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Question:

    Why does chopping an onion make you cry?

Answer:    

    Unstable chemicals.

Onions produce the chemical irritant known as syn-propanethial-S-oxide. It stimulates the eyes' lachrymal glands so they release tears. Scientists used to blame the enzyme allinase for the instability of substances in a cut onion. Recent studies from Japan, however, proved that lachrymatory-factor synthase, (a previously undiscovered enzyme) is the culprit (Imani et al, 2002).

The process goes as follows:

  1. Lachrymatory-factor synthase is released into the air when we cut an onion.
  2. The synthase enzyme converts the sulfoxides (amino acids) of the onion into sulfenic acid.
  3. The unstable sulfenic acid rearranges itself into syn-ropanethial-S-oxide.
  4. Syn-propanethial-S-oxide gets into the air and comes in contact with our eyes. The lachrymal glands become irritated and produces the tears!
Standard DisclaimerRelated Web Sites
  • National Onion Association - This Web site provides onion information & tips, recipes, growing areas, and industry. "The National Onion Association encourages the United States onion industry to voluntarily exercise all reasonable efforts to supply consumers with the highest quality, most nutritious, and safest onions available; and furthermore, to grant appropriate consideration and respect to the issues of food security, sound pesticide management, and environmental stewardship."
  • NPR Story: Exploring the Stinky Science of Alliums - July 2, 2010 interview with Eric Block author (and chemist) of Garlic and Other Alliums: The Lore and the Science (2010).
  • Onions - This site from Texas A&M University provides all types of information about onions such as planting tips, the varieties, the history and laws, and onion recipes.
  • Onions for the Home Garden - Kansas State University Agricultural Experiment Station and Cooperative Extension Service provides tips for growing onions.

Library of Congress Web SiteFurther Reading
  • Dille, Carolyn and Susan Belsinger. The onion book: a bounty of culture, cultivation, and cuisine. Loveland, CO, Interweave Press: c1996. 96 p.
  • Block, Eric. The chemistry of garlic and onion. Scientific American, v. 252, March 1985: 114-119.
  • Block, Eric. The chemistry of garlic and onion. Scientific American, v. 252, March 1985: 114-119.
  • Frey, William H. Crying: the mystery of tears. Minneapolis, Winston Press, c1985. 175 p.
  • Imani, S. et al. Plant biochemistry: an onion enzyme that makes the eyes water. Nature, v. 419, Oct. 17, 2002: 685.
  • Parsons, Russ. How to read a french fry and other intriguing kitchen science. Boston, Houghton Mifflin, c2001. p 1-3.
  • Rogers, Mara Reid. Onions: a celebration of the onion through recipes, lore, and history. Reading, MA, Addison-Wesley Pub., c1995. 193 p.

SearchFor more print resources...
Search on "onion," "lachrymal gland" and "crying" or "tears" in the Library of Congress Online Catalog.

Photograph of person chopping onions
"Share The Meat" recipes. Ann Rosener, photographer, 1942. Prints & Photographs Division, Library of Congress.

Did you know?
Image: see caption below

According to the Guiness Book of World Records, the largest onion weighed 10 pounds 14 ounces. It was grown by V. Throup of Silsden, England.


Mexican onion-picker in onion field near Tracy, California. Dorthea Lange, photographer, [1935]. Prints & Photographs Division, Library of Congress.

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  August 25, 2010
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