Why Do Onions Make You Cry?
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
Onions make you cry because cutting one triggers a chemical defence system. The onion is not simply pungent by nature. Damaging its cells sets off a reaction that produces an irritant gas specifically effective at attacking eyes, and it happens within seconds.
What cutting releases
An intact onion is harmless. Its cells store sulphur compounds in one compartment and enzymes in another, kept separate as long as the cells stay whole.
Cutting breaks the walls and mixes them. An enzyme converts the stored compounds into an unstable intermediate, and a second enzyme rearranges it into a volatile gas called propanethial S-oxide. That gas floats up towards your face.
What happens to your eyes
When the gas reaches the moisture on the surface of your eye, it reacts to form a small amount of sulphuric acid. The concentration is tiny and does no damage, but nerve endings in the cornea detect it immediately.
Those nerves trigger the tear glands, which flood the eye to dilute and flush the irritant away. The stinging and watering are a protective reflex working exactly as intended, just aimed at something that is not actually a threat.
Why the onion does this
This is a defence mechanism. Onions store energy underground in a bulb, which makes them an attractive target for animals that dig. A chemical that causes immediate eye pain is an effective deterrent.
Related plants use similar tricks. Garlic produces allicin by the same kind of enzyme reaction when crushed, though it irritates differently. In both cases the plant is not pungent until it is damaged, which is why whole garlic cloves smell so mild.
What actually helps
Some kitchen advice is sound and some is folklore. The methods with a real basis are:
- •Chilling the onion first, which slows the reaction and reduces gas evaporation
- •Using a genuinely sharp knife, which ruptures fewer cells
- •Cutting near an extractor fan or open window to move the gas away
- •Wearing goggles, which is unglamorous but completely effective
- •Leaving the root end intact until last, since it holds a high concentration
What probably does not
Holding a spoon in your mouth, chewing bread or lighting a candle nearby have no plausible mechanism and no good evidence. Cutting under running water works but is awkward and washes away flavour.
Plant breeders have taken a different route and developed low tear onions by reducing the activity of the enzyme that produces the irritant. They are sold in some markets, and they show the chemistry is well enough understood to engineer around.
Why cooked onions taste sweet
The same sulphur compounds responsible for the tears are also responsible for the raw pungency, and heat destroys them. That is why a cooked onion tastes nothing like a raw one.
Long slow cooking goes further. The onion's natural sugars concentrate as water evaporates, and both caramelisation and the Maillard reaction produce new flavour compounds. Properly caramelised onions take around forty minutes rather than the ten that many recipes optimistically claim, and rushing the process produces something browned but harsh rather than genuinely sweet.
The takeaway
Onions make you cry because cutting them mixes enzymes and sulphur compounds into an irritant gas that forms a trace of acid on your eye, and chilling the onion, using a sharp knife and moving air away are the fixes that genuinely work.