Why Do Onions Make You Cry?

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.

When the gas reaches the moisture on the surface of your eye, it directly stimulates pain nerves in the cornea, rather than forming sulphuric acid as is often claimed. 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.

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.

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

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.

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.

Onions are not the only plants that keep ingredients apart until they are damaged. Mustard, horseradish, wasabi, cabbage and other members of the cabbage family store compounds called glucosinolates separately from an enzyme called myrosinase. When you chew or grate them, the two meet and produce sharp, volatile chemicals. That is why a whole mustard seed tastes mild while ground mustard mixed with water develops its heat after a few minutes.

The burning feeling from wasabi or horseradish rushes up into the nose rather than lingering on the tongue like chilli, because the irritant compounds are volatile and travel as vapour. Chilli heat comes from capsaicin, which is not volatile and works differently. Cooking also reduces the pungency of mustard family plants, since heat stops the enzyme. This is why freshly grated horseradish is fierce, while boiled cabbage has only a mild, sometimes sulphurous smell.

Not every onion hits equally hard, and growing conditions are a big reason why. The tear-making and pungent compounds are built from sulphur that the plant takes up from the soil. Onions grown in soil low in sulphur, and with plenty of water, tend to produce less of these compounds, so they taste sweeter and sting less. Some famous sweet onion varieties are associated with particular regions partly for this reason. Storage onions, bred to keep for months, are usually more pungent, and their stronger chemistry also helps protect them while stored.

A persistent myth claims that a cut onion left in a room absorbs viruses or bacteria from the air, sometimes linked to stories from the 1918 flu pandemic. There is no good evidence that onions draw germs out of the air, and a cut onion has no special power to soak up illness. A related claim says cut onions become poisonous overnight. They do not, provided they are handled like any other cut vegetable: covered, refrigerated and used within a few days.

The sulphur chemistry that makes your eyes sting has a more serious effect on some pets. When dogs or cats eat onions, sulphur compounds formed from the onion's stored chemicals damage their red blood cells by oxidising haemoglobin, the oxygen-carrying protein. Clumps of damaged protein called Heinz bodies form inside the cells, which are then destroyed or removed from circulation, causing a form of anaemia. Signs such as weakness, pale gums and dark urine often appear a few days after eating. Cats are more sensitive than dogs, and garlic, leeks and chives can cause the same problem.

Cooking does not make onions safe for animals. Fried onions, onion powder in gravy, stock or baby food and leftovers from takeaway meals can all contain enough to matter, and dried powder is more concentrated than fresh onion. A single small taste is unlikely to harm a large dog, but repeated small amounts or one large meal can. People are far less affected, partly because a typical portion is tiny relative to human body weight and partly because species differ in how well their red blood cells resist oxidative damage. Cattle and horses have also been poisoned after eating large quantities of wild or surplus onions.

For most of the twentieth century, chemists assumed that the irritant gas formed on its own once the first enzyme, alliinase, had broken down the onion's stored sulphur compounds. In 2002 researchers at a Japanese food company showed that this was wrong. A separate enzyme, now called lachrymatory factor synthase, is needed to turn the unstable intermediate into the tear-making gas. That finding revealed a fork in the chemistry. When the second enzyme is present, much of the intermediate becomes irritant gas. When it is missing, the same molecules pair up instead into compounds called thiosulfinates, which carry strong savoury, pungent flavours.

This explains why garlic, a close relative, is so much gentler on your eyes. Garlic stores a slightly different sulphur compound and lacks the tear-making enzyme, so crushing it produces allicin, a thiosulfinate, rather than a tear gas. It also explains a side effect of efforts to breed tearless onions. When researchers in New Zealand used gene silencing to switch off the second enzyme in onions, the plants made much less irritant and more thiosulfinates, the kind of compounds that dominate the flavour of garlic. Chopping an onion still sets off the same chemistry, but the reaction is steered down a different branch.

Onions make you cry because cutting them mixes enzymes and sulphur compounds into an irritant gas that stings the nerves of your eye directly, and chilling the onion, using a sharp knife and moving air away are the fixes that genuinely work.