Decoded

Why onions make you cry

You're cutting onions wrong. Your eyes are being gassed.

1
Two chemicals, kept apart.
Inside every onion cell, sulfur compounds and the enzyme that attacks them are stored in separate compartments. Separate rooms.
2
Your knife breaks the walls.
They mix. A second enzyme, lachrymatory-factor synthase (LFS), turns the product into a gas: syn-propanethial-S-oxide, the onion's tear gas.
3
The gas finds your eyes.
It irritates the nerve endings in your cornea. Your eyes think something is burning and flood with tears to wash it out.

2002

The tear-gas enzyme was found this recently.

Imai et al. described it in Nature. In 2013 the team won an Ig Nobel Prize for it.

How to win

Chill it first
20–30 minutes in the fridge. Cold slows the reaction, so less gas escapes.
Use a sharp knife
A clean cut breaks fewer cells than a blunt one crushing them.
Cut the root last
The root end is especially rich in the compounds. Leave it.
Bonus: airflow
A fan or a cooker hood blows the gas away from your face.

Next time, the onion loses.

Sources

Imai S. et al., “An onion enzyme that makes the eyes water,” Nature 419, 685 (2002)
Ig Nobel Prize in Chemistry, 2013

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