A chemical in onions called syn-propanethial-S-oxide is why you can’t chop an onion without tearing up.
How Does It Work?
Sometimes a strange but simple fact—that onions make you cry, for example—is so accepted that you forget to ever stop and wonder…why? That is, until you’re chopping with a 3-year-old who gets mad at you because you made her weep for no reason. For example.
So what’s going on with onions that make you cry, or make you shell out $20 for a pair of onion goggles? As Eric Block, author of the book Garlic and Other Alliums (onions are part of the plant genus allium) explained to NPR, “the onion is a perennial bulb that lives in the ground with lots of critters who are looking for a snack.” And so they have a chemical defense system, which, like a skunk fending off predators with a stench, releases the tear-jerker chemical to protect itself from being eaten. Within each cell of an onion is a sealed vacuole filled with enzymes that rupture when cut into. The enzymes then mix with other chemicals in the onion cell and, in Blocks words, “a whole cascade of chemical processes happen within an instant.”
The result of all those chemical processes are syn-propanethial-S-oxide molecules, the irritating culprit making you (or your 3-year-old) cry. “[It] really is quite beautiful from a scientific viewpoint,” Block said.
So, What Can I Do?
You’ve probably heard plenty of tips: Hold a silver spoon in your mouth, hold a piece of bread in your mouth, cut the onion underwater, use a super sharp knife… But Block’s tip is simple, and makes plenty of sense once you understand the onion science. “If you just cut the onion in a stream of air blowing away from your face,” he says, “then you’ll pull the molecules away and they won’t get to your eyes.”