Science
Why does a cut onion make you cry but a whole one does not?
- Cutting releases a gas that reaches your eyes
- The juice is acidic
- The smell is very strong
The answer is Cutting releases a gas that reaches your eyes. Cutting breaks cells that mix two chemicals into an irritating gas.
An intact onion is harmless. Cutting ruptures cells and lets two things meet that the plant keeps carefully apart - a sulphur compound and an enzyme - which react in about thirty seconds to make a volatile gas that drifts up to your eyes.
There, it meets the water on the surface of the eye and forms a weak sulphuric acid. Your eye responds exactly as it should to an acid, by flooding to wash it out. The defence is aimed at animals that would eat the bulb underground, and it works on us by accident.
That explains why the usual remedies do and do not work. Chilling the onion slows the reaction; a sharp knife bursts fewer cells than a blunt one crushing its way through; cutting under running water carries the gas away. Goggles work perfectly and look ridiculous. There is also a tearless onion, bred over decades in New Zealand by suppressing the enzyme, which is sold commercially and is reported to be slightly sweeter.