You probably already have it in your kitchen.

Sushi gets dipped in it.

Ramen gets seasoned with it.

Teriyaki wouldn't be the same without it.

But that little bottle of soy sauce is hiding something surprisingly complicated.

Soybeans.

Wheat.

Koji.

Salt.

Water.

And months of fermentation.

Meet shoyu (醤油) — one of the foundations of Japanese cooking.

What Is Shoyu?

Shoyu (醤油) is the Japanese word for soy sauce.

Pronounced approximately:

show-yoo.

Japanese soy sauce is traditionally made by fermenting soybeans and wheat with koji, then combining them with salt water and allowing the mixture to mature.

The result is the dark brown seasoning we recognize as soy sauce.

Salty?

Definitely.

But there's much more happening inside that bottle.

Shoyu can contain layers of:

umami

sweetness

acidity

aroma

saltiness

That's why a tiny splash can change an entire dish.

Wait... Soy Sauce Is Fermented?

Yes!

It's easy to think of soy sauce as simply a salty liquid.

But traditionally brewed Japanese soy sauce is a fermented food.

And one of the most important ingredients is something we've met before:

Koji (麹).

Koji is made by cultivating the mold Aspergillus oryzae on grains such as rice, barley or soybeans.

For typical Japanese soy sauce production, roasted wheat and cooked soybeans are inoculated with koji mold.

This creates soy sauce koji.

And that's where the transformation begins.

How Is Japanese Soy Sauce Made?

Traditional soy sauce production takes time.

Step 1: Prepare the Soybeans and Wheat

Soybeans are cooked until soft.

Wheat is roasted and crushed.

The two are combined.

Step 2: Add Koji

The soybean-and-wheat mixture is inoculated with Aspergillus oryzae or related koji cultures.

Over the next few days, the koji grows through the mixture.

Its enzymes will become incredibly important during fermentation.

Step 3: Make Moromi

The koji mixture is combined with salt water to create a thick mash called:

Moromi (もろみ / 諸味).

This is where the long fermentation and maturation process happens.

Microorganisms including yeasts and lactic acid bacteria participate in transforming the mixture.

Meanwhile, enzymes from the koji break larger molecules into smaller ones.

Proteins become amino acids.

Starches become sugars.

And gradually...

soybeans and wheat become something completely different.

Step 4: Let It Mature

Moromi can mature for months.

During this time, its flavor, aroma and color continue developing.

This slow process creates the complexity associated with traditionally brewed soy sauce.

Step 5: Press It

When the moromi is ready, it's pressed.

The liquid extracted from the mash eventually becomes soy sauce after additional processing such as clarification and, for most commercial Japanese soy sauces, pasteurization.

From pale beans and wheat...

to a dark, intensely savory liquid.

Fermentation did a lot of work.

Why Does Soy Sauce Have So Much Umami?

One important reason is protein.

Soybeans contain plenty of it.

During fermentation, enzymes produced by koji help break those proteins down into amino acids.

One of those amino acids is glutamate.

Glutamate contributes to the savory sensation we call:

umami (うま味).

That's why soy sauce doesn't simply make food taste salty.

It can make food taste deeper, fuller and more savory.

A few drops can do a surprising amount of work.

Does Japanese Soy Sauce Contain Wheat?

Most standard Japanese soy sauce does.

This sometimes surprises people.

The most common Japanese style, koikuchi shoyu, is generally brewed using both soybeans and wheat.

But not every soy sauce uses the same proportions or ingredients.

For example, tamari traditionally contains little or no wheat, although modern products vary.

So if you need to avoid gluten, don't assume that every bottle labeled soy sauce or tamari is gluten-free.

Check the label.

The Main Types of Japanese Soy Sauce

Walk through a Japanese supermarket and you'll quickly discover:

There isn't just one shoyu.

Japan recognizes several major varieties.

Koikuchi Shoyu (濃口醤油)

Koikuchi means “dark” or “strong-flavored.”

This is the standard everyday soy sauce used across much of Japan and the type you're most likely to encounter.

It's versatile and balanced, making it useful for:

Cooking

Dipping

Sauces

Soups

Marinades

If a Japanese recipe simply says shoyu, koikuchi is often what it means.

Usukuchi Shoyu (淡口醤油)

Despite the name, usukuchi doesn't mean less salty.

It refers to its lighter color.

Usukuchi is particularly associated with the Kansai region and is useful when cooks want seasoning without making a dish too dark.

You'll often find it in dishes where the natural colors of the ingredients are meant to remain visible.

Tamari (たまり醤油)

Tamari is a darker, rich soy sauce historically associated with the liquid produced during miso making.

Compared with standard koikuchi, it generally contains a higher proportion of soybeans and less wheat.

Its deep flavor makes it popular for dipping and cooking.

Shiro Shoyu (白醤油)

Now things flip.

Shiro means white.

Shiro shoyu uses a much higher proportion of wheat than soybeans and has a very pale amber color.

It's useful when you want soy sauce flavor without dramatically darkening the food.

Saishikomi Shoyu (再仕込醤油)

Saishikomi roughly means “re-brewed.”

Instead of using ordinary salt water for part of the brewing process, soy sauce is used again in the fermentation.

