Most wasabi outside Japan is horseradish, mustard and green dye
The green paste beside your sushi probably never touched a wasabi plant. Real wasabi is fussy, shunning direct sun and demanding cool, flowing water, so supply never meets demand. Even respected sushi bars abroad mostly use a powdered blend of horseradish and mustard oil, and many packets labelled wasabi contain none at all.
Wasabi is a member of the cabbage and mustard family that grows wild along stream beds in Japanese mountain valleys, and is also native to Korea and Russia's Far East. Its oldest recorded use as food dates to the eighth century. The part sold as the root is more accurately the rhizome, a thickened stem base, which is grated into a paste. The traditional grater was coarse sharkskin; today it is usually a metal oroshigane.
The sting comes from allyl isothiocyanate, which forms only when grating ruptures the cells and releases an enzyme that breaks down a sulphur-containing sugar compound. The plant uses the same reaction to defend itself when damaged, and it is the same chemical that gives horseradish and mustard their kick. Because it is not oily, the burn fades fast and hits the nose rather than the tongue, unlike chilli. That volatility earned researchers the 2011 Ig Nobel Prize in Chemistry for working out how much airborne wasabi would wake sleepers, part of an effort to build a fire alarm for deaf people; one test subject woke within 10 seconds.
Growing it is hard. Air needs to stay between roughly 8 and 20 °C, above 28 °C the plant suffers heat damage and rot, and ideal water runs at 10 to 13 °C with plenty of dissolved oxygen. Japanese growers developed soilless beds of rock, pebbles and sand fed by spring water, including the terraced Tatamiishi style of 1892, which amounts to an early form of hydroponics. Nagano's Daio Wasabi Farm, the world's largest, needs no added fertiliser thanks to runoff from rice paddies upstream.
Freshly grated paste loses flavour quickly in open air, which is why sushi chefs tuck it between the fish and the rice. Horseradish, called western wasabi in Japan, tastes similar but sharper and earthier.
Source: Wasabi
