Science

Why onions make you cry

An onion builds its irritant only when you cut it. Knowing the reaction tells you which of the usual kitchen remedies actually work.

By Updated 5 min read
Chopped onion on a wooden board beside a kitchen knife.
Photo: HungryHuy, via Wikimedia Commons (CC BY 2.0).

Key takeaways

  • The irritant is created when cutting mixes enzymes with their substrates.
  • Syn-propanethial-S-oxide is volatile and irritates the surface of the eye.
  • Tearing is a protective reflex rather than an injury.
  • Chilling the onion, using a sharp knife and moving the air all reduce the effect.

Cutting an onion makes you cry, and the reason is that the onion is defending itself. Not passively, the way a thorn does, but with a chemical reaction that only assembles the moment the plant is damaged. The whole sequence takes a few seconds, and understanding it tells you which kitchen remedies are worth bothering with and which are folklore.

The weapon is built on demand

An intact onion is harmless. You can hold a whole one to your face indefinitely. What it contains is two things kept carefully apart: a set of sulphur compounds called amino acid sulphoxides, stored in the cell fluid, and an enzyme called alliinase, held separately in the cell structures.

Cutting ruptures the cells and lets them mix. The alliinase acts on the sulphoxides, and through an intermediate step involving a second enzyme, lachrymatory-factor synthase, the reaction produces syn-propanethial-S-oxide. That compound is volatile, so it rises straight off the cut surface towards your face.

The reaction that only assembles when an onion is cutIn an intact onion, sulphur compounds called amino acid sulphoxides sit in the cell fluid while the enzyme alliinase is held separately in the cell structures, so nothing happens. Cutting ruptures the cells and lets the two mix. Alliinase acts on the sulphoxides, and a second enzyme, lachrymatory-factor synthase, produces syn-propanethial-S-oxide. That compound is volatile and rises to the eye, where it dissolves in the tear film and forms a small amount of sulphuric acid, which the cornea registers immediately and the lacrimal glands flush away with tears.A weapon that is built on demandINTACT ONIONsulphoxides, in cell fluidalliinase, held separatelykept apart: harmlesscutCELLS RUPTUREDthe two mix; alliinase actsthen lachrymatory-factorsynthase completes itsyn-propanethial-S-oxidevolatileTHE EYEdissolves in thetear film, formingsulphuric acidcornea reacts; tearsWhere each remedy acts — and why some cannotFewer cells rupturedA sharp knife slices ratherthan crushes. Leave the rootend until last.Less gas reaching youChill the onion. Use anextractor or open window.Cut under water. Goggles.Acts nowhere on the pathBread in the mouth. Breathingthrough the mouth. A candle.Contact lenses.The gas travels through the air to the eye, so anything acting on the mouth cannot interrupt it.
Laying the reaction out as a path makes the remedies sortable: a method only works if it removes cells from the cut, keeps the volatile compound away from your face, or blocks the eye. Nothing that acts on the mouth touches any step. Diagram by HRITU.

This is a genuinely elegant defence. Producing an irritant only at the moment of injury costs the plant nothing while it is undisturbed, and onions grow underground where the threat is a burrowing animal taking a bite.

What it does to your eye

The gas reaches the surface of the eye and dissolves in the film of tear fluid, where it forms a small amount of sulphuric acid. The concentration is tiny and it does no lasting damage, but the cornea is one of the most densely innervated tissues in the body and it registers the irritation immediately.

Nerve endings signal the discomfort, and the reflex response is to flush the irritant away, so the lacrimal glands produce tears in quantity. The stinging and the crying are the same event: the eye detecting an acid and washing it off.

What actually reduces it

Ranked by how much difference each makes in an ordinary kitchen.

  • A genuinely sharp knife. The most effective single change. A sharp blade slices cell walls cleanly; a blunt one crushes and tears them, rupturing far more cells and releasing far more of the precursors. If you do one thing, sharpen the knife.
  • Chill the onion for 15 to 30 minutes. Both the enzyme reaction and the evaporation of the gas slow down at lower temperature. Do not freeze it fully, which damages the texture.
  • Cut near an extractor fan, or with a window open. You are dealing with an airborne compound, so moving air away from your face works exactly as you would expect.
  • Leave the root end until last. The sulphur compounds are concentrated towards the base. Cutting from the top and keeping the root intact until the end genuinely helps.
  • Cut under running water or in a bowl of water. Effective, because the compound is water-soluble and gets absorbed before it reaches you. Also awkward and bad for your knife skills.
  • Swimming goggles. Faintly ridiculous, and completely effective. A physical barrier is the only method that removes the problem entirely.

What does not work

  • Holding bread in your mouth. The gas reaches your eyes through the air, not your mouth. This one is pure folklore.
  • Breathing through your mouth. Same reason.
  • Lighting a candle nearby. Sometimes suggested on the grounds that the flame burns off the compound. The volume of air a candle processes is far too small to matter.
  • Contact lenses. Often claimed to help. They cover the cornea but not the whole eye surface, and the tear film carries the acid anyway.

Why some onions are worse than others

The amount of the precursor compounds depends on the variety and on how much sulphur was available in the soil. Sweet onions, such as Vidalia varieties, are grown in low-sulphur soil and are noticeably milder to cut as well as to eat. Older onions that have been stored a long time tend to be harsher than fresh ones.

Plant scientists in New Zealand and Japan have produced onions with the lachrymatory-factor synthase gene suppressed, which do not trigger tears at all. They have not become common in shops, but they exist.

More kitchen chemistry

The onion step in the masala omelette is where this becomes practical. For more kitchen chemistry, see why soap works and why bread goes stale faster in the fridge.

Sources

  1. Syn-Propanethial-S-oxide
  2. Alliinase
  3. Imai, S. et al., “An onion enzyme that makes the eyes water”, Nature 419, 2002, which identified lachrymatory-factor synthase
  4. Eady, C. C. et al., work on silencing the lachrymatory-factor synthase gene in onion, Plant Physiology, 2008

Frequently asked questions

Yes. Heat deactivates the enzymes and drives off the volatile compound, which is why a cooking onion smells sweet rather than sharp, and why nobody cries over a pan of frying onions. It is also part of why cooked onion tastes so different from raw.

Usually the onion, not you. Variety, age and storage conditions all change the concentration. A blunter-than-usual knife is the next most likely explanation.

HRITU Editorial Desk is the byline for pieces produced in-house by the HRITU editorial team. We write about cooking, travel, science and everyday knowledge, and we explain the reasoning behind what we publish rather than just stating conclusions. Every article is checked against primary or reference sources before it goes out, dated on publication, and updated in the open when we learn something is wrong. Corrections and questions are welcome at info@hritu.com.

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