The result is generally darker, richer and more concentrated.

This is soy sauce saying:

What if I became... even more soy sauce?

Japanese Soy Sauce at a Glance

Type	Japanese	Character
Koikuchi	濃口醤油	Balanced, versatile, everyday
Usukuchi	淡口醤油	Lighter color, relatively salty
Tamari	たまり醤油	Rich, dark, soybean-forward
Shiro	白醤油	Very pale, wheat-forward
Saishikomi	再仕込醤油	Dark, rich, concentrated

 

How Is Soy Sauce Used in Japan?

The easier question might be:

Where isn't it used?

Shoyu appears throughout Japanese cooking.

You'll find it in:

🍣 Sushi and sashimi

🍜 Ramen

🍚 Rice dishes

🐟 Simmered fish

🥩 Marinades

🍲 Soups and hot pots

🥢 Dipping sauces

🥬 Vegetable dishes

🍗 Teriyaki-style sauces

It can be used during cooking or added at the table.

But Japanese cooking often treats soy sauce as one part of a larger balance.

Dashi.

Mirin.

Sake.

Sugar.

Miso.

And shoyu.

Different combinations create completely different flavors.

Is All Soy Sauce Made the Traditional Way?

No.

Production methods vary considerably between manufacturers and products.

Traditional brewing relies on microbial fermentation and maturation over time.

Some modern soy sauces or soy-sauce-style seasonings may use different or accelerated production techniques.

If you're curious about how a particular bottle was made, look for information from the producer and terms indicating traditional brewing or fermentation.

Because two bottles that look almost identical on the supermarket shelf...

may have very different stories behind them.

Why Is Soy Sauce So Important in Japanese Food?

Soy sauce does several jobs at once.

It adds saltiness.

It adds aroma.

It contributes color.

And most importantly, it brings umami and complexity.

But its importance is bigger than flavor alone.

Shoyu represents one of the recurring ideas behind traditional Japanese food:

simple ingredients transformed through microorganisms, technique and time.

Soybeans.

Wheat.

Salt.

Water.

Koji.

Nothing particularly dramatic.

Until you wait.

Fun Facts About Japanese Soy Sauce

• Shoyu (醤油) means soy sauce in Japanese.
• Traditional Japanese soy sauce is a fermented food.
• Koji plays an important role in its production.
• The fermenting mash is called moromi (もろみ / 諸味).
• Most standard Japanese soy sauce contains wheat.
• Koikuchi is the most common style in Japan.
• Usukuchi is lighter in color but isn't necessarily lower in salt.
• Tamari typically contains proportionally more soybeans and less wheat than koikuchi.
• Shiro shoyu contains a high proportion of wheat and is unusually pale.
• Fermentation helps create the amino acids responsible for soy sauce's deep umami flavor.

Frequently Asked Questions

What is Japanese soy sauce called?

Soy sauce is called shoyu (醤油) in Japanese.

How do you pronounce shoyu?

Approximately:

show-yoo.

Is soy sauce fermented?

Traditionally brewed Japanese soy sauce is fermented. Koji, yeasts and bacteria help transform soybeans and wheat during the brewing and maturation process.

What is soy sauce made from?

Typical Japanese soy sauce is made using soybeans, wheat, salt and water, with koji cultures beginning the fermentation process.

Does soy sauce contain gluten?

Many Japanese soy sauces contain wheat and therefore contain gluten. Some tamari products are gluten-free, but not all are, so always check the label if gluten is a concern.

What is the most common Japanese soy sauce?

Koikuchi shoyu (濃口醤油) is the dominant everyday style in Japan.

What is moromi?

Moromi (もろみ / 諸味) is the fermented mash created from soy sauce koji and salt water. After fermentation and maturation, the mash is pressed to extract the liquid used to make soy sauce.

Is soy sauce the same as tamari?

Not exactly. Tamari is a type of Japanese soy sauce with its own ingredients, proportions and flavor characteristics.

🇯🇵 Keep Discovering Japan

If you enjoyed discovering how soy sauce is made, explore our beginner's guide to Japanese fermented foods, from miso and natto to koji, nukazuke and shiokara.

 

👋 Meet a JapPop Resident

Soy Mate

しょうゆさし君

A loyal little soy companion who's happiest swimming beside a plate of sushi. 🍣

Soy Mate knows his place.

And that place is usually right beside the sushi.

But inside his tiny fish-shaped bottle is the result of something much bigger: soybeans, wheat, koji, salt and months of transformation.

He may look like a simple little condiment.

But never underestimate the little guy responsible for the umami.

Wherever sushi goes, Soy Mate follows.

👉 Meet Soy Mate and discover more residents from the JapPop Universe.

🇯🇵 About JapPop

At JapPop, we celebrate Japanese culture through playful illustrations, original characters, and stories inspired by everyday life in Japan.

From beloved foods and traditional festivals to folklore, historical figures, seasonal customs, lucky gods, and language, every article introduces another piece of Japan—and another story worth discovering.

Sometimes Japanese culture is hidden in an ancient tradition.

Sometimes it's growing quietly in a fermentation barrel.

And sometimes...

it's sitting in the little fish-shaped bottle next to your sushi. 🐟

Discover Japan, one story at a time. 🇯🇵✨