Why do onions make you cry?

onion close-up
Oddly enough, the volatile compound that makes you cry is also responsible for the great taste in onions. See more vegetable pictures.
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Whenever George Washington felt a telltale tickle in his throat or sniffle sneaking up, he tucked in to a particularly pungent bedtime snack: an onion. As the popular tale goes, Washington would eat a cooked onion in order to ward off colds. The Founding Father's homeopathic regimen makes nutritional sense. Pearly white onions are chock-full of vitamins C, B1 and B6, along with a healthy dose of potassium, phosphorus and fiber. Though onions aren't universally appealing to the human palate, they have the potential to lower bad cholesterol, relieve hypertension and minimize blood clotting [source: McNamee].

On the flip side, onions exact a brief (but uncomfortable) physiological toll to those who dare to slice through their white, yellow or red-tinted skins. Within a few moments of opening up an onion, the tangy scent wafts up to our noses, and our eyes begin to water. Sometimes, the onion reaction is just a mild ocular irritant; other times, it's a full-on cry fest.


Unless you're a chef with a freshly broken heart, the tears you shed when chopping onions aren't emotional ones. That leaves two other categories of tears: basal and reflexive. Since basal tears are the ones that hang around our eyes and eyelids to act as a lubricant, that leaves us with reflex tears. The lachrymal glands above the eyelids regulate the release of tears. In the case of reflex crying, an external irritant, such as dust or smoke, triggers nerve endings in the cornea to communicate with the brain stem. The brain registers the irritation in the eye then alerts the lachrymal gland to stimulate tear production to flush away the invader.

If we're chopping onions a few feet away from our eyes, what's causing this weepy reaction? The answer begins in the soil. Onions are part of the plant genus Allium, along with garlic, chives, leeks and about 400 other cousins. These vegetables absorb sulfur in the earth, which helps form a class of volatile organic molecules called amino acid sulfoxides. These sulfoxides are the real tear-jerkers when onions go under the knife.


Onion's Tear Jerker

onion slicing
Slicing open an onion releases enzymes that irritate the eye.

As we chop up an onion, it releases lachrymatory-factor synthase enzymes. These catalysts instigate the chemical chain reaction that ends with you crying over the kitchen counter. These enzymes react with the sulfoxides and convert them into sulfenic acids [source: Scott]. Sulfenic acids are highly unstable and rearrange into a compound called syn-propanethial-S-oxide [source: Library of Congress].

When syn-propanethial-S-oxide (a pesky combination of sulfuric acid, sulfur dioxide and hydrogen sulfide) enters the air around our faces and approaches our eyes, it elicits the reflexive tear response described on the previous page [source: Norton]. Multiple nerve endings in the cornea register the sensation of the syn-propanethial-S-oxide as a substance that could harm our eyes. Consequently, the brain stem phones the lachrymal glands, and we commence to sniveling.


For those fearing runny mascara and tear-stained cheeks, researchers in New Zealand may have found a reason to return to the cutting board. By isolating and handicapping the lachrymatory enzyme, the scientists have created a tear-free onion. What's the first trick to growing the genetically modified Supasweet onion? Low-sulfur soil [source: Highfield].

Onion lovers have probably noticed a difference in the tear reaction depending on the type of onion they carve up. Georgia's Vidalia onion and other sweet varieties that are harvested in the spring and summer won't induce as many tears, compared to the effect of tart fall and winter onions. The higher sugar content and water concentration in sweet onions diminish the irritable enzymes.

When recipes call for yellow Spanish onions or other sharp relatives, you can try out a few tricks to ward off crying in the presence of onions:

  • Chop an onion beneath running water.
  • Turn on a fan while cutting an onion to scatter the sulfur compounds.
  • Chill or cook an onion before chopping it.
  • Use an onion chopping container.
  • Wear goggles or glasses to protect your eyes.

But dry your eyes: Despite all of these tears, there is some good news. Supposedly, the more you cook with onions and endure their sting, the less they'll affect you over time [source: Hillman]. Just keep some lemons nearby to scrub the vegetable's strong signature scent off your hands.


Lots More Information

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More Great Links

  • Highfield, Roger. "GM tear-free onion created by scientists." Daily Telegraph. Feb. 2,
  • 2008. (April 6, 2009)http://www.telegraph.co.uk/scienceandtechnology/science/sciencenews/3323954/GM-tear-free-onion-created-by-scientists.html
  • Hillman, Howard. "The new kitchen science." Houghton Mifflin Harcourt. 2003. (April 6, 2009)http://books.google.com/books?id=BCLT3hH84GoC
  • Library of Congress. "Why does chopping an onion make you cry?" Feb. 12, 2009. (April 6, 2009)http://www.loc.gov/rr/scitech/mysteries/onion.html
  • McNamee, Gregory. "Movable Feasts." Greenwood Publishing Group. 2007. (April 6, 2009)http://books.google.com/books?id=96wzC3B0TJUC
  • National Onion Association. "About Onions: Seasonality." (April 6, 2009)http://www.onions-usa.org/about/season.php
  • Norton, James. "Kitchen Science 101." Popular Science. Oct. 23, 2007. (April 7, 2009)http://www.popsci.com/scitech/gallery/2007-10/kitchen-science-101
  • Scott, Thomas. "What is the chemical process that causes my eyes to tear when I peel an onion?" Scientific American. Oct. 21, 1999. (April 6, 2009)http://www.sciam.com/article.cfm?id=what-is-the-chemical-proc