Wasabi is a plant that grows naturally in the marshy edges of clear mountain streams and when cultivated also needs clear, running water. Such cultivation usually takes place on mountain terraces. Due to the difficulty of growing wasabi, it is an expensive product.
Although conveniently called "horseradish" in English, it is in fact very different from Western horseradish: wasabi is more fragrant and less sharp. The pale green flesh of the root is made into a paste by rubbing the root on a fine metal grater (oroshigane). After grating, wasabi has to be used immediately as it soon loses its flavor.
Authentic wasabi has a fresh and cleansing taste - even a certain sweetness. The burning sensation works on the nasal passage rather than the tongue and can be easily washed away with liquid. Wasabi helps to prevent food poisoning and that is the reason why wasabi is eaten with raw fish (sashimi) as well as sushi containing raw fish as a topping. In exclusive sushi bars the chef grates the wasabi roots with a sharkskin grater (samegawa-oroshi). With sushi, wasabi is added by the customer to the soy-based dipping sauce, but also used by the chef, who always puts some wasabi between the rice and the slice of raw fish.
Wasabi has been long known in Japan - the oldest record dates from the 7th century, but it was mostly used for its medical properties. Wasabi is not used as a general condiment in traditional Japanese cooking, which does not know any sharp flavors. As stated above, its main function is for its anti-microbial properties with sashimi and sushi, and besides that in dipping sauces for cold soba, in chazuke and sometimes on steak. Wasabi can also be used for pickling vegetables (wasabizuke).
The best wasabi roots come from the Izu Peninsula in Shizuoka or from Nagano Prefecture.
[Wasabi with metal grater - photo Wikipedia]
Real wasabi is a luxury product even in Japan, and at home mostly wasabi paste (neri-wasabi) or wasabi powder (kona-wasabi) is used. This in itself would not be so bad, were it not that most of these products contain little or no authentic wasabi but instead Western horseradish (called Seiyo wasabi) mixed with mustard, starch and green coloring. The paste is sold in tubes in supermarkets, the less common powder is sold in cans.