Why onions make you cry — and the three kitchen tricks that actually turn the chemistry off
The tears are not sentimental. They are a tiny, very fast chemistry experiment happening in your eye, and three of your kitchen habits can shut it down before it starts.
In short
What happened. When you cut an onion, you set off a fast chain of chemistry that ends with a tiny amount of a volatile gas drifting into your eyes.
What it means. The crying is not sensitivity. It is a measurable chemical reaction, and understanding it lets you stop it.
Risks and impact. None. This is a kitchen inconvenience, not a health problem. The gas is harmless in the amounts a cook ever meets.
What can be done. Three habits — a chilled onion, a very sharp knife, and moving air across the board — remove most of the discomfort. The folk tricks (bread in your mouth, a match between your teeth) do not.
What to watch. Whether the kind of onion you buy matters. The science says yes, and newer low-tear varieties are beginning to appear in supermarkets.
Shown as a summary because of your reading settings.
What happened
Cutting an onion makes you cry because of a four-step chemistry sequence that was only fully understood in 2002.
Inside an intact onion cell, a compound called isoalliin sits in one compartment, and an enzyme called allinase sits in another. They do not meet. When you cut the onion, you break the cell walls and the two mix. Allinase converts isoalliin into an unstable intermediate, 1-propenesulfenic acid. A second enzyme — lachrymatory-factor synthase, usually shortened to LFS — then converts that intermediate into a tiny, highly volatile molecule called syn-propanethial-S-oxide.
That molecule drifts into the air. If it reaches your eye, it dissolves in the tear film and the surface of the cornea. There, it is converted into small amounts of sulphuric acid. The nerve endings on the cornea notice. The brain tells the tear glands to flood the eye to wash the irritant away.
The LFS enzyme was identified by a Japanese team led by Shinsuke Imai and reported in the journal Nature on 17 October 2002. Before that paper, allinase had been assumed to do the whole job, which is why early no-tear-onion breeding experiments did not work. Once scientists knew about LFS, they knew what to switch off.
What the evidence supports
The best-supported parts of the story are the ones with published chemistry.
The two-step enzyme pathway, the identity of syn-propanethial-S-oxide as the tear-causing agent, and the role of the lachrymatory-factor synthase enzyme are all documented in peer-reviewed journals, with Nature’s 2002 letter the most-cited reference. The Royal Society of Chemistry’s Chemistry World magazine has written up the same mechanism for a general readership, as has Encyclopaedia Britannica’s standing entry on the onion plant.
What is less firmly established is the comparative effectiveness of the various kitchen tricks. Here, the evidence is a mix of chemistry (which tells you what should work in principle) and informal cook’s experience (which tells you what people find helps). The three habits in the practical section below are all consistent with the chemistry; the folk remedies, including putting a slice of bread in your mouth or holding a match between your teeth, are not. No controlled study has found them to help, and there is no plausible mechanism by which they should.
What we do not know with confidence is the precise amount of LFS and syn-propanethial-S-oxide produced by different onion cultivars under different storage conditions. The chemistry is clear; the agriculture is still being worked out.
How the story is being framed
The engineering view. This is a problem to be solved. Cool the onion to slow the gas. Sharpen the knife to break fewer cells. Move the air. Each step attacks a different point on the pathway. This is the frame that cooks and food scientists find most useful.
The genetic view. If LFS is the problem, breed it out. A no-tears onion was announced by researchers in New Zealand and Japan in the 2000s, grown as “Sunions” in parts of the United States since 2017, and is now stocked by several European supermarkets under different names. The onion still tastes like an onion but does not kick off the LFS reaction.
The folk view. Every kitchen has a trick: a chilled onion (works), a wet knife (works a bit, by trapping the gas in water droplets), a candle on the board (works a little, if it moves air), a slice of bread in the mouth (does not work). It is useful to notice that the folk tricks which work are the ones that happen to disrupt the chemistry; the ones that do not work are the ones that assume the problem is in your mouth or breath, which it is not.
The nostalgic view. Some cooks like a bit of mild crying. It feels like real cooking. There is nothing wrong with this view, provided it is held knowingly. The point of understanding the chemistry is not to abolish feeling.
The background
An onion is a defensive plant. The LFS pathway almost certainly evolved to deter animals that dig for the bulb; the gas is strongly irritating to the eyes and nose of a mammal, which is why it works on humans. Garlic, which is a close relative, does not make you cry — it uses a different enzyme pathway that produces allicin, the smelly compound, but has no LFS. This is why garlic does not sting your eyes, but does leave you smelling of it. Chives and leeks have small amounts of the same tear chemistry; they rarely cause problems because they are rarely cut in large quantities.
The chemistry also explains three practical details.
First, a sharp knife matters because it cuts cells cleanly instead of crushing them. Fewer broken cells means less contact between allinase and isoalliin, which means less syn-propanethial-S-oxide released.
Second, chilling the onion for twenty minutes in the fridge slows the enzymes (they move more slowly at lower temperatures) and lowers the vapour pressure of the gas (so less of it leaves the surface and reaches your eyes). An hour in the freezer works even better but risks changing the texture.
Third, moving air helps. A kitchen fan, an open window, or chopping near the extractor hood over your stove all pull the gas away from your face before it reaches your eyes. Goggles work for the determined; most cooks do not bother.
What does not work, despite being endlessly repeated: bread in the mouth, matches between the teeth, chewing gum. All of these assume the gas comes from your mouth. It does not — it comes from the cutting board. Rinsing the onion in water after cutting does help a little, because syn-propanethial-S-oxide is water-soluble, but by then the gas is already in the air.
The deeper story
There is a quiet puzzle under this. For as long as people have cooked, they have known that a chilled onion and a sharp knife reduce the crying. They did not know why. The chemistry of LFS is twenty-three years old; the kitchen trick is older than any country on the map.
How is it that cooks were correct about a mechanism they could not describe? The usual answer — “trial and error” — is true but incomplete. It is more precise to say that the kitchen rewards accurate description of what happens without ever requiring an explanation of why. A cook who notices that a cold onion stings less, and tells their apprentice, has passed on a true fact in a form that is useful even though no one at any stage knows about enzymes.
Science then arrives, late and useful. It does two things that the kitchen could not do by itself. It separates the folk remedies that happen to work from the ones that happen to be rituals. And it explains the mechanism well enough to allow the next step — in this case, breeding an onion that keeps the taste but drops the LFS. Neither of these would be possible from inside the kitchen tradition alone.
The point is not that science is better than cooking, or the other way round. It is that the two kinds of knowing need each other more than they usually admit. Someone has to notice; someone else has to explain. Dinner is better when both have had a go.
Something to sit with
- What else in your kitchen do you do correctly without being able to say why?
- When a scientific explanation arrives for something you already knew, does it feel like a gain, or like something slightly spoiled?
Sources
- Nature — https://www.nature.com/articles/419685a
- Encyclopaedia Britannica — https://www.britannica.com/plant/onion
- Royal Society of Chemistry — Chemistry World — https://www.chemistryworld.com/features/the-chemistry-of-the-crying...
We report facts from the sources above in our own words and link to the originals. Interpretation is ours, not theirs.
Which enzyme, discovered by Japanese researchers in 2002, is the key reason chopping onions makes you cry?
Allinase is the first enzyme in the chain, but it is the LFS enzyme — identified by Imai and colleagues in Nature in 2002 — that converts the intermediate into the tear-making gas syn-propanethial-S-oxide.
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