Tokyo: this world-class metropolis is a veritable gourmet wonderland.

Discover the stories behind the ingredients that make this city so delicious - so "oishii."

This time, we feature koya-tofu, a traditional Japanese ingredient made by using tofu.

Koya-tofu has the outstanding ability to absorb and hold broth and umami like a sponge.

It's full of broth.

Excellent. So "oishii!"

Using the harsh cold of winter, people have been making koya-tofu for about 800 years.

This nutrient-rich preserved food has long supported people's health.

It's hard!

Couple of frozen blocks!

Yeah, looks like soap.

Koya-tofu, which pairs well with any flavor, is thought to be the origin of freeze-dried foods in Japan.

Let's bite into this traditional Japanese superfood: koya-tofu!

Trails to Oishii Tokyo.

Good morning, everyone.

My name is Kyle Card, and I come from Canada.

And today's theme is koya-tofu the traditional Japanese processed food.

Just how exactly is this food made, and what makes it so special?

Let's go find out together.

This is the retail area of Toyosu Market.

It has stores aimed at food professionals which specialize in condiments, processed foods, utensils and more.

Kyle visits a dry food wholesaler that's been in business for 80 years.

They have all kinds of dried foods, like beans, kelp, and shiitake mushrooms.

Third-generation owner Akiyama Ryugo gives Kyle a look at koya-tofu.

- This is it.
- This?

It's processed tofu.

- Made from tofu, huh?
- Exactly.

Give it a feel.

It's really light.

So light.

A block this size weighs only 15 grams, and is dry to the touch.

- Basically no aroma.
- Right.

I expected the smell of tofu.

- Can I break it?
- Sure.

There it goes.

That's like a really tough biscuit. Wow.

- Kind of crumbly.
- Right.

It's dehydrated tofu.

Isn't tofu good enough as-is?

Tofu doesn't last long.
This helps preserve it.

It's rehydrated with hot water before use.

Getting plump.

- Yes, bigger and bigger.
- This is fun.

Squeeze it.

Like a sponge.

So strange.

After just one minute, it expands this much.

- Can you eat it after rehydrating?
- Sure.

What a texture!

It's like eating a sponge.

It’s really like I’m eating a sponge. This is amazing.

But this is an edible sponge.

And a bit mochi!

But no. No flavor.

- None at all.
- Right.

So how do you make it flavorful?

How indeed.

The answer is broth. You soak it.

- I can see how that would work.
- Right?

Kyle's never seen such a mysterious ingredient.

Into what kinds of dishes will koya-tofu transform?

Kyle heads to Nagano.

It's three-and-a-half hours northwest of Tokyo.

Over 95 percent of all koya-tofu is produced in this area.

His first stop is Zenkoji Temple, founded in the 7th century.

Koya-tofu has long been used in the Buddhist cuisine called shōjin-ryōri.

At a shukubo, an inn for temple visitors, he'll get his first taste of koya-tofu dishes.

Koya-tofu is rich in things like protein, calcium, and iron, making it highly nutritious.

It's long been a key source of protein in shōjin-ryōri,

in which the eating of meat and fish is forbidden.

- Beautiful.
- Thank you.

- What is this?
- Stewed vegetables.

The koya-tofu has a different color.

It absorbs the color of the vegetables as it cooks.

So koya-tofu absorbs everything
around it like a sponge.

- That's right.
- Interesting.

It's full of broth.

Excellent. So "oishii!"
It soaks up the dashi broth and vegetable flavor.

- Before it's cooked, it feels like eating a sponge.
- True.

But once it's cooked, it's quite tasty.

Wakaomi also recommends koya-tofu tempura.

It's seasoned with kombu kelp and dried shiitake mushroom broth, then deep-fried in oil.

It's seasoned then fried, so you can taste that flavor.

Still spongy and juicy.

"Oishii."

I thought the broth would disappear when fried.
But it retains the flavor.

Exactly.

This is finely crushed koya-tofu.

It's lightly simmered in dashi broth then poured over rice topped with aromatic plants.

If you pour on a lot, it tastes better.

Like green tea over rice?

Yes. This is served at Zenkoji Temple
during traditional events.

It has a long history handed down for centuries.

"Itadakimasu."

That is some wonderful dashi.

Strong dashi flavor and aroma.
Seems very nutritious, too.

Right.

People in the past did a great job making such dishes,
thinking about nutritional balance and so on.

Koya-tofu: a key ingredient in Japanese culinary culture.

But where does it get its unique absorption power?

Kyle visits the plant that produces the largest amount of koya-tofu in Japan.

It's a mechanical maze in here.

Soybeans.

Yes. We make koya-tofu from them.

