Why do onions make us cry?

By far the biggest drama queens of the fruit and vegetable world are the alliums: onions, garlic, leeks, shallots and various bulbous cousins. Oh, they may look calm and innocent when they’re sitting pretty in the grocery store or in your closet, but as soon as you cut into them, the waterworks come out.

Of course, it’s not the vegetables themselves that are crying; that’s it us. Somehow, onions and their kin are able to exact revenge on us, their overlords, by cursing us with stinging eyes, running noses, and virtually unstoppable tears. But how do they do that?

The answer is more interesting than you might think.

The science of sobbing

So, what’s behind the tears when we chop an onion? As you may know, there are three types of tears our bodies make: basic, emotional and reflexive – and we can rule one of those out pretty quickly, because unless you’re empathetic to a frankly absurd degree, you probably are. I don’t cry with sorrow because I have to mutilate some poor, defenseless vegetable.

Onions are also not responsible for basal tears, which you are actually shedding straight away. “These are your basic tears. It makes your eyes roll around all day,” Cleveland Clinic explains. “They contain oil, mucus, water and salt and help fight infections.”

And so, through elimination, we have our answer: Chopping onions causes us to reflexively sob tears: “your eyewater tears,” according to Cleveland Clinic.

“The glands under your eyebrows push them out when you peel an onion, throw up or get dust in your eyes. They flush away the material that irritates your eyes,” they write. “These are the kind of tears that stream down your face when your allergies kick into high gear.”

But what makes the vegetables so annoying? It’s actually a pretty neat (though clearly unsuccessful) onion side defense strategy – and the science behind it is so complicated that it took us until the 21st century to figure out how it worked.

So why onions?

Onions may not feel pain, but that doesn’t mean they are Cheerful to be eaten: they are actually the bulb of a perennial plant, so their purpose is to survive underground for as long as possible.

It is, chemist and author of Garlic and other alliums: the lore and the science Eric Block told NPR in 2010, “a very tough world” for these vegetables; “a world where there are many worms in the ground and animals that would devour something that exists as a sphere and has to survive in the ground.”

“If you live in the ground as a perennial plant […] you have to defend yourself, and you can’t run,” he explained. “Plants can’t run. So they stay and fight, and they’re great at it.

Indeed, it turns out that the backup plan for ‘running from a worm’ is all-out chemical warfare. Cutting an onion – or crushing it, or just chewing it whole like an apple if that’s your jam – triggers a cascade of reactions, all of which start with the release of a certain amino acid called S-1-propenyl- L-cysteine ​​sulfoxide (on a side note: chemists, please come up with some snappier names for the things you discover).

This amino acid starts to react with water and enzymes released from the broken cells of the onion, producing – well, a lot of new chemicals that we’re not interested in right now, to be honest, but Also something called 1-propenylsulfenic acid. That in turn breaks down into a gas called propanethial S-oxide – and it is this that is the culprit of our crying.

Why? Because propanethial S-oxide – chemical formula C3H6SO – reacts with water to form H2SO4, also called sulfuric acid. And guess where there’s a lot of water? That’s right: in those basic tears that constantly cover our eyeballs.

Fortunately (or unluckily, depending on how you look at it) our corneas are incredibly good at detecting these kinds of things – they actually have about 400 times as many pain receptors per square millimeter as the skin – so this acidic intruder immediately triggers our reflex tears to start flowing.

“[It’s] part of what’s so fun about studying the alliums is that the chemistry is absolutely fascinating,” Block said. “Everything is, I think, very Darwinian […] They are not there for our pleasure. They are there to help the plant survive.”

How to chop onions without tears

Well, that’s all very interesting, you might say, but it doesn’t help me prepare this lasagna. Is there a way to bypass all this chemistry and chop onions without tearing them?

In fact, there are – and again, it all comes down to science. “You have to consider the chemistry of what’s involved before you can come up with a solution,” Block said. “So the molecule that causes cracks is a very small molecule. It is very soluble in water and because it is a small molecule, it can enter the gas phase relatively easily.”

“So what you do is cool it before you cut it, which reduces its volatility, use an extractor hood to pull out the fumes, or you chop it under water,” he advised. “Or [do] anything else that takes a water-soluble small molecule out of the air.”

Some more inventive solutions you may have heard of — chopping them up with a match between your teeth, for example, or using a piece of bread — are virtually useless, he added.

But if you really are Real suffer, then perhaps there is hope on the horizon. Scientists long thought that the culprit of our crying was an enzyme called alliinase, one of the many chemicals responsible for the onion’s characteristic taste. However, that hypothesis was proven wrong in 2002, when a team of researchers from Japan discovered a previously unknown substance in the vegetable they named tear factor synthaseor LFS.

Why is that important? Well, as the team suggested in their paper: “It might be possible to develop a non-teardrop onion that still retains its characteristic taste and high nutritional value by downregulating the activity of this synthase enzyme.”

In other words, maybe one day we can grow onions that don’t make us cry every time we cut them. But until then, we’ll stop by the gas mask shop on our way to the production aisle today.

Leave a Reply

Your email address will not be published. Required fields are marked *