I’m the kind of person who cries at the slightest provocation; films, books, blog posts, emotional songs, TV adverts about cancer and donkey sanctuaries, kids cartoons… you name it, I have probably cried at it. You only need to say the words Goodnight Mister Tom to me in the street, and I’ll be off. So I’m experienced with tears, I’ve got them down. But onion tears? Onion tears don’t make sense to me. Onions don’t make me sad, or happy, or angry, or any of the other emotions that provoke tears, so why am I crying? And also, why does it HURT? Normal tears don’t hurt; I should know, I’ve cried enough of them. In the past I’ve been given a wide variety of answers to this question in the past, and I’ve been advised of a whole host of ways to stop it happening, including, strangely, having a teaspoon in your mouth while you chop (which, unsurprisingly, does not work, and yes, I did try it, just to see). However, now I have found out the real answer to what those pesky onions are up to.
During their growth, onions absorb sulphur from the earth, and this sulphur combines with amino acids in onion cells to form amino acid sulfoxides. These molecules, once produced, are harmlessly contained within the cells of an unchopped onion, their tear-inducing properties not yet apparent.
Meanwhile, elsewhere in the onion cell, there lurks an enzyme called lachrymatory-factor synthase (LFS). This enzyme specifically converts the previously harmless amino acid sulfoxides into sulfenic acid. Sulfenic acid itself is highly unstable, and can easily have a mini molecular identity crisis and rearrange itself chemically to become a devilish little molecule known as syn-propanethial-S-oxide.
Most of the time, this isn’t an issue because LFS doesn’t come into contact with amino acid sulfoxides. There is some serious segregation going on inside onion cells, and LFS is kept under lock and key. When you chop an onion, however, you damage the cells; you break them apart and release the contents of individual cell compartments to mingle freely with one another. The crucial result of this molecular get together is that LFS finds the amino acid sulfoxides and converts them to sulfenic acid. Sulfenic acid in turn wobbles about in an unstable manner, wailing that it just doesn’t know who it IS anymore, before rearranging itself into syn-propanethial-S-oxide. And this molecule is released into the air, where it can float upwards towards your unsuspecting eyes.
As soon as syn-propanethial-S-oxide reaches your cornea, it is detected by the sensory nerve endings of the ciliary nerve. The ciliary nerve is a branch of the much larger trigeminal nerve, which is responsible for transmitting sensations of touch, temperature, and pain from your face to your brain. Once the presence of syn-propanethial-S-oxide has been detected by the ciliary nerve, a message shoots to the central nervous system, which interprets the stimulation as a sharp, painful burning sensation. This triggers a reflex pathway, sending another message back to the eye, specifically to the lachrymal glands, telling them to produce water to wash away the irritant in your eye. And hey presto, suddenly you’re crying.
Interestingly, wobbly and unstable sulfenic acids can also form another type of compound, thiosulfinates, which give raw onions their distinctive smell. Since thiosulfinates are produced by sulfenic acids when an onion is chopped, there has been a persistent myth that it is the smell of onions that causes you to cry. This is not at all true, although the true culprit does come from the same starting molecule.
So, that’s why chopping onions make you cry. Once you start to cook the onion, the heat you apply causes the bonds that hold proteins together to break. This effectively destroys the structure of proteins, meaning that enzymes can no longer do their jobs. As all the LFS is rendered useless, syn-propanethial-S-oxide stops being produced, and the crying stops. I find this fascinating, this interplay between molecules and the effects it has, and it’s left me pondering whether there’s a reason onions make you cry when you cut them… is it a defence mechanism, because they don’t wish to be chopped into my dinner?! Enquiring minds must, and will, try to find out…
Further questions aside though, this is pretty useful knowledge, because once you know what causes onion tears, it’s a lot easier to think about stopping them. When temperatures drop, reactions take place more slowly because all the molecules have less energy for running around and doing stuff. So, chilling onions in the fridge before chopping them reduces the amount of syn-propanethial-S-oxide that can be produced. Freezing the onions works even better, in fact it completely disables the enzyme, although it might make chopping a bit more challenging. Alternatively, of course, if you forget to freeze your onions, you can always chop them wearing goggles so that the chemicals can’t get to your eye. You may look ridiculous if you take this course of action, but your eyes will be blissfully pain-free. It’s your call.