Wasabi is such an expensive condiment that the genuine article is rarely available. True wasabi is almost never offered in restaurants outside of Japan, and even in that country it has been found that only about 5% of restaurants provide the real thing. Indeed, powdered horseradish is used to adulterate wasabi. Such fraudulent substitution is inevitable because of the limited supply and great cost. Generally, the dab of pale-green paste served with sushi is imitation wasabi, usually a combination of horseradish, mustard, cornstarch, and artificial food coloring. An easy way to tell if genuine wasabi is being served is based on the fact that the isothiocyanate, which produces the sensation of heat, dissipates rapidly, and a fresh lump on the plate will lose most of its heat within 15 minutes. In a high-end restaurant that serves genuine wasabi, a waiter may be expected to come to the table occasionally to freshen up the condiment. Only enough material that can be consumed within approximately 15 minutes should be grated.

[p. 596-597]