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Asked by IzzyClifton to Sarah, Sankar, Jo, Becky, Alan on 13 Mar 2015.
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Jo Sadler answered on 13 Mar 2015:
When you cut an onion, you break apart some of the cells. This means that parts of the cell can mix that couldn’t mix before. This allows enzymes to react with amino acid sulfoxides to make a chemical called propanethiol s-oxide. this chemical wafts up to your eyes and reacts with the water in your tears to make sulfuric acid. The sulfuric acid burns your eyes, so your eyes make more tears to try and get rid of if, so it feels like you’re crying!
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Becky Gregory answered on 13 Mar 2015:
As Jo has said, it is because of the sulfur compound that is produced by two enzymes mixing when you break open the onion’s cells upon chopping. The sulfur compound reacts with the water in your eyes, making you cry.
However, when you then start to cook the onions, the heat deactivates the enzymes, so the sulfur compound is no longer produced and you are finally able to stop crying!
In theory, putting your onions in the fridge should be able to slow the reaction down, preventing you from crying. However I’m not sure this works that well, as I keep my onions in the fridge, and they still make me cry! Personally I like to stick my tongue out when I chop onions – it seems to work. It absorbs the compound before it reaches your eyes, due to the water on your tongue.
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Sarah Kirk answered on 16 Mar 2015:
In short, the onion is gassing you! The gas reaches your eyes and makes acid, which stings! So as Becky said, sticking your tongue out (or putting a wet cloth in your mouth) will absorb this before it reaches your eyes.
I find that some onions make me cry and some don’t, I guess some of them aren’t so good at producing the gas!
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