Wasabi (Wasabia japonica , Cochlearia wasabi, or Eutrema japonica) is a member of the Brassicaceae family, which includes cabbages, horseradish and mustard. Known as "Japanese horseradish", its root is used as a spice and has an extremely strong flavour. Its hotness is more akin to that of a hot mustard than the capsaicin in a chili pepper, producing vapors that irritate the nasal passages more than the tongue. The plant grows naturally along stream beds in mountain river valleys in Japan. There are also other species used, such as W. koreana, and W. tetsuigi. The two main cultivars in the marketplace are W. japonica cv. 'Daruma' and cv. 'Mazuma', but there are many others.
Wasabi is generally sold either in the form of a root (real wasabi), which must be very finely grated before use, or as a ready-to-use paste (horseradish, mustard and food colouring), usually in tubes approximately the size and shape of travel toothpaste tubes. Once the paste is prepared it should remain covered until served to protect the flavour from evaporation. For this reason, sushi chefs usually put the wasabi between the fish and the rice.
Fresh leaves of wasabi can also be eaten and have some of the hot flavour of wasabi roots. They can be eaten as wasabi salad by pickling overnight with a salt-and-vinegar-based dressing, or by quickly boiling them with a little soy sauce. Additionally, the leaves can be battered and deep-fried into chips.
The burning sensations it can induce are short lived compared to the effects of chili peppers, especially when water is used to remove the spicy flavour.
Wasabi is often served with sushi or sashimi, usually accompanied with soy sauce. The two are sometimes mixed to form a single dipping sauce known as Wasabi-joyu. Legumes (or peas) may be roasted or fried, then coated with a wasabi-like mixture (usually an imitation); these are then eaten as an eye-watering "in the hand" snack.
Almost all sushi bars in America and Japan serve imitation (seiyō) wasabi because authentic wasabi is usually expensive, but it is becoming widely available even in the United States. Wasabi loses much of its flavour if exposed to air for even a short time, so genuine powdered wasabi, while it does exist, typically contains horseradish and other ingredients to approximate the nasal spiciness of fresh wasabi. Because of this, most powders use no real wasabi and instead turn to just horseradish, mustard seed, and green food colouring (sometimes Spirulina). Whether real or imitation, the powder is mixed with an equal amount of water to make a paste. Few people, even in Japan, realise that most of the wasabi that they consume is in fact an imitation. While not considered equal with the freshly grated product, preserved wasabi is available in tubes and, in larger quantity, frozen bags. Like powder, tubed wasabi often contains no real wasabi at all, so verification of the ingredients is needed.