Why do onions make you cry?

An intact onion is harmless because its weapon is stored in two separate places. The cell vacuoles hold sulfur-containing amino acid derivatives, chiefly a compound abbreviated PRENCSO, while the enzyme alliinase sits in the surrounding cytoplasm. Slicing ruptures the cell walls and lets them meet. Alliinase cleaves the substrate into an unstable sulfenic acid, which lachrymatory-factor synthase, or LFS, promptly rearranges into syn-propanethial-S-oxide. That molecule is small and volatile, so it drifts up to your face, dissolves in the film of moisture on your eye, and stimulates nerve endings in the cornea. The eye reads this as an irritant and floods itself with tears to wash the compound away.

The deeper principle is compartmentalization as a defense strategy. A plant that stored a finished irritant would have to tolerate it constantly and would lose it to evaporation. Storing a harmless precursor and a separate enzyme means the reaction only runs where tissue is torn, which is exactly where a grazing animal or insect is. Garlic uses the same architecture with different chemistry, producing allicin rather than a tear gas. For a long time the sulfenic acid was thought to rearrange spontaneously, and the discovery of LFS as a dedicated enzyme in the early 2000s showed the plant actively directs the pathway toward the lachrymatory product rather than letting chemistry take its course.

Knowing the enzyme is responsible made it a target. Researchers have used gene silencing to suppress LFS in onions, producing plants that keep their flavor chemistry but generate far less of the tear compound, and low-pungency onion lines have reached commercial development through both engineering and conventional breeding. Ordinary kitchen tactics work on the same mechanism. Chilling the onion slows the enzymes and lowers volatility. A very sharp blade crushes fewer cells than a dull one. Cutting under running water or near an extractor carries the gas away, and goggles simply keep it off the cornea. Storing the root end for last helps because that region concentrates the precursor.

Common misconceptions usually name the wrong chemical. People say sulfuric acid forms in the eye, but the irritant is syn-propanethial-S-oxide, and while it can hydrolyze, the tears are a nerve response, not an acid burn. People say holding a metal spoon or a piece of bread in the mouth stops it, which does nothing to a gas travelling to your eyes. People say all onions are equally strong, when precursor levels vary widely by variety and growing conditions. People say cooking makes the problem worse, when heat destroys the enzymes and stops production entirely. And crying does not mean the onion is fresher; a dried-out onion simply has less of the volatile compound left to release.