Hello, and welcome to Japanology Plus.
I'm Peter Barakan.

Our theme for today is sweet potatoes.

I suspect I'm not the only one
who was under the misapprehension

that sweet potatoes
and yams were the same thing.

It turns out
that they're not related at all.

Now this is a sweet potato,
and it's a freshly baked one.

I'm going to break it in half and...

that looks absolutely delicious,
doesn't it?

And...it's really sweet.

Nice, moist texture to it as well.

Sweet potatoes have been
a popular wintertime snack food in Japan

for as long as I can remember,
and much longer than that too.

But these days, they're being turned
into all kinds of sweets.

There are even sweet potato festivals.

We're going to try and find out

what the secret is of the popularity
of sweet potatoes in Japan.

Around 60 types of sweet potato
are cultivated in Japan.

They come in a wide range
of appearances and flavors.

Five or six varieties
are especially common.

Let's have a look at how they're used.

First, they're eaten as a snack.

Stone baking is a classic cooking method.

Heated stones cook the potatoes
with a far-infrared effect,

making them sweeter
than those cooked on an open fire.

Baking is a good way to enjoy
the textures of different varieties.

Some are firm and substantial,
with a sweet, straightforward flavor.

Others have a soft and moist texture,
with a rich sweetness.

For centuries,

sweet potatoes have been used
to make Japanese confectionery.

These are made
by combining strained potato and sugar.

This snack is called daigaku imo.

Sweet potato is diced, fried,

and coated
with syrup and black sesame seeds.

Sweet potato is
also a classic element of tempura.

Deep-frying in batter
accentuates the sweetness.

In soups,
stews and countless other dishes,

the sweet potato is a familiar
feature of Japanese cooking.

Sweet potatoes can also be used
to make an alcoholic beverage.

The starch ferments
before distilling begins.

The result is shochu, a strong drink.

Beyond the world of food and drink,

sweet potato plants themselves are admired

for their varied leaves
and pretty flowers.

This humble vegetable is deeply rooted
in everyday Japanese life.

To learn more,
we've come to Kasumigaura, in Ibaraki.

Hello, Hashimoto-san.

Hello.

Nice to meet you.

Nice to meet you, too.

I see you're well armed
with your sweet potatoes.

You actually even look
like a sweet potato.

In order to tell you about them,
I've turned into one!

OK.

Our guest is Hashimoto Ayuki.

He's a sweet potato expert,

offering consulting services,
and promoting the vegetable.

Satsumaimo is what sweet potatoes
are called in Japanese.

Imo means “potato,”

and “Satsuma” is the historical name
for what is now known as...

well, it's the southern part of Kyushu,
and it's called Kagoshima.

Why are we in Ibaraki,

which is a couple of hours north of Tokyo,

rather than going southwest to Kagoshima?

This prefecture is actually Japan's
second-biggest producer of sweet potatoes.

They're grown
in fields throughout Ibaraki.

I didn't know that. OK.

Our first destination is a shop
in Kasumigaura.

The sweets sold here are all made
from sweet potato.

Everything they have is made
out of sweet potatoes?

Yes.

Really?

Specialist sweet shops
are becoming more common.

Shall we go in?

Sure. OK.

Come in. Welcome.

Are these all different kinds?

These are a soft, moist variety.

Chilled and ready to eat.

You eat them cold?

They've already been baked,

so you can take them
out and eat them right away.

They're velvety,
with a luscious sweetness.

Smooth, moist varieties are delicious,
even when served cold.

Lately, you see them in
ice cream sundaes, and so on.

Those are dried potatoes.

These ones are flat slices,
and these ones are whole dried potatoes.

OK.

The whole ones have a high water content,
so the texture is different.

They're quite firm,
but a bit softer than flat slices.

The whole ones
are more challenging to make.

The shop also uses sweet potato
to make Western confectionery.

This is a potato-based dessert
with yuzu citrus and ginger.

It features one of the cooled
baked potatoes you saw just now.

So...just cut into this, yeah?

Yes. Eat it with the ice cream.

And you can eat the skin too?

