Why Do Onions Make Us Cry?

By far the biggest drama queens of the fruit and vegetable world are the alliums: the 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 your cupboard – but as soon as you cut into them, out come the waterworks.

Of course, it’s not the veggies themselves that are crying – it’s us. Somehow, onions and their relatives are able to exact revenge upon us, their overlords, by cursing us with stinging eyes, running noses, and nigh-unstoppable tears. But how do they do it?

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 that our bodies make: basal, emotional, and reflex – and we can rule one of those out pretty quickly, since unless you’re empathic to a frankly absurd degree, you probably aren’t crying out of grief at having to mutilate some poor defenseless vegetable.

Neither are onions responsible for basal tears, which you are in fact shedding right now. “These are your basic tears. Your eyes roll around in them all day,” explains Cleveland Clinic. “They contain oil, mucus, water and salt, and help fight infection.”

And thus, by process of elimination, we have our answer: onion-chopping prompts us to start sobbing reflex tears: “your eyewash tears,” per Cleveland Clinic. 

“The glands under your eyebrows push them out when you peel an onion, vomit or get dust in your eye. They flush out the material that’s irritating your eyes,” they write. “These are the kind of tears that stream down your face when your allergies are kicking into high gear.”

But what is it that makes the vegetables so irritating? It’s actually a pretty neat (if evidently unsuccessful) defense strategy on the onions’ part – and the science behind it is so intricate that it took us until the 21st century to figure out how it worked.

So why onions?

Onions may not be able to feel pain, but that doesn’t mean they’re happy to get eaten: they’re actually the bulb of a perennial plant, so their goal 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 hardscrabble world” for these veggies; “a world where there are lots of worms in the ground and animals that would devour something that exists as a bulb and has to survive in the ground.”

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

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

This amino acid starts reacting with water and enzymes that have been released from the broken cells of the onion, and that produces – well, a bunch of new chemicals which we’re not interested in right now, to be honest, but also something called 1-propenyl sulfenic acid. That, in turn, decomposes into a gas called propanethial S-oxide – and it’s this which is the culprit for our crying.

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

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

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

How to chop onions without the tears

Well, that’s all very interesting, you might be saying, but it doesn’t help me prepare this lasagna. Is there any way to get around all this chemistry, and chop onions without tearing up?

In fact, there are – and again, it all comes down to the science. “You have to consider the chemistry of what is involved before you can come up with a remedy,” Block said. “So, the molecule that causes tearing is a very small molecule. It’s very soluble in water, and being a small molecule, it’s relatively easy for it to go into the gas stage.”

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

Some more inventive solutions that you may have heard – chopping them with a match between your teeth, for example, or using a piece of bread – those are pretty much useless, he added. 

But if you’re really really suffering, then there may be hope on the horizon. You see, for a long time, scientists thought the culprit for our crying was an enzyme called alliinase, which is one of the many chemicals responsible for the onion’s characteristic flavor. That hypothesis was proven wrong, however, in 2002, when a team of researchers from Japan discovered a previously unknown substance in the vegetable which they named lachrymatory factor synthase, or LFS.

Why does that matter? Well, as the team suggested in their paper, “it may be possible to develop a non-lachrymatory onion that still retains its characteristic flavor and high nutritional value by downregulating the activity of this synthase enzyme.”

In other words, we may one day be able to grow onions that don’t make us weep every time we cut into them. Until then, however – well, we guess we’ll be stopping by the gas mask shop on our way to the produce aisle today.

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