Allyl isothiocyanate
The volatile compound that gives mustard, horseradish and wasabi their nose-clearing sharpness.
Also: mustard oil compound, AITC, horseradish heat, wasabi heat
Allyl isothiocyanate forms when plant enzymes meet glucosinolate compounds in crushed mustard, horseradish or wasabi. In intact seeds the two are kept apart; crushing and wetting them starts the reaction, which is why mustard seeds are nearly odourless until ground, and why the heat is strongest minutes after preparation. Acid, such as vinegar, halts the enzyme and fixes the flavour at a milder level, while hot water destroys the enzyme and produces bitter mustard. The sensation is felt through the nose and sinuses, and fades quickly. It activates the TRPA1 receptor and is a natural defence against herbivores.