MOST PEOPLE THINK they know what wasabi tastes like. A small blob of the pale green, fiery, sinus-clearing 'Japanese horseradish' is always served with sushi and sashimi, and is mixed into a dipping sauce (along with minced spring onions and quail egg yolk) for cold soba or green-tea noodles. But unless you've been dining at the best Japanese restaurants, chances are you've never tasted the real stuff. Most eateries use an imitation wasabi made from ground, dried Armoracia rusticana horseradish, mustard powder and blue and yellow food colouring to turn it that familiar pale green. This is made into a ready-to-use paste squeezed from tubes, or into a powder, which the chef mixes with water. The imitations are convenient and much cheaper than the genuine article but the taste is sharp, acrid and nothing like the comparatively mild Wasabia japonica (also known as Eutrema japonica), which belongs to a different botanical family (Japan-ese horseradish is a misnomer). Freshly grated wasabi even tastes different to the '100 per cent real' wasabi paste in tubes, which is mixed with stabilisers to prolong its shelf life. Fresh wasabi is served at top Japanese restaurants only because it is so expensive. The plant is difficult to cultivate: it must grow in cool, shaded places along clear, running streams and takes about two years to reach maturity. The whole plant is edible but it is the pale green root that is relieved of its thin skin and grated. The first time you encounter real wasabi it is a revelation - everything about it is so different from what you might expect. At first glance it doesn't look any different, but closer inspection reveals the tiny shreds created when the root is rubbed against a fine grater. Taste it and instead of a harsh, burning flavour and odour, it is fragrant and much more mild. Instead of obliterating all other flavours, real wasabi complements whatever it is served with. Even the texture is different: it is light, almost fluffy, and it breaks apart easily into fine strands when dipped in liquid. People who are used to eating the imitation sometimes find it difficult to appreciate the subtlety of real wasabi because they think it tastes bland by comparison.