Why do onions make you cry?
Quick answer
When you cut an onion, you break its cells and mix chemicals that are normally kept apart. Enzymes turn sulfur compounds into a volatile gas called syn-propanethial-S-oxide. It drifts up and irritates nerves in your eyes, which respond by producing tears to wash it away. It’s a defense the onion evolved against animals.
The chain reaction
An onion keeps its ingredients in separate compartments until a knife breaks them open. Then this happens:
- Onions take up sulfur from the soil and store it as compounds called amino acid sulfoxides.
- Cutting releases an enzyme, alliinase, which converts them into sulfenic acids.
- A second enzyme, lachrymatory-factor synthase, turns these into syn-propanethial-S-oxide.
- The gas reaches your eyes within about 30 seconds.
- Nerve endings in the eye signal the tear glands to start flushing.
Tips to cry less
| Tip | Why it helps |
|---|---|
| Chill the onion for 30 minutes | Cold slows enzymes and the gas evaporates less |
| Use a sharp knife | Crushes fewer cells than a blunt one |
| Cut the root end last | It has the highest concentration of the compounds |
| Work near a fan or extractor hood | Blows the gas away from your face |
| Wear goggles | Physically blocks the gas |
Popular tricks like holding bread in your mouth have little evidence behind them.
A tearless onion
Scientists in Japan identified the tear-causing enzyme in 2002. Since then, breeders have developed tearless onion varieties, such as the Sunions sold in the US, which have lower levels of the irritating compounds and grow milder in storage.
Cooking removes the sting
Heat destroys the enzymes, which is why cooked onions don’t make you cry and taste much sweeter. Slow-cooked onions brown and caramelize through the same reactions described in why toast turns brown.
Relatives that do it too
Shallots, leeks, chives and garlic belong to the same plant family, Allium. Garlic releases a different compound, allicin, which gives it its sharp smell but doesn’t cause tears.
For a fermented dish full of onion relatives, see what kimchi is.
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