as only salty.

There are also several styles of Japanese soy sauce, including koikuchi, usukuchi, tamari, saishikomi, and shiro shoyu. Those styles vary in color, intensity, ingredients, aging, and intended use. We'll go deeper into those in our separate guide to choosing soy sauce, but for now, the key idea is this: “soy sauce” is not one thing. It's a whole category of fermented flavor.

Read next: How to Choose Soy Sauce

A Very Short History of Soy Sauce in Japan

The roots of soy sauce reach back into older Asian traditions of fermented, salted foods. In Japan, soy sauce is closely connected to the development of fermented seasonings such as miso and earlier forms of salted fermented pastes and liquids.

That connection is important because it helps explain soy sauce’s character. Shoyu is not just a sauce that happens to contain soy. It belongs to a broader family of fermented Japanese seasonings that use salt, koji, aging, and microbial transformation to preserve food and create flavor.

By the time soy sauce became widely established in Japan, it had evolved into a seasoning that could do many jobs at once: preserve, season, deepen, balance, and bring out the flavor of other ingredients. That is still what good soy sauce does today.

The Ingredients: Soybeans, Wheat, Salt, Water & Koji

Traditional soy sauce is a great reminder that simple ingredient lists can hide enormous complexity.

Soybeans: The Source of Deep Umami

Soybeans provide much of soy sauce’s savory foundation. Their proteins are broken down during fermentation into amino acids, which help create the deep umami flavor we associate with shoyu.

This is one of the reasons soy sauce can make food taste more complete without necessarily making it taste “like soy sauce.” A small amount can make mushrooms taste meatier, soup taste rounder, grilled vegetables taste deeper, and marinades taste more integrated.

Wheat: Aroma, Sweetness & Roundness

Wheat brings carbohydrates that help build aroma and subtle sweetness during fermentation. Before it's mixed with soybeans, the wheat is typically roasted and crushed. That roasting matters—it adds a warm, grain-like aromatic foundation that shows up later in the finished soy sauce.

This is part of what makes many Japanese shoyu styles so fragrant. Good soy sauce can smell roasted, malty, nutty, or even faintly fruity. It's not only salty, it has a nose.

Salt: Preservation, Structure & Control

Salt does more than season the final sauce. It controls the fermentation environment, helping prevent spoilage while allowing the right microorganisms and enzymatic activity to shape the moromi over time.

This is where soy sauce gets interesting: salt is both ingredient and guardrail. It creates the conditions that make long fermentation possible.

Water: The Unassuming Ingredient

Water seems invisible, but it matters. Brewing water affects extraction, fermentation, and the final character of the soy sauce. In artisan brewing, makers often speak with real affection about their local water because it becomes part of the house style.

Yugeta’s organic shoyu, for example, is naturally brewed using underground water flowing from the Chichibu mountains. That kind of detail should not be overlooked. In fermentation, the unassuming ingredients count.

Koji: The Engine of Transformation

Koji is the ingredient that makes the whole process feel slightly magical.

To make soy sauce koji, steamed soybeans and roasted crushed wheat are inoculated with koji mold. Over a few days, the koji grows through the mixture and begins producing enzymes that will later help break down proteins and carbohydrates during fermentation.

If soy sauce has a secret, it is this: the ingredients do not become shoyu by sitting passively in salt water. Koji unlocks them. It turns the potential flavor inside soybeans and wheat into something the fermentation can develop.

Step 1: Steaming the Soybeans

The soybeans are first soaked and steamed until they are soft enough for fermentation. This makes their proteins accessible and prepares them to be combined with the roasted wheat.

It's a humble step, but it sets up everything that follows. If the soybeans are not properly prepared, the koji cannot do its work as effectively. Traditional fermentation often comes down to details like this: not glamorous, but essential.

Step 2: Roasting and Crushing the Wheat

The wheat is roasted, then crushed. Roasting develops aroma; crushing makes the wheat more accessible to the koji and fermentation process.

This is one of the reasons shoyu can have such an appealing aroma. There is roasted grain hiding underneath the salt. In a really good soy sauce, you may notice warmth, nuttiness, and a faint sweetness that makes the flavor feel round rather than harsh.

Step 3: Making Shoyu Koji

Once the soybeans and wheat are prepared, they are mixed with koji culture. This mixture is held under carefully managed conditions so the koji can grow.

This stage usually takes only a few days, but it is one of the most important steps in the process. The koji becomes the enzymatic foundation for the entire fermentation. It is the difference between a mixture of cooked beans and grain, and the beginning of soy sauce.

