Why cutting an onion makes you cry

A whole onion doesn't sting. The tears come from a chemical reaction that starts the moment the knife breaks its cells, and ends in your eyes.

A whole onion sitting on the counter doesn't make anyone cry. The trouble starts with the knife.

An onion's cells hold sulphur-containing compounds and enzymes called alliinases. Cutting or crushing damages the cells and releases the enzymes, which break the sulphur compounds down into substances called sulfenic acids.

Then a second enzyme steps in. It is called lachrymatory-factor synthase, and it quickly turns one of those acids into syn-propanethial-S-oxide, a volatile compound that escapes into the air. Lachrymatory means tear-making, and the name is earned.

From the board to your eyes

The compound drifts up from the cutting board and reaches your eyes, where it stimulates nerves and stings. Your tear glands answer by producing tears to dilute it and flush it out. The crying is a defence on your side; the gas is generally described as a defence on the onion's, released only when something damages the plant.

The second enzyme was described in 2002, in a one-page paper in Nature with a title that says it all: "An onion enzyme that makes the eyes water". It came from researchers at House Foods, a Japanese food company.

The chemistry explains the usual kitchen advice. Cutting the onion under running water, or in a bowl of water, keeps much of the compound from reaching the air. Leaving the root end intact also helps, since the base holds more of the sulphur compounds than the rest of the bulb.

Breeders have gone after the enzyme itself. In 2008, researchers in New Zealand used genetic modification to make onions that don't produce it. In 2018, Bayer released Sunions, onions in which the enzyme is silenced, made through about 30 years of cross-breeding rather than genetic modification. Not everyone is won over: some people find tearless onions weaker in flavour.

So the tears are not a sign that you are cutting badly. They are your eyes cleaning up after a small chemical defence that the onion keeps ready for whoever breaks into it.