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Why do onions make you cry?

Onion's Tear Jerker
Slicing open an onion releases enzymes that irritate the eye.
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.