There is something wonderfully strange about this. Before shoyu is liquid, before it is dark, before it is fragrant, it starts as a living, cultured mixture of soybeans, wheat, and mold. That may not sound romantic, but it is exactly the kind of culinary magic we love.

Step 4: Mixing Koji with Brine to Make Moromi

After the koji is ready, it's mixed with salt water to create moromi, the thick mash that will become soy sauce.

This is where the long transformation begins. Moromi is not beautiful in an obvious way. It is not a jewel-toned jam or a bubbling pot of sauce. It is a fermenting mash. But inside that mash, the flavor is being built molecule by molecule.

Over time, enzymes break down proteins and starches. Microorganisms contribute acidity, aroma, alcohol, and complexity. Color deepens. The raw materials stop tasting like soybeans and wheat and start becoming shoyu.

Step 5: Fermenting and Aging the Moromi

Moromi is where patience becomes flavor. During fermentation and aging, the mash develops the qualities that make soy sauce so compelling: umami, aroma, sweetness, acidity, bitterness, salt, and color. Good shoyu is not one-dimensional because fermentation is not one-dimensional. Different transformations are happening at once.

This is why time matters. You cannot simply shortcut your way into the same kind of layered flavor. You can make something salty and brown quickly. You cannot rush the particular roundness of a carefully fermented soy sauce.

Depending on the maker and style, fermentation may happen in tanks, wooden vats, or other vessels. Each choice affects the process. Temperature, airflow, resident microorganisms, vessel material, and aging time can all influence the final character.

Why Wooden Vats Matter

Wooden-vat soy sauce has a particular romance, but the romance is not just visual. Wooden vats can hold onto a brewery’s microbial life over time, becoming part of the character of the fermentation environment.

That doesn't mean every good soy sauce must be made in wood, or that every wooden-vat soy sauce is automatically excellent. Method matters, but skill matters more. Still, when a producer maintains old vats and works slowly, it often signals a commitment to flavor that goes beyond efficiency.

Yugeta is a good example, aging the moromi for their Organic Shoyu in cedar barrels, with some barrels more than 100 years old. Their Cherry Smoked Shoyu is fermentated and aged for about a year in open pinewood vats, some used since the late 1800s.

That is the kind of detail we pay attention to. Not because old wood is automatically magic, but because it tells us the maker is working inside a living tradition.

Step 6: Pressing the Aged Moromi

Once the moromi has aged, it is pressed to separate the liquid soy sauce from the solids.

This is another place where patience matters. Pressing too aggressively can affect clarity and flavor. Traditional pressing is slow because the goal is not simply to extract liquid. The goal is to extract good liquid.

The pressed liquid at this stage is often called raw soy sauce. It is fragrant, intense, and not yet fully stabilized. From here, it may be clarified and heated before bottling.

Step 7: Clarifying, Heating & Bottling

After pressing, raw soy sauce is typically allowed to settle so oil and sediment can separate. The clarified soy sauce may then be heated. Heating helps stabilize the sauce, stop enzymatic activity, and adjust aroma and color.

This step is one reason finished soy sauce can be shelf-stable before opening. But it is also a flavor step. Heating can change aroma, roundness, and color. Like everything else in soy sauce brewing, it is not just procedural. It affects the final experience in the bottle.

What Traditional Brewing Gives You

So what does all of this mean when you actually taste the soy sauce?

Traditional brewing can give shoyu a layered flavor that feels integrated rather than assembled. Instead of salt first, salt second, and salt third, you may experience:

• Umami: The deep savory quality that makes food taste more complete
• Roasted aroma: Warm grain, toasted wheat, nuts, or malt
• Sweetness: Not sugary, but round and balancing
• Acidity: A subtle brightness that keeps the flavor from feeling flat
• Bitterness: A tiny balancing bitterness that can make the finish more complex
• Color: From amber to deep brown, depending on style and brewing
• Texture: Some soy sauces feel thin and sharp; others feel rounder, smoother, or more viscous

This is why two soy sauces with similar ingredient lists can taste completely different. Fermentation is not just about what goes in. It's about what happens over time.

The Five Major Styles of Japanese Soy Sauce

Japanese soy sauce is often grouped into five major styles. Each has its own character and best uses.