Yes, you can eat the skin if you like.

Mmm. OK.

Mmm! That's really good.

You've got the natural sweetness
of the potato,

plus the slightly tart feel
of the yuzu and the ginger...

and the kind of spicy feel
to the ginger too.

That's a really nice balance.

It's amazing

that there's so many different kinds
of sweets all made out of sweet potatoes.

Indeed.

Japan is said to be the best place in
the world for processing sweet potatoes.

New products are being created
all the time.

Why do you think that these sweet potatoes
are as popular as they are in Japan?

Well, I think that Japan's circumstances
contribute to that.

Japan has relatively little land available
for cultivation.

Typhoons and other natural disasters
make it difficult to grow many crops.

But sweet potatoes are usually easier.

For one thing,
they're resistant to disease.

Also, sugarcane or sugar beet can only
be grown in a very few places in Japan.

Sweet potatoes can
be planted pretty much anywhere,

and for centuries
they were a kind of sugar substitute.

Ah, I see.

These dried potatoes
I've seen going back quite a long time.

But some of the other things,
like the ones we were eating earlier on,

those seem to be
a much more recent phenomenon.

What's brought about this sudden vogue

for having all these different kind
of sweet potato sweets?

Baked sweet potatoes
have enjoyed waves of popularity.

We're in the so-called fourth boom,
which started in the early 2000s.

Before that, they commonly
had a somewhat dry texture.

That type was not easy for elderly people,
or children, to eat.

But then we started to get a softer,
moist texture.

In the early 2000s, a rich,
sweet variety became popular.

Then we saw creamy types,
and some with a silky texture.

Also, there were new ways to bake them.

That made a difference.

People could simply plug a device
into a typical electric outlet.

And so, baked sweet potatoes
also became available in shops,

like supermarkets and convenience stores.

That boosted their popularity.

The history of sweet potatoes in Japan
dates back to the early 17th century.

They came from China to Satsuma,
in Kyushu,

and became known as “satsumaimo.”

At the time, they were
a valuable source of sweetness.

In the mid-18th century,
Japan suffered repeated famine.

Sweet potatoes proved to be invaluable.

They're resistant
to bad weather and pests,

and can be grown almost anywhere.

They offered clear benefits,
and were soon widely cultivated.

By the end of the 18th century,

there were over
a hundred sweet potato dishes.

The devastation of the Second World War
led to serious food shortages in Japan.

The government issued instructions
for sweet potatoes to be planted,

even in school grounds and parks.

Once again, they helped in a time of need.

In the 1950s, as the economy boomed,

mobile sales of stone-baked potatoes
were embraced by the nation.

From the 1980s,
sweet potato snacks diversified,

and became part of everyday life
in the home.

Our next stop is Namegata.

Even within Ibaraki,
it's a sweet potato hotspot.

A storehouse here plays a key role
in producing high-quality sweet potatoes.

This is Kaneta-san.

Hello.

Hello. Nice to meet you.

Welcome. It's good to see you.

Let's go.

When farmers harvest sweet potatoes
from the fields,

the potatoes are sometimes grazed,
like this.

If you store them as-is,
bacteria will get in, and they'll spoil.

To prevent that, they must be cured.

Curing heals the wounds,

and the potatoes can be stored
in a good condition.

This potato used to be covered in scrapes.

But it's been cured.

OK.

The appearance is good.

Whoa.

That's steam.

Yes, it's steam. You're right.

To cure the potatoes, we raise
the humidity to one hundred percent.

Whoa.

We leave them at 32 or 33 degrees Celsius
for 72 to 80 hours.

That stimulates the cells of the potatoes.

They start to repair their own injuries.

So what happens
when you keep sweet potatoes

at that temperature and humidity
for around 80 hours?

A protective layer known
as cork forms under the surface

to prevent damage by harmful bacteria.

Once this curing process is complete,
the potatoes can be placed in storage.

This storage area is kept
at around 13 or 14 degrees Celsius.

That's pretty much the same
as the temperature in the ground

when sweet potatoes are growing.

If they're too hot, they sweat, just like
we do, and lose moisture content.

