Why does chopping onions make your eyes water?

Why is it that cutting up an onion makes us cry? You can be fine when you start and yet by the time the task is finished we’re left with tears running down our faces. What’s going on?

While onions are growing they absorb sulphur from the soil. When we cut through an onion the blade ruptures the onion’s cells, freeing the sulphuric compounds contained within them. These mix with the onion’s enzymes (protein molecules found in cells) to form a gas called propanethiol S-oxide. This gas wafts upwards and comes into contact with the moisture in our eyes, forming sulphuric acid and causing the burning, stinging sensation. Your body responds to this by producing tears to wash the irritant away. This only happens with raw onions; cooking an onion deactivates the enzymes, so the gas doesn’t form.

(Image: Jessica Spengler).
(Image: Jessica Spengler).

How can you chop onions without getting watery eyes? I’ve heard of several methods that are said to work, although I admit I’ve never attempted any of them! You might like to try them though, so here are some suggestions:

  1. Chop the onions beneath an extractor fan (switched on, obviously). This may remove the propanethiol S-oxide gas from the air around you.
  2. Wear goggles. This prevents the gas reaching your eyes at all but probably isn’t advisable if you need glasses to see what you’re cutting.
  3. Chop the onion in a bowl of water. This supposedly stops the gas escaping into the air, but it affects the onion’s taste and I suspect may prove rather tricky to do.
  4. Freeze the onion for a short time before chopping it. The idea is that lowering the onions temperature stops the enzymes reacting with the sulphuric compounds, in much the same way that cooking does.

Good luck!

NB: Samantha Gouldson does not accept liability or responsibility for any injuries incurred while chopping onions without adequate vision and/or grip. 😉

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