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.
Allyl isothiocyanate serves the plant as a defense against herbivores; since it is harmful to the plant itself, it is stored in the harmless form of the glucosinolate, separate from the myrosinase enzyme. When an animal chews the plant, the allyl isothiocyanate is released, repelling the animal.
Allyl isothiocyanate can also be liberated by heating during distillation. The product obtained in this fashion is known as volatile oil of mustard and is usually around 92% pure. It is used principally as a flavoring agent in a variety of foods.
Synthetic allyl isothiocyanate is used as an insecticide, as an animal repellent, to denature alcohol, and as a topical anesthetic. It is also highly antibacterial, and can also help combat mold growth.
- Piperine, the active piquant chemical in black pepper
- Capsaicin, the active piquant chemical in chile peppers
- Allicin, the active piquant flavor chemical in uncooked garlic and onions (see those articles for discussion of other chemicals in them relating to pungency, and eye irritation)