Allyl isothiocyanate is the chemical compound responsible for the pungent taste of mustard, horseradish and wasabi. It is a colorless to pale yellow liquid that is slightly soluble in water, but well soluble in most organic solvents.
Allyl isothiocyanate itself does not occur in the mustard seed. Instead, when ground mustard seeds are mixed with water, vinegar, or other liquids or when they are chewed, a chemical reaction between the enzyme myrosinase and a glucosinolate known as sinigrin from the seeds of black mustard (Brassica nigra) or brown Indian mustard (Brassica juncea) leads to the production of allyl isothiocyanate.