synpropanethialSoxide

SynpropanethialSoxide, or syn-propanethial-S-oxide, is a volatile sulfur-containing compound produced by onion and related Allium vegetables when tissue is damaged. It is best known as the primary lachrymatory factor that triggers tearing during cutting or crushing.

Biosynthesis and formation occur in two steps. First, enzymes called alliinases act on S-alk(en)yl-L-cysteine sulfoxides (such as alliin) to yield sulfenic acids. In a spontaneous rearrangement, two sulfenic acid molecules condense to form syn-propanethial-S-oxide. The result is a highly volatile product that readily escapes from damaged plant tissue into the surrounding air and toward the eye.

Mechanism of irritation is through contact with the ocular surface. When syn-propanethial-S-oxide reaches the eye, it acts as an irritant that stimulates the sensory nerves of the cornea and conjunctiva via the trigeminal pathway, inducing reflex tearing. The effect depends on the concentration and volatility of the compound, as well as the degree of tissue disruption. While commonly associated with onions, similar lachrymatory agents can be produced by other Allium species under mechanical damage.

In scientific and culinary contexts, syn-propanethial-S-oxide is sometimes referred to as the lachrymatory factor. The term syn reflects the stereochemical arrangement produced during the sulfenic acid rearrangement. Variability in lachrymatory response among cultivars and preparation methods has led to strategies such as chilling, sharp knife use, or processing approaches to minimize tearing.