Horseradish
The distinctive pungent taste of horseradish is from the compound allyl isothiocyanate. Upon crushing the flesh of horseradish, the enzyme myrosinase is released and acts on the glucosinolates sinigrin and gluconasturtiin, which are precursors to the allyl isothiocyanate.  The allyl isothiocyanate serves the plant as a natural defense against herbivores. Since allyl isothiocyanate 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 is an unstable compound, degrading over the course of days at 37 °C.  Because of this instability, horseradish sauces lack the pungency of the freshly crushed roots.