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When you cut them you damage a bunch of their cells, which mixes chemicals that are not supposed to be mixed. The mix causes a chemical reaction that creates a chemical (syn-Propanethial-S-oxide) which irritates our eyes in a way similar to, but much less intense than, tear gas.
I am not sure if this is an effect that evolution produced in onions as an adaptation or if it is just a pure coincidence though.
Pest repellent/deterrent is my guess. Bulbs with higher concentrations if sulphur would likely be less appealing
Maybe, but the specific chemical may not effect pests, as the way they interact with it, and how it effects us, are completely different.
The chemical in question does not exist in the plant until it is cut, where it is a multi-stage chemical reaction. So it is very possible to be a pest repellent, even if it is a complicated strategy, but our reaction to it might not indicate that.
Again though, I do not know. Pure speculation. I am just pointing out why I was uncomfortable assuming that it helped repel pests. It probably does, after thinking about it I think it would be difficult for it not to have an evolutionary pressure.
not supposed to be mixed
Weirdly moralising way to phrase it!
Well, in my head it was more anthropormophizing. If my blood and my stomach fluids were suddenly mixing because of a knife, I would probably phrase it the same way.
It was just a lighthearted turn of phrase though, as it is a vegetable.
Sweet. My five years of studying for my PhD in garlic and onion chemistry getting put to good use.
Onions contain an amino acid called cysteine sulfoxide. This amino acid lives in a compartment separate from the rest of the cell tissues of the onion. When onions are cut or sliced, the cysteine sulfoxide amino acid is mixed with an enzyme that lives in another compartment. This enzyme is called alliinase, and it is found in both garlic and onion.
The alliinase enzyme cleaves apart cysteine sulfoxide and forms a molecule called a sulfenic acid. Two of these molecules come together and spontaneous form what is called a thiosulfinate. The main thiosulfinate in garlic is known as allicin, and is responsible for its flavor.
Onion sulfenic acids would normally form a thiosulfinate, but because of a slight quirk in their molecular structure, there’s actually a second enzyme that converts the sulfenic acid into propanethial S-oxide. And because scientists are super creative, they call this compound the lachrymatory factor, and the enzyme is called lachrymatory factor synthase (LFS). It does it very fast, within a few seconds most of the sulfenic acids are converted. This is the compound released by onion that causes your to cry.
Essentially, the compound spontaneously breaks down into a very dilute sulfuric acid in your eyeball. It’s not enough to cause any damage, but it hurts like hell. It is believed to be a defense measure, as these compounds are irritating to most mammals, and thiosulfinates and other sulfur compounds also possess antimicrobial and antifungal properties.
They’re quite good at warding off pests as well.
I also love those moments where my niche(ish) PhD topic becomes relevant in a Reddit post...
Makes the pain of chopping all those onions worth it.
Most my PhD did for my eyes was help me realize my eyes see certain colors differently from looking through a microscope lol
Any advice to lessen the effects?
Very sharp knife - cuts more effectively and causes less cellular damage. Less cellular damage > less release and mixing of the two chemicals. A blunt knife rips and tears, which causes more damage so it'll be worse.
But honestly, you kinda just have to deal. Even the sharpest knife will still release a lot of those compounds so it's happening regardless.
The chemical in onions that makes you cry is syn-propanethial S-oxide. When an onion is cut, it releases this volatile compound, which then reacts with the water in your eyes, creating a sulfuric acid-like substance that irritates the nerves and triggers tear production.
They contain sulphur based compounds. When you break the cells of an onion, these are released into the air. When some of them reach your eyes, the liquid on your eyeballs basically becomes dilute sulphuric acid, and that hurts.
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One tip is to sharpen your knife, because a blunt knife will crush more cells while cutting, releasing more of these sulphur compounds.
You of course still release some when using a sharp knife, but not quite as many.
The onion is doing chemical warfare on you. No, seriously.
Many living things have mechanisms to defend themselves against anything that wants to eat them, and as plants don't move, they tend to use chemicals that are irritating or poisonous.
In the case of onions, garlic, leeks, and basically all the plants on the Allium genus, they have some chemicals with sulphur in their cells. When something pierces those cells, those chemicals mix and release into the air, spitting out an irritant gas that hopes it deterrs anything that is trying to cut the plant.
That is why if you grow onions in soil without sulphur, like the ones grown in the Vidalia valley in the USA, those onions don't sting that much and even taste a bit sweeter.
When you cut an onion it causes an enzyme inside to react to air and produce an irritating gas. It's thought to have been an evolutionary defense but humans don't care for those (see peppers)
Edit: chill them or cut under/inside water to help with the eye watering.
The real question is why don’t people tear up when they cut onions? Was it my exposure to CS gas that gave me an immunity to it?
If you were exposed to it a lot, maybe. You can build a tolerance.
Otherwise, people's sensitivity to it is genetic. Usually with CS Gas that is just going to lower its overall effect, but not eliminate it. With onions though, which are much less strong but work similarly, it might just not be noticable.
Interesting. I’ve actually always wondered this, as I’ve never had any adverse effects to super heavy aromatics since my military service.
basically it release a gas that mix with your eyeball surface fluid to form sulfuric acid. Your eye is being melting away ... slowly. But if you have a sharp knife or cut onion under a vent where the gas is being expunge before it reach your eye. Then it's fine. Btw weawr google is not enough, it can come through your nostril as well.
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