You set out to make a simple pot of soup. A few slices into the onion, your eyes start to sting, the tears roll, and suddenly you look like you're watching the end of a tearjerker. What on earth is that onion doing to you?
The short answer is that cutting an onion sets off a chemical chain reaction. Breaking the onion's cells releases enzymes that end up producing a volatile chemical called syn-propanethial-S-oxide. It drifts through the air into your eyes, irritates nerves there, and your tear glands kick in to wash it away.
A Chain Reaction in Every Slice
An intact onion is fairly calm. The trouble starts when a knife damages its cells. That releases enzymes called alliinases, which break down sulfur-containing compounds in the onion and create substances known as sulfenic acids.
Then a second enzyme steps in. It's called lachrymatory factor synthase, from the Latin word for tears. It quickly turns one of those sulfenic acids into syn-propanethial-S-oxide, the actual tear-maker. Because it evaporates easily, it rises from the cutting board as a fine vapor.
Why Your Eyes Fight Back
When that vapor reaches your eyes, it activates sensory nerves, which is why you feel the sting. Your lacrimal glands, the tear factories near your eyes, respond by producing tears to dilute the irritant and flush it out.
In other words, those aren't sad tears. They're cleanup tears. Your body is simply rinsing away something it doesn't want in your eyes. The whole chain of reactions serves as a defense mechanism for the onion. That's why it only gets going once the onion's cells are damaged, whether by a knife or by something trying to take a bite.
Tricks That Actually Help
There are a couple of easy ways to cut down on the tears. One is to slice onions under running water or while they're submerged in a bowl of water, which keeps more of the irritating vapor from reaching your face.
Another is to leave the root end intact until last. The base of the onion has a higher concentration of sulfur compounds than the rest of the bulb, so keeping it whole means less of that tear-making chemistry gets started.
Not every onion relative hits you equally hard, either. The amount of irritant released differs among different species in the onion family. In 2008, researchers in New Zealand even created a no-tears onion through genetic modification, by blocking the enzyme that makes the tear-causing chemical.
Onions and Tears Through History
People have been crying over onions for a very long time. The onion is known only as a cultivated plant, and its closest wild relatives grow in Central Asia and Iran. Ancient Egyptians revered onion bulbs, seeing their round shape and rings within rings as symbols of eternal life.
Even ancient poets seemed to know about the tears. In Homer's Odyssey, a shining tunic is compared to the skin of a dried onion. One scholar has suggested the onion was chosen partly because of its reputation for producing tears, a quiet hint at Penelope's sorrow.
So the next time you're blinking through a pile of chopped onions, don't take it personally. The onion is just doing what it evolved to do, and your eyes are doing their job too. A little running water, an intact root end, and a sense of humor go a long way.
