Wasabi is made from a type of radish similar to Horse radish.
I found this from Chowhound:
Constituents of wasabi
Isothiocyanates (ITCs) are a group of naturally occurring sulphur compounds responsible for the chracteristic flavour of wasabi.
The compounds are volatile and are evolved from plant tissues when they are disrupted e.g. in the preparation of food, grating, cutting, chewing etc. However, plant tissues do not contain ITCs but contain glucosinolates (GSLs), which are the precursors of ITCs. GSLs are a group of glucosides, stored within the cell vacuoles of all Crucifereaeplants.
GSLs coexist in plants, but are not in contact with the hydrolytic enzyme myrosinase in the intact cell. When plant tissues are mechanically disrupted or injured (e.g. by chewing, crushing or grating in the preparation of food) the enzyme is released from the cell wall, and in the presence of adequate moisture myrosinase rapidly hydrolyses the GSLs to yield glucose and an aglucone.
The organic aglucone is unstable and undergoes Lossen Rearrangement to produce sulphate and a variety of products. The nature of products is dependent on a number of factors, including the structure of GSL side chain, the reaction conditions (e.g. pH), the presence of cofactors (e.g., metal ions, specific protein), temperature and durationas well as the age and condition of plant tissues.
Isothiocyanates (ITCs) are formed from GSLs under neutral and alkaline conditions. However, GSLs, which contains a â-hydroxyl group (I in Figure 2) in their side chain, give rise to ITCs that spontaneously cyclise to form oxazolidinethiones. Some aromatic and heterocyclic GSLs (II in Figure 2) produce ITCs which are unstable at pH 7 or higher and break down to release the corresponding alcohol and inorganic thiocyanate ions.
However, once formed ITCs are more stable under acidic conditions. In weakly acidic pH’s or in the presence of Fe+2and/ or in presence of endogenous nitrile factor, nitriles are produced from aglucone by autolysis instead of ITC, with the liberation of elemental sulphur. The relative proportion of ITC to nitriles can vary widely depending upon the conditions of autolysis.
Different ITCs have been reported from previous investigations in wasabi and each ITC has a specific flavour profile and so the complete taste of wasabi derived from the combined tastes and odours of all the ITCs present .