Why do onions make you cry? — the chemistry and health benefits of onions

Español: Cebolla siendo cortada.

Many of us have cried over chopping onions. Do you know why they make your eyes water, though? Yes, yes, they have that pungent juice spraying up into your eyes. But do you know what’s in that misty juice? And don’t stop that chopping – chopping and crushing onions release health-benefiting compounds.

Let’s get straight to the point. The component in an onion that makes your eyes water is called lachrymatory factor, from Latin lacrima, or “tear.” (How appropriate is that!) Lachrymatory factor irritates the eyes and stimulate the tear glands to produce tears, much like the action of a tear gas.

Onions are rich in two health-benefiting compounds: flavonoids and sulfur-containing compounds (or alk[en]yl cysteine sulfoxides, if you really want to know the name). Flavonoids are potential antioxidants that could protect us against cancer, heart disease, and aging. They are often found at high concentrations in the skins and outer layers of onions with yellow or brown color (but some are masked by red or purple color). (Read more on the benefits of flavonoids here.) The sulfur-containing compounds are the flavor precursors that give rise to the characteristic odor and flavor of onions, which will be discussed below.

For a long time, it was thought that lachrymatory factor was produced by alliinase, a critical enzyme in a pathway of onion flavor synthesis. (An enzyme is a protein that helps speed up a reaction, in this case a conversion of one compound into another.) However, in 2002, a previously unknown enzyme – not alliinase – was shown to be responsible for synthesizing lachrymatory factor. This new enzyme was termed lachrymatory-factor synthase, a very straightforward name. The discovery was significant because inhibiting alliinase would lead to reduction of both lachrymatory factor and other flavor compounds, whereas inhibition of lachrymatory-factor synthase would only prevent the synthesis of lachrymatory factor, thus making you no longer tearful, while keeping the flavor intact.

Chemical reactions when an onion is chopped. Adapted from Imai et al. (2002) Nature.

Let’s look at the chemistry of onion flavor synthesis more closely. When an onion is damaged through cutting or crushing, a volatile sulfur compound is released into the air. This compound is broken down into an unstable intermediate with the help of alliinase. This product can then either turn into lachrymatory factor with the help of lachrymatory-factor synthase or spontaneously turn into thiosulfinate. This thiosulfinate is not only responsible for the onion’s distinct odor and flavor, but also gets converted into other sulfur-containing compounds with potential health benefits, namely anti-inflammation, anti-blood clotting, anti-cancer, anti-asthma, and lowering cholesterol levels. Now you see why inhibiting alliinase all together is a bad idea?

As a matter of fact, inhibiting lachrymatory-factor synthase would not only stop onions from making your eyes water, but would also increase the yield of thiosulfinate because all the sulfur compounds released from onions will be converted into thiosulfinate. So the onions lacking lachrymatory-factor synthase activity would be tear-free but retain that odor and flavor distinct to fresh onions.

Onions on a neutral, mostly white background

Tear-free onions have been developed by Crop & Food Research of New Zealand and House Foods Corporation of Japan in 2008. These onions look and taste like regular onions but have lowered activity of lachrymatory-factor synthase through genetic modification and thus do not make your eyes water upon chopping or crushing, while increasing the production of beneficial thiosulfinate. The research groups are considering marketing these tear-free onions, but commercialization of genetically modified foods is no simple process. It is said that commercialization of such onions are at least 6 years away.

But let’s not forget “tear-less” onions at hand, the sweet onions. Vidalia, a brand of sweet onions, have been around since 1930’s. These onions are usually grown in sulfur-deficient soils so that the production of lachrymatory factor, along with other sulfur compounds responsible for the pungent and hot flavor, is reduced, resulting in a sweeter flavor.

Nonetheless, if you like that kicking, pungent flavor of fresh onions but don’t want to cry over chopping one, you would have to wait for the lachrymatory-factor-synthase “silenced” onions to become available on the market.

Thank you for reading the post. See you next week! :)

References:
Plant biochemistry: An onion enzyme that makes the eyes water
Silencing Onion Lachrymatory Factor Synthase Causes a Significant Change in the Sulfur Secondary Metabolite Profile
Onions—A global benefit to health
Tear-free onions
Vidalia Onions

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13 Comments

Filed under Health, Q&A

13 responses to “Why do onions make you cry? — the chemistry and health benefits of onions

  1. Jeanie

    I didn’t know the mechanics behind why cutting onions make you cry, but y’know what else I didn’t know? That chopping jalapenos with your bare hands make your hands burn so, so badly afterward.

    • It burns because of capsaicin in jalapeño and TRPV channels in your skin. Maybe I can cover this topic one day, but it’s good that you’ve had a hands-on experience of it! ;) (I hope your hands have recovered from the pain, though!)

  2. [Adapted from Imai et al. (2012) Nature.]:
    The published year of the article might be 2002. Please, see: http://www.nature.com/nature/journal/v419/n6908/full/419685a.html

    • Thank you very much for the correction! The typo in the figure legend has now been corrected. Comments/suggestions that help my posts improve are greatly appreciated! :)

  3. kkj

    i thoughth onions made you cry because they have sting rays

  4. Pingback: 5 Simple Steps to Chop Onions without Tears | Health200

  5. munil singh

    plz sent me which of acid to smell in onians and gralic.,….help me

  6. thank you very much thy explaination was more clear and i get thy point……………………

  7. Pingback: Make Eating Vegetables Fun | Sourcelight Template

  8. Thank you, I’ve recently been looking for info about this topic for ages and yours is the best I have discovered till now. But, what in regards to the conclusion? Are you positive in regards to the source?|

  9. Clyde Herreid

    Is there an evolutionary significance to onion smell and the lachrymatory factor?
    CFH

    • That is a very interesting question, Clyde! It is said that hot chili peppers have capsaicin (stuff responsible for the hotness) which birds do not have the receptor for, so that mammals leave them alone while birds eat them and help distribute undigested seeds. (Too bad for the hot chili peppers that a lot of humans love them for the spiciness!) It is very possible that onions have adopted a similar mechanism to discourage mammals from devouring them. If I come across a more specific evidence or examples, I’ll post them for sure! Thank you for leaving a comment :)

  10. Pingback: The Chemistry of an Onion | Compound Interest

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