If you have ever sliced an onion, you have probably experienced the burn and shed a few tears. But, why do onions make you cry?

SHORT ANSWER: Onions make you cry because the chemical irritant known as syn-Propanethial S-oxide is released into the air during slicing. When this irritant comes in contact with the cornea – the front surface of the eye, it is detected by the central nervous system and eventually causes the lacrimal (tear) glands to release tears in order to wash the irritant and neutralize the effect.

LONG ANSWER: Onions are vegetables that belong to large genus of perennial and biennial pungent bulbous called Allium. Along with 400 other variations that include garlic, chives, leeks and scallions, onions are also member of the family Liliaceae (lily).

Onions, like other vegetables of this lily family, absorb sulfur in the earth to form amino acids sulfoxides – sulfur compounds that readily turn into a gas. So when you slice through into an onion, you break open lots of individual cells. These ruptured cells spontaneously release enzymes – alliinase and lachrymatory-factor synthase, and amino acid sulfoxides, which form unstable sulfenic acid. The sulfenic acid then reacts with onion enzymes to form a chemical irritant known as syn-Propanethial S-oxide. And, this gas (the chemical irritant) floats up from the onions and eventually reaches the eye.

The cornea, the front surface of your eyes, is equipped with special sensory nerves that detect and protect eyeballs from dirt, germs, and other physical and chemical irritants. These special sensory nerves respond to this gas, syn-Propanethial S-oxide, by signaling the central nervous system as – a burning sensation. The signal is then carried back to the lachrymal glands, which release tears in order to dilute the irritant.

Onions do not actually contain Propanethial S-oxide. It is the product of a series of chemical reactions that takes place once the onion has been sliced or damaged. The reactions are shown below:

Chemical Reactions: Formation of propanethial S-oxide (in onions)

Propanethial S-oxide is relatively volatile sulfur compound. Some studies have suggested that when its vapor comes in contact with the eye, it reacts to form sulfuric acid. Researchers think this strong acid could be the culprit behind the burn and itching sensations as well. Also, different people react differently when they chop onions, because the corneal sensitivity varies from people to people.

Scientists have now produced genetically modified tear-free onions that do no not contain the lachrymatory-factor synthase enzyme. Well, unless you can get one of these GM tear-free onions, these are some of the tips to avoid weepy onion slicing sessions:

  • Refrigerating the onions before slicing. The cold prevents some of the volatile onion compounds from turning into a gas.
  • Running water over onion while cutting. This can stop the gas from reaching your eyes.
  • Using a decent sharp knife. This can easily make fine cuts, and thus releasing less gas. Less gas = Less tearing.
  • Keeping your mouth busy such as chewing gum, or holding an object such as spoon in the mouth. The idea is that this will encourage you to breathe in the lachrymator to prevent it from reaching your eyes.
  • Wearing goggles or contact lenses can help, too.

What was your experience with onions lately? What do you do to prevent crying while chopping onions? Tell us about it in the comments.

Video via Reactions.

15 thoughts on “Why Do Onions Make You Cry?

  1. My cousin told me once about the chewing gum thing, but we don’t have very many chewing gums so everytime I cut onions I simply pretend to chew on something you know, it works too. I chew on air, it’s crazy lol but at least I don’t hurt my eyes haha:)

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