Why Cutting Onions Makes You Cry

When we cut an onion, we are not simply releasing a ready-made “tear gas” stored inside it. An intact onion keeps key reactants and enzymes apart. Cutting damages the tissue, allowing them to mix and begin a chain of reactions.

First, the enzyme alliinase converts a sulfur-containing precursor into an unstable sulfenic acid. Lachrymatory-factor synthase then produces the volatile compound propanethial S-oxide. When it reaches the surface of the eye, it irritates sensory nerves. The lacrimal glands respond by producing extra tears to dilute and wash away the irritant. This is a protective reflex, not an emotional response.

The mechanism also explains why finer chopping usually causes more irritation: more cells are broken. Chilling an onion may slow the reactions and evaporation, but it does not change the underlying process. The key finding in 2002 was that the lachrymatory factor is not merely an accidental by-product of the flavour pathway. A specialised enzyme makes it.

References
Nature, An Onion Enzyme That Makes the Eyes Water
https://doi.org/10.1038/419685a
ACS Chemical Biology, Enzyme That Makes You Cry
https://doi.org/10.1021/acschembio.7b00336