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 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!

Related Web Sites

Crying for Onion Chemistry by Megan FitzPatrick - This article from the Chem Ecology Education Center explains why onions make you cry. It gives suggestions on how to avoid tearing up when you cut an onion. 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." 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.

Further Reading

Dille, Carolyn and Susan Belsinger. The onion book: a bounty of culture, cultivation, and cuisine. Loveland, CO, Interweave Press: c1996. 96 p. 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.

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

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

Did you know?

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

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