Curiosities: Why do onions make us cry when chopping them?

By Eddy Montilla

Some people have different reasons to cry when they chop onions: Perhaps they don’t want to cook or wash the dishes. Perhaps they had a quarrel with somebody or don’t want to go to work tomorrow. However, from the scientific point of view, the reasons are quite different. Shinsuke Imai, a Japanese scientist, found the answer to this mystery and shared it to the world in 2002. But chemical terms are not easy to understand, and the situation is more complicated when many years has passed since your chemistry classes (the last sentence pictures me!). So, explained in its most simple and fundamental form, we cry when chopping onions to protect our eyes and help relieve the pain produced by sulfuric acid that enters into the eyes by diluting it with tears.

For a more specific answer, we should review the following terms first: Enzyme is a substance produced by a living organism that acts as a catalyst to bring about a specific biochemical reaction without itself being destroyed or altered upon completion of the reaction. Amino acid is any of the substances that combine to form the basic structure of proteins. And now, let’s see the process:

When you cut an onion, you break its cells, which will release chemical substances into the air. Among these substances, there is an enzyme called lachrymatory-factor synthase. This enzyme reacts with amino acids of the onion and coverts the amino acids into sulfenic acid. The sulfenic acid rearranges itself into (be ready for this name!)… syn-propanethial-S-oxide, volatile sulfur compound (gas) that reacts with the water inside your eyes and forms sulfuric acid. This sulphuric acid causes the burning sensation in your eyes and this is the moment when the lachrymal glands located there will release tears to clean your eyes, dilute the acid and relieve the pain.

And how can you avoid crying when chopping onions? If you are not an excellent cook, I do not recommend you to close your eyes to avoid tears when chopping onions, especially if you are using a sharp knife! Refrigerate the onion before cutting instead.

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