Most of the Wasabi You've Eaten Wasn't Wasabi

The sharp green paste served with a piece of sushi almost everywhere outside Japan is not, with rare exceptions, wasabi. It is European horseradish — Armoracia rusticana — ground into a paste with mustard powder and a drop of green food colouring. By most estimates, more than 95 percent of "wasabi" served outside Japan, and a fair amount served inside it, is this substitute. Real wasabi appears at high-end restaurants and as a small luxury item in shops, and that is roughly it.

The plant being substituted for is Eutrema japonicum, native to Japan and a member of the same family as horseradish, mustard, and cabbage. It grows along cold, shaded mountain streams, wants temperatures between 8 and 20 °C, and dislikes direct sun. The largest producers are in Nagano and Shizuoka. A rhizome — the part that gets grated — takes 18 months to two years to mature. It is among the most expensive crops in the world by weight.

The chemistry is what makes the substitution easier. The compound responsible for that nose-kick is allyl isothiocyanate, generated when grating ruptures the plant's cells and the enzyme myrosinase reacts with stored allyl glucosinolate. Horseradish does the same thing, with the same compound. So does ordinary mustard. The molecule is small and volatile — within fifteen minutes of grating, much of the pungency has evaporated — which is why high-end sushi counters grate wasabi to order on a sharkskin-covered paddle, the oroshigane, with ridges fine enough to break out as much enzyme as possible.

What real wasabi has, on top of that fading kick, is a sweet, vegetal undertone the horseradish lacks. That is the part the dye cannot fake.