We place washed beans in a tank
with water and let them soak it up.

Soaked overnight, the beans more than double in size.

They're then ground into small pieces and heated to release soy milk.

I'll show you the soy milk.

Soy milk? Amazing.

Wow, this is awesome. Take a look at this.

Freshly squeezed soy milk.

Making koya-tofu means first making tofu.

This soy milk is mixed with a coagulant then compressed.

It looks like wooden boards.
But it's tofu.

Yes.

Beautiful.

Freshly made tofu. Touch it.

Nice. May I?

Sure.

More springy than I thought.

Break it in half.

It's dense.

It's a special tofu for koya-tofu.

This tofu is made with less water than normal.

It's firmer and has nearly twice the concentration of soybeans.

Next, an essential part of koya-tofu production.

Oh my god! It's frozen!

This room is negative nine degrees Celsius.

Freezing!

Placed in this subzero room for three hours, the tofu is slowly frozen to its core.

I thought I'd be flash-frozen there for a moment.

Oh my goodness.

So cold.

Once completely frozen, the tofu is stored in a low-temperature warehouse, where it's aged for three weeks.

Shiny. Nice.

Welcome back.

Nice and warm.

Here's the thawing process.
These are ready to be thawed.

Wait. You freeze it, age it,
and then thaw it back?

Right.

The tofu is slowly thawed in room-temperature water for three hours.

This process is what gives koya-tofu its distinctive properties.

This is thawed koya-tofu.

It's soft now.

Squeeze it.

Oh! It's turned into a sponge. Wow.

Fascinating.

The blocks are squeezed then placed in a drying room.

After half a day to remove all moisture, the koya-tofu is finished.

This is the final product.
Freshly made.

Dry to the core.
Hard to find such an ingenious ingredient!

Yes.

So many steps.
That's part of what gives it its value.

Right.

Well, I feel like I've been on quite a long journey.

I watched the small little soybean turn into this freeze-dried goodness.

And, I have to tell you, I've never seen anything that has so many steps involved in its processing.

There's got to be something truly special about koya-tofu.

Why all these steps?

To find out, Kyle visits a laboratory on the factory grounds.

This is our koya-tofu.

Let's look at its surface with this microscope.

Great. Can't wait.

The surface of koya-tofu looks smooth,
but it actually has many large holes.

It literally is a sponge.

Exactly.
Each of these holes was formed by ice crystals.

When tofu is frozen, small ice crystals form inside.

When it's thawed, countless holes form, just like a sponge.

These holes help release moisture,
even using a simple drying method.

When you dry raw, unfrozen tofu, only the outside dries.
The moisture inside stays put.

I see.

Koya-tofu isn't just spongy so it can absorb broth.

Its sponge-like holes are also key in drying it thoroughly for preservation.

How did people centuries ago know this?

They were so wise. I study koya-tofu
production from a scientific perspective.

The wisdom of those who devised this process
back then never ceases to amaze me.

It might've been through a long process
of trial and error.

Amazing.

Koya-tofu, this well-preserved source of protein, has been used in surprising ways.

Kyle speaks to company president Kinoshita Hirotaka to learn more.

Koya-tofu is light and nutritious.

It was served on advanced Antarctic
expeditions beginning in the 1960s.

The latest research says it may be good for space food.

Studies have shown that it prevents muscle weakness.

When astronauts return to Earth,
their muscles are too weak to walk.

It's thought koya-tofu may improve this situation.

Koya-tofu has been used in Japanese space food.

In addition, new shapes make it easier to use, and researchers have even devised new ways to eat it.

Pour boiling water and wait one minute.

- It puffs up right away.
- Yes.

Getting bigger every second.

Now just add seasoning.

Soup with broth and vegetables.

This gives a new meaning to instant-well, instant anything, really. Wow.

It smells great.

Lovely vegetable broth going on here.

Ah, wonderful. Lovely.

Great texture.

The more you chew, the more broth comes out.
Very nice.

Koya-tofu is remarkably preservable and nutritious.

No doubt this ever-evolving superfood will continue to be used in new and exciting ways.

Factory production of koya-tofu began in the early 20th century.

Before then, it was a side business for farmers in mountainous areas,

who utilized the harsh winter to create it.

Kyle will meet an artisan who still makes koya-tofu in the traditional way.

- Good morning.
- Hello.

- I heard you make koya-tofu by hand.
- Yes.

Like these ones?

We put this tofu here yesterday evening.
It froze overnight. It's naturally frozen koya-tofu.

Kobayashi Tetsuro once worked for a company in Tokyo.

He's been making koya-tofu for 15 years, since he took over his wife's family tofu shop.

