The green paste next to your sushi probably isn't wasabi.

The story you've heard

Order a plate of sushi almost anywhere in the world and it arrives with a neat little blob of green paste. Most of us call it wasabi, dab a bit on the fish, and wait for that famous rush up the nose.

It feels like part of the ritual, as Japanese as the chopsticks. But in most restaurants, supermarkets and handy squeezy tubes, that green paste contains little or no wasabi at all.

A plant that hates to be rushed

Real wasabi is a plant native to Japan, a member of the mustard and cabbage family. The part people grate is often called a root, but it is really a thick rhizome, a stem that grows at the base of the leaves. According to Nippon.com, wild wasabi lives in the shallow water of clear, cold streams in cool mountain areas, and it takes between eighteen months and three years to reach a usable size.

Growing it on purpose means copying those streams. Japan's tourism agency describes farms in Azumino, in Nagano, built around springs fed by snowmelt from the Northern Alps, with the water kept at about 13°C and black cloth hung over the beds in summer for shade. Nippon.com adds that the water should stay below about 16°C even in midsummer. In Shizuoka, farmers use a terraced method called tatamiishi, layering large stones, pebbles and sand so spring water can trickle through. That system has been used for around 400 years and was recognised by the United Nations Food and Agriculture Organization as a Globally Important Agricultural Heritage System in 2018.

History runs deep too. Nippon.com says records show wasabi being used as a medicinal herb in the Asuka period, which ended in 710. Shizuoka Prefecture traces cultivation to the Keicho era, roughly 1596 to 1615, when wild plants from the mountains above Utogi were replanted beside a village spring. Local tradition says the shogun Tokugawa Ieyasu was so fond of it that he banned taking it out of the area.

Why it must be grated at the last minute

Here is the surprising part: bite into a whole wasabi rhizome and it is not really hot. The heat only appears when the cells are broken. Grating mixes an enzyme with other compounds in the plant, and together they make allyl isothiocyanate, a sharp mustard oil. The finer the grating, the stronger the kick, which is why the traditional tool is a grater covered in sharkskin, rubbed in slow circles.

That kick does not last. Japan's tourism agency says the peak flavour lasts only about three to fifteen minutes after grating, and Mental Floss gives a similar window of fifteen to twenty minutes. Either way, real wasabi is a sprint, not a marathon. Smithsonian Magazine notes another clue: unlike chilli, whose heat sits on the tongue, wasabi's compounds mostly hit the nasal passages.

Real wasabi also tastes different. It has a sweet, fresh, almost green aroma, and grated wasabi is a soft green, while grated horseradish is white.

So what is in the tube?

Because the real thing is slow to grow, hard to farm and quick to fade, most kitchens use a substitute. Mental Floss describes the usual version as a dyed blend of horseradish powder and mustard, sometimes with a small amount of real wasabi added. Smithsonian Magazine found the same mix of horseradish, mustard and food colouring in a supermarket pack. Horseradish is a close cousin, so it brings a similar heat, just a louder and less fragrant one. In Japan horseradish is even called seiyō-wasabi, Western wasabi.

Mental Floss reports an estimate that only around five per cent of the wasabi served in restaurants worldwide is the real plant. Treat that figure as a rough guess rather than a measurement, but the direction is clear.

Wasabi's link with sushi is old, but not ancient. Nippon.com explains that it was first a condiment for soba noodles, and its popularity surged in the early 1800s with the invention of nigirizushi in Edo, today's Tokyo, where a dab sits between the rice and the topping.

On your plate today

The paste in a tube is not a fake in a sneaky sense. It is a practical stand-in that keeps for months and gives a similar jolt. But if you are in a serious sushi bar and see the chef rubbing a knobbly green stem over a small grater, you are getting the real thing, and it is worth tasting on its own first. Look for a paler green, a softer heat and a fresh, sweet smell that vanishes almost as quickly as it arrived.

Timeline

1. 593–710Records show wasabi used as a medicinal herb in Japan's Asuka period.
2. 1596–1615Wild wasabi is replanted beside a spring in Utogi, Shizuoka, traditionally seen as the start of cultivation.
3. Mid-1700sCultivation techniques spread from Utogi to the Izu region.
4. Early 1800sNigirizushi becomes popular in Edo, with grated wasabi tucked between rice and topping.
5. 2018Shizuoka's traditional wasabi terraces are recognised as a Globally Important Agricultural Heritage System.