Why Do Onions Make You Cry?

Onions are chemical defence systems. Cutting one sets off a reaction designed to drive away animals that bite it — including you.

The chain reaction

1. Onion cells store sulfur-containing compounds and, separately, enzymes.
2. Cutting breaks the cells and mixes them. An enzyme turns the sulfur compounds into reactive acids.
3. A second enzyme, called lachrymatory-factor synthase, quickly converts those into a volatile gas: syn-propanethial-S-oxide.
4. The gas drifts up to your eyes and activates nerve endings that sense irritation. Your eyes respond by making tears to wash it away.

Tricks that help

• A sharp knife: clean cuts break fewer cells than crushing with a blunt one.
• Chill the onion for 20–30 minutes: the reaction and the gas are slower when cold.
• Cut the root end last: it has a higher concentration of the compounds.
• Ventilation: a fan or an open window carries the gas away.
• Goggles: they look silly and work perfectly.

Tearless onions

Plant breeders have developed onion varieties that produce much less of the irritating gas, and scientists have studied switching off the enzyme altogether. Some cooks miss the flavour these compounds also bring.

More kitchen science in Wacky Facts.