Freshly cut onions frequently elicit uncontrollable tears and a stinging sensation in the eyes of those in the vicinity. Syn-propanethial-S-oxide, a volatile liquid, and its aerosol, which stimulate eye nerves, are to blame for this. A series of reactions that act as a defense mechanism produce this gas: hacking an onion makes harm cells which discharges catalysts called alliinases.

Sulfenic acids are produced when amino acid sulfoxides are broken down by these. A second enzyme, lacrimatory factor synthase (LFS), quickly reacts with a specific sulfenic acid, 1-propenesulfenic acid, resulting in syn-propanethial-S-oxide. This gas diffuses through the air and before long arrives at the eyes, where it initiates tactile neurons. Tears are produced by the lacrimal glands to remove the irritant.

Cutting onions while submerged in a basin of water or under running water can prevent eye irritation. Because the onion base contains a higher concentration of sulfur compounds than the rest of the bulb, leaving the root end intact also reduces irritation.

Allium species differ in terms of the amount of sulfenic acids released, lacrimal factor, and irritation effect. In order to prevent the synthesis of lachrymatory factor synthase in onions, the New Zealand Institute for Crop and Food Research created “no tears” onions in 2008. According to one study, consumers prefer onions with less LFS in their flavor. Since the cycle hinders sulfur ingestion by the plant, some track down LFS− onions mediocre in flavor.