Ask Yan - Why do onions make us cry?

Humans are a bit funny in that we like tangy, spicy things like onions and peppers. The taste we like is actually from irritant chemicals that evolved in these plants to discourage animals from eating them. The effects are pleasant on the tongue, but irritating to other sensitive parts, like your eyes.

When you chop onions, the cells break open and release volatile sulfurous acids. The chemical that stings our eyes (propanthial-S-oxide) is produced when this acid cocktail reacts and is broken down by an enzyme. This evaporates easily so it can waft through the air up to your eyes.

The gas reacts with the water in the lubricating tears, which are always on your eyes, to form sulfuric acid, propanol, and hydrogen sulfide. These chemicals irritate the eyes, and that’s why you cry – the irritation triggers the washing type of tears to flow and clear the irritant away. Another reflex kicks in too - to rub your eyes with your hands. This will just bring more of the irritating chemical to your eyes making things worse.

To enjoy the taste without the tears, try cooling your onions in the fridge. The lower temperature slows down chemical reactions, so less of the irritating chemical evaporates. Also, using a good sharp knife limits the amount of cells opened up by slicing – as does cooking with bigger chucks of onion.

If you still despair of eating onions, dry your eyes; a group of Japanese scientists have recently discovered that the enzyme that makes the tasty chemicals is different from the one that makes the weepy chemical - meaning we might well have tear-free, yet tasty onions in future.

- Answered by Daniel Ridley-Ellis, 25 March 2010