- Completely frozen. May I?
- Go ahead.

I want to try too.
Wow, very hard.

Listen to that.

That is frozen solid. Wow.

We call it shimi-tofu, meaning frozen tofu.
Customers thaw it and use it for cooking.

That's what makes this region's shimi-tofu unique.

Here, koya-tofu is called shimi-tofu, or frozen tofu.

Peak production comes in January and February, the coldest months of the year.

They start at five in the morning.

First, Kobayashi extracts soy milk from local soybeans to make tofu.

Try some freshly squeezed soy milk.

Lucky me.

- Thank you.
- It's hot.

Beautiful color.

Oh, that's hot.

Nice soybean smell.

Sweet, right?

Yes. Strong soybean flavor and sweetness.

They pour coagulant into the soy milk all at once,

then stir it 100 times with the split end of a bamboo stick.

This prevents lumps from forming.

The day's temperature and humidity affects how the tofu hardens.

They fine-tune based on the sensations they feel in their hands from the bamboo mixers.

Next, they carefully remove excess water.

By now it's pretty firm.

Wow, amazing.

Look at that lovely curd.

Finally, wrap it with a cotton cloth.

So many steps.

Indeed.

40 kilograms.

Add weights and wait for 20 minutes.

It's tofu!

- Quite firm.
- Yes.

- So dense.
- Indeed.

Amazing.

The tofu is finished.

At sunset, they cut the tofu into blocks, then place it on grids to freeze outside in the freezing overnight temperatures.

The more we make, the more we're impressed by the
wisdom of our ancestors and the richness of nature.

I make it with the hope that more people
learn about it, and to pass it to the next generation.

Considered by some as the origin of today's koya-tofu,

shimi-tofu symbolizes how people centuries ago made the most of their harsh natural environment.

Kobayashi's wife Chitoshi prepares some homemade shimi-tofu dishes.

I'll start with miso soup.

"Itadakimasu."

Enjoy.

Each piece is large.
Very nice.

It's a bit springy, but not super spongy. Interesting.

Very smooth.

People often say that.

Next, shimi-tofu simmered in broth with Chinese cabbage and shimeji mushrooms with beaten eggs on top.

Lots of shimi-tofu.

Very filling. Perfect springiness,
and the broth flavor blends well.

Right?

What's hard about making shimi-tofu?

-6 to -10 degrees Celsius is best.
Getting that timing right is difficult.

Working in the cold mornings and nights is tough.
I hope there are successors who want to keep it going.

Back from Nagano, Kyle will meet a cooking specialist who uses koya-tofu in new, unique ways.

Sakai Yukako is known for sharing her creative dry food recipes in cookbooks and media appearances.

Let's start with small koya-tofu pieces.
We'll thaw with milk instead of water.

Milk works?

That's right.

It's like cereal.

Let the pieces soak up milk for about five minutes.

In the meantime, fry ginger, then add chicken broth, sake and canned scallops.

They soaked it all up.

Indeed.

Let's put them in here.

Add some shiitake mushrooms,
then let the koya-tofu soak up the soup.

Nice aroma.

The koya-tofu isn't breaking at all.

Right. Normal tofu would break into pieces.

Nice.

Add milk and edible chrysanthemums, then season with salt and pepper.

Finally, thicken with water-soluble potato starch, and it's ready to serve.

Koya-tofu with a modern twist.

So different from what I've tried so far.

Not just traditional Japanese cuisine.

I agree. Nice and creamy.
Koya-tofu has so many possibilities.

Right?

This is quite mind blowing.

I'm glad.

Next, gapao rice, the famous dish from Thailand.

Unthawed koya-tofu is stir-fried with minced pork and vegetables.

"Oishii?"

The texture is close to meat.
But the flavor really pops out.

Kind of bursts forth, right?

Tiny pieces, but they pack a punch.

This is amazing. You've got to try it.

Finally, believe it or not, dessert!

Koya-tofu thawed in soy milk with sugar, then cooked like French toast.

Ooh. It's jiggly.

- This is fascinating.
- Is it?

Yeah. "Oishii."

Okay, so there's no eggs used in this, right?

So the texture of the koya-tofu with the soy milk inside gives it this

juicy and kind of chewy texture like there was actually egg used in it.

This is phenomenal. This is like a magic trick.

Another new way to use koya-tofu!
Making desserts really works.

Right.

Just like usual tofu, its taste is plain and simple.

When you combine it with other ingredients,
it brings out a totally different charm.

Koya-tofu: by freezing normal tofu, this ingredient takes on a whole new personality.

The possibilities for this frozen Japanese ingredient are practically limitless!

In Tokyo, every ingredient has its own story.