This Is the Real Reason Onions Always Make You Cry

An onion is a true utility player in the cooking world. Grilled, fried, caramelized, or raw, it will always provide affordable flavor. But why must we cry?

Onions are great. The versatile vegetable can be a volcano, a simile, the site of a Zach Galifianakis comedy special, or the edible centerpiece for any visit to an Outback Steakhouse. Its distinctive structure makes it unlike most other produce, and its affordable price point to flavor ratio makes it an ingredient beloved by soup kitchens and top-tier chefs alike. But why, oh why, do onions make you cry? (Not that that’s always a bad thing—there are health benefits for crying it out.)

This question is so important, that it warranted a Library of Congress web page. The short, government-doled answer provided by the Library is “unstable chemicals.” But that could also be provided as an oversimplified answer to the question “why does nuclear fission happen?” So, a little bit more context is needed.


Pinpointing the precise unstable chemical responsible for turning your tear ducts into water slides has been a long process, with the most recent development coming just 15 years ago. Originally, it was believed that the alliinase, an enzyme also found in garlic, was the culprit. But anyone who has chopped garlic will tell you two things about the flavorful bulbs; first, that they protect you from vampires, and second, that they do not make you cry.

Now, onion weeping theory has been narrowed down to two chemicals: syn-propanethial-S-oxide, an irritant which stimulates the eyes’ lachrymal glands, which release tears, and lachrymatory-factor synthase. The latter culprit is a little-known enzyme which was discovered thanks to several Japanese studies circa 2002.

According to the Library of Congress, the chemical process is 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 produce the tears!

All of these irritants are released in greater volume if you choose to use a dull knife, because the blunt blade essential hacks away at the cell wall of the onion with a baseball bat, causing excessive damage and releasing extra chemicals. A sharp knife provides surgical precision, less cell wall damage, and in turn, fewer chemicals released.

Be free, and enjoy onions with a more informed mind. And if you want to avoid crying while making that Philly cheesesteak, try one of these five hacks to halt your tears in their tracks.

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