If you’ve ever sliced or chopped an onion before, I’m pretty sure you’ve already experienced at some point that sudden feeling of your eyes stinging and tears running down your face.
Now you might wonder why cutting one can make you cry.
Well, let me tell you a secret. It comes down to fascinating biochemistry. But first, let’s talk about what’s inside an onion.
Each onion cell has a small sealed vacuole or the cells’ storage bubbles commonly found at the center. These vacuoles are filled with enzymes, such as amino acid sulfoxides. And when you cut an onion, these enzyme-filled vacuoles break open and release their contents.
Think of this process as the onion’s defense mechanism to protect itself from hungry animals.
Propyl sulfoxide, a sulfur compound found in onions, escapes into the air and changes into sulfuric acid, which is a strong mineral acid when it comes into contact with moisture. Then, these compounds combine to produce a chemical known as syn-propanethial-S-oxide.
Propanethial S-oxide is a tiny irritating gas that quickly becomes vapor and floats into the air after cutting the onion. It’s also a lachrymatory agent that causes a burning sensation and generates tears when it comes into contact with the eye.
Like onions, our eyes have a defense mechanism against chemical irritants to protect them from damage. When the eye nerves detect a lachrymatory agent, the brain triggers a tear response to flush out the irritant. In short, we cry when our eyes are exposed to these irritants.
As much as onions may sting, you can try some home remedies to avoid this onion-chemistry drama.
A great way is to refrigerate or freeze onions for about 30 minutes before cutting them. This slows down the enzymes and chemicals when escaping into the air.
You can also cut off and throw away the root end with stringy roots hanging because this is where the sulfuric compounds escape.
Choosing sweeter onion types, such as green onions, also help produce fewer tears because they are less pungent and have less sulfur.
However, be careful when using onions that are white, yellow, or red in color because they contain a lot of sulfur compounds.