Onion Thiosulfinates: Some Like Them Hot, Others Not.

Onion (Allium cepa) efficiently acquires and accumulates sulfur.  In most plants, the majority of sulfur is located in proteins in the amino acids cysteine and methionine; however in onion a significant proportion of the sulfur is found in non-protein amino-acid derivatives.  In onion, sulfur is most commonly taken up by the roots as sulfate, transported via the xylem to the leaves, reduced to sulfide, and incorporated into organosulfur compounds.  The flavor precursors, Alkyl-L-Cysteine Sulfoxides (ACSOs), are derived from cysteine and are physically separated in cells from the enzyme alliinase.  When onion tissue is crushed or chopped, alliinase acts upon the ACSOs to produce complex suites of thiosulfinates.  Volatile thiosulfinates collect on the outer surface of the eye as sulfenic acids, causing the well-known tearing effect of onion.  Thiosulfinates are also responsible for the pungent flavor of onion and have well established health benefits.  The pungency of onion is strongly influenced by sulfur availability, as well as genetic factors controlling the relative amounts of thiosulfinates produced by mild and pungent onions.