If they're too cold,
they get soft and inedible.

The fibers break down.

So we have to keep them at
just the right temperature.

If we do that,
they can be stored for around a year.

By curing sweet potatoes,

and keeping them
at a set temperature and humidity,

they can be shipped
to market all year round.

Here I have a raw sweet potato.

Would you like to take a bite?

You can eat them raw?

Yes, you can.

Hmm...

it's not sweet.

I was expecting it to be
a little bit sweet, and it's not.

That's right.

When raw, they aren't sweet,
or very tasty, actually.

There are various reasons
why they become sweet and good to eat.

I'd like to take a look
at some of those reasons.

At this long-established business
in Hokota,

potatoes are baked in large terracotta
containers using a time-honored method.

Certain cooking methods bring
out the sweetness.

The sugar content of a raw sweet potato
is about eight percent.

When baked, that rises to 40 percent.

That much?

Yes.

We're going to ask the shop's owner
why that happens.

-Let's go.
-OK.

Nice to see you.

Hello.

Hello. Nice to meet you.

How does it work?
Perhaps you can explain it for us.

Oh, it's charcoal in there. I see.

Oh wow, and the sweet potatoes
are just suspended.

So how does it work?

We use charcoal or a similar fuel,

make a fire,
and simply suspend the potatoes.

Right.

Then let them cook.

How hot is it inside there?

It gets incredibly hot.

Around 400 to 500 degrees Celsius.

Thermal radiation bakes the potatoes.

The key to creating
delicious sweet potatoes

is to keep their internal temperature
at around 67 degrees Celsius.

To make sure they cook evenly,
their position is regularly adjusted.

Sweet potatoes contain starch and amylase.

At 67 degrees, amylase breaks
starch down into maltose.

After two hours of baking,
the amylase has done its work,

and the potato has become much sweeter.

This is all quite fascinating
because like...

normally you go out
and you buy a baked potato,

and you think, “Oh, this is nice,”

and it's hot, and it's
sweet, and it tastes great.

But there's so much that goes
into making it taste like that.

And you normally just wouldn't
even imagine most of it, I think.

In Japan, a lot of research has been
done on sweet potatoes.

We don't just eat them.

Purple varieties are used
to produce a pigment for other foods.

A variety was newly developed with leaves
and stems that were better to eat.

The greens help
to lower blood sugar levels,

and they're being used
in vegetable juice drinks.

There's global interest in
various aspects of sweet potato research.

And so, looking ahead, we can expect
to see other new developments.

The cry of the potato seller

has long been a familiar
feature of the Japanese winter.

But this business has been changing
with the times.

Let's look at some of the latest ways
to sell baked potatoes on the move.

First, Osaka.

Thanks to its modern music
and stunning lighting,

this unique creation has come
to be appreciated as a work of art.

In 2015,

it actually received the
Taro Okamoto Award for Contemporary Art.

Next, a distinctive song.

Sophisticated sweet potato sales!

Our next unusual sweet potato seller is
in Hinode, Tokyo.

The melody is familiar, but it's coming
from a bright red sports car!

The trailer it pulls
actually contains an oven.

Customers are drawn
to the car's eye-catching appearance.

The packaging has a clear car race appeal.

They're delicious.

Our son's always asking
for a potato from the sports car.

The owner is Harada Terukazu.

His main job
is running a car repair workshop.

He and his wife have
been selling sweet potatoes

on Saturdays and Sundays since 2018—
from a stall, and from the car.

They sell around 100 per day.

I've always liked this kind of car.

And my wife was always buying
baked sweet potatoes at the supermarket.

I was thinking of a way that
we could both enjoy the things we love,

and this idea just emerged naturally.

Harada came up
with the plan of selling baked potatoes

from a trailer pulled
by his much-loved sports car.

He spent a month researching
ovens and baking techniques,

and then went into business in 2018.

At weekends,

they sell sweet potatoes
from a stall outside the workshop.

To achieve a great taste,
they use specific cooking materials.

Let me show you the stones
I use for baking.

They're quartz.

I learned that quartz results
in sweeter potatoes,

so I decided to use them for the business.

