a dried powder, or as a ready-to-use paste in tubes similar to toothpaste tubes. The part used for wasabi paste is variously characterized as a rhizome, a stem, or the "rhizome plus the base part of the stem". In some high-end restaurants, the paste is prepared when the customer orders and is made using a grater to grate the stem; once the paste is prepared, it loses flavor in 15 minutes if left uncovered. In sushi preparation, chefs usually put the wasabi between the fish and the rice because covering wasabi until served preserves its flavor. Fresh wasabi leaves can be eaten raw, having the spicy flavor of wasabi stems, but a common side effect is diarrhea. Legumes (peanuts, soybeans, or peas) may be roasted or fried and then coated with wasabi powder mixed with sugar, salt, or oil and eaten as a crunchy snack. In Japan, it is called . Surrogates Wasabi favors growing conditions that restrict its wide cultivation – among other things, it is quite intolerant of direct sunlight, requires an air temperature between , and prefers high humidity in summer. This makes fully satisfying commercial demand impossible for growers, which makes wasabi quite expensive. Therefore, outside Japan, finding real wasabi plants is rare. A common substitute is a mixture of horseradish, mustard, starch, and green food coloring or spinach powder. Often packages are labeled as wasabi while the ingredients do not include any part of the wasabi plant. The primary difference is color, with wasabi being naturally green. Fresh horseradish root is described as having a similar (albeit simpler) flavor and texture to that of fresh wasabi. In Japan, horseradish is referred to as . In the United States, true wasabi is generally found only at specialty grocers and high-end restaurants. Chemistry The chemical in wasabi that provides for its initial pungency is the volatile compound allyl isothiocyanate, which is produced by hydrolysis of natural thioglucosides (conjugates of the sugar glucose and sulfur-containing organic compounds); the hydrolysis reaction is catalyzed by myrosinase and occurs when the enzyme is released on cell rupture caused by maceration – e.g., grating – of the plant. The same compound is responsible for the pungency of horseradish and mustard. Allyl isothiocyanate can also be released when the wasabi plants have been damaged because it is being used as a defense mechanism. The sensory neural target of mustard oil is the chemosensory