Why Onions Make You Cry

When an onion's layers are breached, a fascinating chemical cascade is set in motion. Inside its cells, previously separated enzymes, such as alliinase, meet sulfur-rich amino acids, specifically (E)-S-(1-propenyl)cysteine S-oxide. This initial encounter produces unstable sulfenic acids. However, the real culprit for our tears is a specialized enzyme called lachrymatory factor synthase (LFS), which then swiftly converts these sulfenic acids into syn-propanethial-S-oxide, a volatile sulfur compound that readily vaporizes and wafts into the air.

This airborne compound serves as a potent irritant. When syn-propanethial-S-oxide reaches the eyes, it reacts with the water covering the eye's surface, stimulating the lachrymal glands to produce tears as a protective reflex. This seemingly inconvenient reaction is actually an ingenious defense mechanism developed by the onion plant over millions of years. Unable to flee from threats, the onion relies on these harsh sulfur compounds to deter pests, microbes, and foraging animals from consuming its bulb, ensuring its survival and propagation.

Despite this formidable chemical defense, humans have cultivated and cherished onions for millennia, valuing their distinctive flavor and nutritional benefits. Ancient civilizations, like the Egyptians, even revered onions as symbols. Today, various methods are employed to mitigate the tearing effect, from chilling the onion to slow down enzyme activity, to using a very sharp knife to minimize cellular damage and thus the release of the irritant. Scientists have even explored creating "tearless" onions by suppressing the LFS enzyme, showcasing our ongoing fascination with this tear-jerking vegetable.