Style	General Character	Best Uses
Koikuchi	The most common general-purpose Japanese soy sauce; balanced salt, umami, sweetness, acidity, and aroma.	Everyday cooking, dipping, marinades, stir-fries, soups, sauces.
Usukuchi	Lighter in color, often saltier, used when you want seasoning without darkening food too much.	Broths, simmered vegetables, pale sauces, dishes where color matters.
Tamari	Often richer, darker, and more soybean-forward, with deep umami.	Dipping, sashimi, glazes, richer sauces.
Saishikomi	Refermented or double brewed; deeper, rounder, often more viscous and intense.	Finishing, dipping, sashimi, dumplings, grilled meats, roasted vegetables.
Shiro Shoyu	Extra-light-colored soy sauce with a pale amber color and gentle fragrance.	Clear soups, chawanmushi, pickles, rice crackers, delicate dishes.

This is where “best soy sauce” becomes the wrong question. The better question is: what do you want the soy sauce to do?

Do you want a daily workhorse? A darker finishing sauce? A pale seasoning that will not stain the broth? A rich dip for sashimi? A smoky accent for grilled food? A soy sauce that can make vanilla ice cream taste like salted caramel?

There is a soy sauce for that.

Featured Maker: Yugeta Shoyu

Yugeta Shoyu is one of the traditional soy sauce makers we love because their range makes the craft easy to taste.

Yugeta has made soy sauce in Tawame, Saitama Prefecture since 1923. The company is now led by fourth-generation craftsman Yoichi Yugeta. Their soy sauces are made with Japanese-grown soybeans and wheat, natural sea salt, and water, and their products show how one producer can create very different expressions of shoyu.

Yugeta Organic Shoyu

Yugeta Organic Shoyu is the everyday bottle with serious depth. It's made with organic soybeans, organic wheat, salt, and water, and naturally brewed using underground water from the Chichibu mountains. This is the bottle to reach for when you want a traditional shoyu that can move easily between cooking and finishing.

Try it with: rice, tofu, brothy soups, mushrooms, roasted vegetables, dressings, marinades, noodle bowls, and eggs.

Yugeta Double Brewed Shoyu

Yugeta Double Brewed Shoyu is where things get especially interesting.

In a typical batch of soy sauce, koji is mixed with salt water to create moromi. For double brewed soy sauce, Yugeta first brews a traditional batch, then uses finished soy sauce instead of plain salt water for the second fermentation. The result is deeper, rounder, more layered, and more viscous without simply tasting saltier.

This is a finishing soy sauce. Use it where you can really taste it.

Try it with: sashimi, dumplings, steak, roasted mushrooms, grilled vegetables, soft-boiled eggs, avocado, tofu, and vanilla ice cream.

Yugeta Cherry Smoked Shoyu

Yugeta Cherry Smoked Shoyu takes naturally brewed soy sauce and smokes it over sakura, or cherry blossom wood. The smoke is not shy. It gives the shoyu a deep, woodsy aroma that can change a dish with just a few drops.

This is not the bottle to pour casually into everything. It is very smoky! It's the bottle to use sparingly when you want a finishing accent that announces itself.

Try it with: grilled vegetables, fish, meats, mushrooms, dumplings, tempura, cheese, soft eggs, avocado toast, and roasted squash.

How to Taste Traditional Soy Sauce

If you really want to understand soy sauce, taste two or three side by side. You don't need a formal tasting setup. You just need curiosity and something neutral.

1. Smell first. Look for roasted grain, malt, smoke, fruit, alcohol, acidity, or deep savoriness.
2. Taste a drop plain. Let it sit on your tongue. Notice whether the salt feels sharp, round, sweet, or balanced.
3. Taste with rice or tofu. Soy sauce often reveals itself better with food than on its own.
4. Compare styles. Try everyday shoyu next to double brewed shoyu, or smoked shoyu next to unsmoked shoyu.
5. Notice the finish. Does it disappear quickly? Does it become bitter? Does it linger? Does it make you want another bite?

The best soy sauces do not just season food. They make food taste more dimensional.

How to Use Traditional Soy Sauce in Everyday Cooking

Once you start thinking of soy sauce as a fermented flavor tool, it becomes much more useful.

Use	Best Soy Sauce Style	Why It Works
Everyday cooking	Traditional koikuchi-style shoyu	Balanced salt, umami, aroma, and color.
Dipping	Double brewed shoyu, tamari-style shoyu, richer shoyu	More depth and body when the soy sauce is tasted directly.
Grilling and glazing	Traditional shoyu or double brewed shoyu	Fermented depth plus browning-friendly savoriness.
Finishing	Double brewed shoyu or smoked shoyu	A few drops add aroma and complexity at the end.
Delicate soups or pale dishes	Shiro shoyu, shiro tamari, or light-colored soy sauce	Adds umami without darkening the dish as much.
Desserts	Double brewed shoyu	Salt, umami, and caramel-like depth can balance sweetness.

