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How Come? Why onions make our tears flow

How come onions make you cry? asks reader Jessica R.

Elaborate precautions and instructions. Special patented peeling devices. Goggles with foam seals and anti-fog lenses. All to produce a slice of something you'll happily eat on a sandwich.

Onions are a vegetable that seems to resist being eaten. Cut into an onion, and it's like a mini-mace attack. The result is stinging, watering eyes, which you struggle to keep open as you keep chopping. But hold an unpeeled, uncut onion to your face, and . . . nothing. Little to no scent, zero tear gas.

How -- and why -- does an innocent-looking onion accomplish its tiny assault? Breaking an onion's cells starts the ball rolling. Broken cells release molecules called amino acid sulfoxides, along with a special enzyme. The enzyme changes the sulfoxides into sulfenic acids. The acids rearrange themselves into a chemical called syn-propanethial-S-oxide -- which stealthily spreads as a gas. Syn-propanethial-S-oxide is also called "lachrymatory factor'' because of its effects on the lachrymal, or tear, glands.

About 30 seconds after an onion is cut, its gassing lachrymatory factor peaks. The gas reacts with the eyes' thin film of water. One of the chemicals produced in the reaction is a mild form of hydrochloric acid, the same acid our stomachs use to break down food. Irritated nerve endings immediately fire off messages to the brain; the eyes begin to sting. Meanwhile, the nerve signals cause tear glands to release a flood of tears to dilute and wash away the offending chemical.

Scientists in Japan discovered the enzyme that synthesizes an onion's lachrymatory factor, publishing their findings in 2002.

(The newly discovered enzyme was named, not surprisingly, "lachrymatory-factor synthase.")a mystery. Some say that the gas is a kind of plant-y defense against animals, the onion version of a rose's thorns.

But some point out that an onion's chemistry is designed to protect it from other threats -- invading bacteria, viruses and fungus. The irritating gas, they say, may just be a byproduct of a kind of chemical "bomb,'' designed to kill invading microbes as quickly as possible.

So don't take an onion's reaction personally. But when cutting an onion, do try some techniques that really work: Chill the onion in the fridge before cutting; cold makes the lachrymatory factor tamer. Cut the onion under running water, which short-circuits the gassy reaction by dissolving the chemical. Or set up a fan to blow the onion vapors away from you.

Onions are so good at what they do that researchers studying tears often use onion vapors to make their volunteers cry. But the "reflex'' tears produced, scientists say, are chemically different from those that emerge when we're upset. Emotional tears rid the body of some stress-produced hormones and contain natural painkillers. This may help explain why we feel better after we cry - but get no emotional lift from chopping onions.


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