Everyone knows cutting an onion makes you cry. But it's not what you think! Onions don't naturally contain the tear-causing chemical, it's a chemical reaction that happens only once the onions are cut!
Yes, that is 5 year old me. And yes, I am wearing swimming goggles while chopping onions. I love to cook, but I don’t like being in pain!
At first glance, uncut Alium cepa, the scientific name for onion, is odorless and benign looking. You can put your nose right up to an onion and smell nothing. But the moment you cut open that onion, a slew of onion-y fumes mercilessly attack your eyes.
My only escape from the searing pain and hot, angry tears was to slap on a pair of blue swimming goggles before slicing. But why do these stinky bulbs make you cry once you cut into them?
The answer isn’t as clear cut as you think. An onion does not inherently contain the tear-causing chemical. Instead, two separate components in different parts of the onion cell are released when it is cut. These chemicals mix to create a natural tear-gas or, in fancy terms, lachrymatory factor.
To understand this chemical process, as well as why this tear-gas stings our eyes, we must first understand onion biology.
Onions are a common root vegetable that absorb lots of sulfur from the ground. This sulfur is used to make amino acids that contain sulfoxides. Onions also contain special enzymes that are responsible for breaking down these sulfurous components as the onion cell needs to.
Cutting an onion open breaks the onion cell walls, allowing all the contents of the cells to spill out and mix - including the sulfur-rich amino acids and the special enzymes. These two components chemically react to form unstable sulfenic acids. This acid then changes into a more stable form, the lachrymatory factor. This is a volatile substance, which means that it readily turns into a gas and floats up to our eyes.
It is actually important that onions produce this painful chemical when they are bitten into or cut. It prevents other herbivores from eating it. This safety mechanism works against humans, too. Who wants to bite into a raw onion?
This lachrymator, whether from a cut onion or a canister of tear gas, is sensed by the cornea of the eye. Some of the lachrymator factor reacts with your eye to form sulfuric acid, which causes the uncomfortable burning and itching. The nervous system quickly responds and triggers the lachrymal (tear) glands to produce tears in an attempt to wash away the irritant.
So how do we prevent onion fumes from making us cry? I’ve tried many internet tricks like holding my breath, eating bread while chopping, and soaking the onion in cold water, but I still believe the most effective prevention is an old fashioned pair of goggles.
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