What to Look for When Buying Traditional Soy Sauce

When choosing soy sauce, the label tells a story. Look for:

• Brewing method: Traditionally brewed or naturally brewed soy sauce will usually have more depth than quick chemically produced versions.
• Simple ingredients: Soybeans, wheat, salt, water, and koji are the classic foundation for many Japanese shoyu styles.
• Origin: Regional and producer differences matter.
• Style: Koikuchi, usukuchi, tamari, saishikomi, and shiro shoyu behave differently in cooking.
• Intended use: Everyday cooking, dipping, finishing, grilling, pale dishes, or dessert accents.
• Allergens: Many Japanese soy sauces contain wheat and soy. Always check the label if allergens matter.

And then taste. That is the part that matters most.

Why Good Soy Sauce Makes Better Food

A good soy sauce does not need to dominate a dish. In fact, some of the best uses are the least obvious.

A little shoyu in a mushroom soup can make the mushrooms taste deeper. A splash in a vinaigrette can make the acidity feel less sharp. A brush of soy sauce on grilled corn can make the sweetness pop. A few drops of double brewed shoyu on vanilla ice cream can make the dessert taste more adult, more savory, more complete.

That is what we love about traditional soy sauce. It's not just a Japanese ingredient. It's a precision tool for flavor.

Once you taste the difference, it becomes hard to go back to thinking of soy sauce as just salt.

Shop Traditional Japanese Soy Sauce at ChefShop

ChefShop carries traditional Japanese soy sauces for cooking, dipping, finishing, and exploring. Yugeta is one of our featured makers, but our larger Japanese pantry includes other traditional seasonings and styles as well.

Start with an everyday shoyu if you want a versatile bottle for cooking. Choose a double brewed shoyu if you want something richer for dipping and finishing. Choose smoked shoyu if you want a few drops of deep, woodsy aroma. Add shiro tamari or shirodashi if you want pale umami for delicate dishes.

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FAQ: Traditional Japanese Soy Sauce

What is traditional Japanese soy sauce made from?

Traditional Japanese soy sauce is usually made from soybeans, wheat, salt, water, and koji. The soybeans and wheat are fermented with koji, mixed with salt water to form moromi, aged, pressed, clarified, and usually heated before bottling.

What is koji in soy sauce?

Koji is the cultured mixture of soybeans and wheat inoculated with koji mold. It produces enzymes that help break down proteins and carbohydrates during fermentation, creating the foundation for soy sauce’s umami, aroma, sweetness, and complexity.

What is moromi?

Moromi is the fermenting mash made by mixing soy sauce koji with salt water. During aging, moromi develops the flavor, aroma, color, acidity, and savory depth that become finished soy sauce.

How long does traditional soy sauce take to make?

The exact time depends on the producer and style, but traditionally brewed soy sauce often ferments and ages for months, and some artisan versions age longer. Time is part of what creates depth, aroma, and balance.

What is the difference between shoyu and soy sauce?

Shoyu is the Japanese word for soy sauce. In English, “soy sauce” can refer to many different sauces from many countries. Japanese shoyu usually refers to fermented soy sauce made with soybeans, wheat, salt, water, and koji, though there are several styles.

What is double brewed soy sauce?

Double brewed soy sauce, also called saishikomi shoyu, is brewed using finished soy sauce instead of plain salt water during a second fermentation. This creates a deeper, richer, rounder soy sauce that is especially good for dipping and finishing.

What is smoked soy sauce used for?

Smoked soy sauce is best used as a finishing accent. A few drops can add deep smoky aroma to grilled vegetables, fish, meat, mushrooms, eggs, dumplings, tofu, avocado, cheese, or roasted squash.

Does Japanese soy sauce contain wheat?

Many Japanese soy sauces contain wheat, especially standard shoyu styles made with soybeans and wheat. If you need gluten-free soy sauce, check the label carefully and look for a product specifically marked gluten-free.

How should soy sauce be stored?

Unopened soy sauce can usually be stored in a cool, dark pantry. After opening, refrigeration helps preserve aroma, freshness, and flavor, especially for higher-quality or artisan soy sauces.

How do I choose the right soy sauce?

Choose soy sauce by use. For everyday cooking, choose a balanced traditionally brewed shoyu. For dipping and finishing, choose a richer or double brewed soy sauce. For smoky accents, choose smoked shoyu. For pale dishes where color matters, consider shiro shoyu, shiro tamari, or another light-colored seasoning.

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