Why Does an Onion Make You Cry?

September 26, 2017

chopping onions

You can sniff an onion or peel the layers without shedding a tear, but once you crush, chop, or cut an onion there’s a good chance those tear ducts will start working. The thing that is causing you to tear up is actually a defense mechanism of the onion to deter intruders trying to eat it when it was in the ground.

An onion bulb is composed of many rectangular cells, and within each of the cells are vacuoles. The vacuoles are spaces enclosed by a membrane that are filled with an enzyme called lachrymatory factor synthase, or LF, which is an irritant. When these vacuoles are broken open, like they would be when they are cut, LF is released. This enzyme then mixes with the contents of the onion cell, which contain sulfenic acid, and a chemical reaction begins to take place. The result of the chemical reaction produces a gas called syn-propanethial-S-oxide which floats into the air and can quickly irritate a person’s eyes. The nerves in the eye then react to the gas and produce tears to try to get rid of the irritant. This is the complex process that makes us cry while cutting onions.

This chemical reaction is more pronounced in some varieties of onion over others. Red, white, and yellow onions have higher concentrations of LF while green onions, scallions and sweet onions contain lower concentrations. That’s why all onions don’t necessarily cause you to tear up.

What’s the best way to not tear up while cutting an onion? There are a couple of simple options. One is to cut onions in a well-ventilated area or with a fan present to blow the gas that is released by the onion away from the face. Another option is to refrigerate the onion before chopping which makes the molecules move slower and the chemical compounds less volatile.

Other onion facts that don’t have to do with tears

According to the National Onion Association, the average American eats 20 pounds of onions per year and 450 semi-truck loads of onions are eaten every day in the United States.

The country with the highest per capita consumption of onions is Libya. People in this country consume 66.8 pounds of onions per person each year.

The largest onion in the world was grown by Tony Glover of Leicestershire, England, in 2014. It weighed 18 lbs 11 oz and was 32 inches around its widest part.

The state vegetable of Texas is the 1015 sweet onion. It was developed by Texas A&M horticultural professor in the 1980s and got its name from the optimal planting date for the onion on October 15. It grows only in the Rio Grande Valley of South Texas.

The state vegetable of Georgia is the Vidalia onion. It is only grown in a small portion of southern Georgia.

The country with the largest onion production in 2016 was China at 20.5 million metric tons. India was next at 13.4 million metric tons followed by the United States at 3.3 million metric tons.

Sources: WebMD, NPR, US Library of Congress, National Onion Association, NY Times, Metro UK, Bristol Farms, State Symbols USA, World Atlas

About the author 

Daniel Ganninger - The writer, editor, and chief lackey of Knowledge Stew, the author of the Knowledge Stew line of great trivia books, and editor of Fact World and the Knowledge Stew sister site on Medium. I hope you find things here to annoy those around you with your new found knowledge.

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