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    Why does chopping an onion make you cry?


    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 amino acids sulfoxides 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
  • Cooking Onions Without Crying External Link - From the University of Nebraska-Lincoln Extension.
  • National Onion Association External Link - This website 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 External Link - July 2, 2010 interview with Eric Block author (and chemist) of Garlic and Other Alliums: The Lore and the Science (2010).
  • Onions External Link - 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.
  • Why Do Onions Make You Cry? External Link- From PennState Extension.

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, Mar. 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.

Photograph of sacks of onions in a field with some loose onions on the ground in the front.
Onions in sacks are drying. Those on ground are culls. Dorothea Lange, photographer, 1939. Prints & Photographs Division, Library of Congress.

Photograph of a male onion picker standing in an onion field with bunches of onions in his hands with other pickers in the background.
Mexican onion-picker in onion field near Tracy, California. Dorothea Lange, photographer, [1935]. Prints & Photographs Division, Library of Congress.

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 September 28, 2018
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