The stall is popular with car lovers,
and with discerning potato fans.

Baked sweet potatoes, and the ways
they're sold, continue to evolve.

Has it ever occurred to you

that sweet potatoes could be
used to generate electricity?

This is a shochu maker
in Miyazaki Prefecture.

Every day,

it produces around 200,000
big bottles' worth of sweet potato shochu—

more than anywhere else
in the country.

That generates a lot of waste,
which has been a headache for years.

Potato scraps that can't be used
in production are brought here.

We receive around 15 tonnes a day.

800 tonnes of shochu lees
are also produced each day.

Previously, the company had no choice
but to treat it as industrial waste,

and pay for its disposal.

But in 2014 they began using the
waste materials to generate electricity.

The scraps and lees are fermented,
and that produces methane.

The methane is fuel for generators.

8.5 million kilowatt hours of electricity
are produced each year—

enough to power 2,000 homes.

The company hopes to recoup its costs

by selling the electricity
to a local power company.

For over a decade,

Professor Suzuki Takahiro
of Kindai University

has been studying the energy applications
of sweet potatoes.

You can store sweet potatoes,
and you can store their methane.

Solar or wind generators depend
on good climate conditions,

but not methane.

We can generate potato power
by using the methane

in the same way as a fossil fuel.

In 2021 the shochu maker
began using electric cars

powered by the electricity it generates.

A future project is to use the electricity

in the shochu
manufacturing process itself.

Sweet potatoes are also being used

to help keep an office block cool
on a hot day.

This building is in central Nagoya.

Today, with the sun beating down,

people who work inside the building
are busy planting 48 potato plants.

I hope we'll get some delicious potatoes!

Go for it!

The sweet potatoes are grown
in eight beds, using hydroponics.

Six months from now,
they will fill the roof.

This is what the roof looks like
in the autumn: a carpet of green leaves.

The leaves play a role
in blocking the rays of the sun.

Also, moisture from the leaves evaporates,
which has a cooling effect.

The sweet potato plant does this
especially effectively.

In summer,

the surface temperature of the roof
can be as high as 50 degrees Celsius.

Studies show that leaf coverage
can reduce the temperature by 27 degrees,

and help to keep the interior
of the building cool.

Rooftop greenery looks nice,
and sweet potatoes are good to eat.

So these plants are useful
in a number of different ways.

What do you think this is?

I haven't a clue.

Looks like some kind of honey or syrup.

It's a syrup of some kind.
It's sweet of course.

It has a distinctive taste to it as well.

I'm guessing
that it comes out of sweet potatoes.

You're right: it's sweet potato syrup.

Wow.

Honey is mainly glucose and fructose.

Maple syrup is mainly sucrose.

But sweet potato syrup
has a lot of maltose.

It's a different kind of sugar?

Yes. It has a soft, gentle sweetness.

It's interesting;

you can actually see the syrup kind
of oozing out of the potato, can't you?

You can.

Sweet potatoes
aren't just a source of sugar.

They provide relatively high amounts
of vitamins and minerals.

In particular, they offer plenty
of vitamins B and C.

As for minerals,
they're high in potassium.

When you eat this syrup,

it has an antioxidizing effect,
and offers other benefits.

I'd like lots of people to try it.

And there are various types
of sweet potatoes as well.

The ones we've been looking
at today are the softer ones

with a more moist texture,
which is very nice.

But there are the sort
of slightly harder ones as well,

which...my favorite, actually,
is the harder ones...

if you slice them quite thinly,
and then use them for tempura.

That is just amazing,
and for anybody that hasn't had that,

I would definitely recommend it.

Absolutely. You can bake them,
you can fry them.

They're delicious.

In due course

I think people all over the world

will come to appreciate
their natural sweetness.

Sweet potatoes may even
play a role in space.

That's because they're so easy to grow.

We are already using hydroponics
to grow them here on Earth.

And it's thought that hydroponics
could also be used in space,

to make sweet potatoes there.

OK. So,
lots of different ways to eat them.

OK. Thank you very much.

Thank you very much.