So as everyone knows, chopping onions makes you cry. I’ve seen people chop onions wearing swimming goggles to try and prevent tears and, interestingly, this can actually work. But not necessarily for the reason you think. Allow me to explain…
Onions, like most plants in the family Alliaceae produce a compound called allicin. It’s the compound responsible for the characteristic pungent smell of raw onions. This chemical is also the plant’s primary defense mechanism. It’s antibacterial, antifungal and repels pests. The thing is though, allicin is only formed in large quantities when onions are damaged — by a chef’s knife, for instance.
When you cut an onion, enzymes rapidly generate allicin. After creating allicin, more enzymes break it down into sulfenic acids. These sulfenic acids are extremely volatile, so they get carried away as vapour. They’re also quite unstable, and will rapidly convert to sulfuric acid on contact with water.
Onions make you cry because while chopping them, you inhale this onion vapour. When sulfenic acids come into contact with your tear ducts, they react and turn into sulfuric acid. It’s this weak sulfuric acid solution in your tear ducts that causes the tears and stinging sensation while chopping onions.
So just covering your eyes isn’t actually enough. Wearing goggles can work, because they can also pinch your nose and close off your tear ducts. A much easier solution though, is to simply take a step back while chopping onions, breathe through your mouth and never ever lean over the chopping board while you’re chopping.