Why Do Onions Make Us Cry?

By Ashton Yoon | June 6, 2017 10:00 am
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Photo Credit: Tastyart Ltd Rob White/Getty Images

We all know that feeling: the burning sensation as we slice into a fresh onion, eyes watering and wincing to relieve the stinging. There are claims that home remedies can solve this problem, including burning a candle, putting the onion in the freezer before chopping, or cutting the onion underwater. In this article we will investigate the culprit behind our onion tears and a possible scientific resolution that has emerged in the 21st century.

The teary-eyed response to cutting an onion is due to the chemical syn-propanethial-S-oxide, which the onion has evolved as a defense mechanism against predators. Each cell inside the onion contains a vacuole filled with enzymes [1] called allinases that convert amino acid sulfoxides that are present in the onion cell to sulfenic acids [2] (namely 1-propenyl-L-cysteine sulphoxide, or PRENSCO)[3]; these are then transformed by another enzyme into syn-propanethial-S-oxide [4] (Fig 1).When the onion cell structure is broken during chopping, the enzymes are released and are then able to interact with other chemicals inside the onion cell, thus catalyzing the chemical reactions that produce syn-propanethial-S-oxide [1] (Fig 1).

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Figure 1. Reaction mechanism for the formation of syn-propanethial-S-oxide. (Burnham, P.M. (1996))

The strong, spicy odor that onions emit when chopped are surprisingly not the culprit chemical that makes your eyes water. This odor is formed by the condensation of syn-propanethial-S-oxide to form odorous thiosulfanates [2]. Because syn-propanethial-S-oxide is a volatile sulfur compound, it easily diffuses into the air [1]. Your cornea contains nerves that relay information to the larger nerves responsible for touch, temperature, and pain detection on your face [2]. The nerves on your cornea detect the presence of syn-propanethial-S-oxide and send the signal to your central nervous system (Fig 2), which then stimulate the autonomic nerve fibers on the lachrymal glands to produce tears [2] in order to dilute the syn-propanethial-S-oxide. This is why referred to as lachrymatory compound or lachrymator, which can be defined as “an irritant that causes the eyes to fill with tears without damaging them” [5].

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Figure 2. The lachrymal response (Tiwari, S., Ali, M.J., and Vemuganti, G.K. (2014)/ScienceDirect)

Whereas previously it was thought that a generic allinase could produce syn-propanethial-S-oxide, recent research has shown that it is synthesized by a specific enzyme called ‘lachrymatory factor synthase’ [3]. In fact, scientists succeeded in producing onions in 2015 with suppressed lachrymatory-factor synthase genes that contained 7.5 times less syn-propanethial-S-oxide as well as eliminated the lachrymatory response among sensory panelists [6]. Additionally, scientists have found that when they mixed only generic allinases and PRENSCO in vitro, the production of thiosulphinate – the chemical responsible for onion’s flavor – increased [3]. No more tears & more flavor? Sounds like a tasty treat we would be down for!

References Cited:

  1. Singh, M. (2016). The science of why onions make us cry. NPR. Retrieved from http://www.npr.org/sections/thesalt/2016/06/22/482032913/the-science-of-why-onions-make-us-cry
  2. Scott, Y. Scientific American. What is the chemical process that causes my eyes to tear when I peel an onion? Retrieved from https://www.scientificamerican.com/article/what-is-the-chemical-proc/
  3. Imai, S., Tsuge, N., Tomotake, M., Nagatome, Y., Sawada, H., Nagata, T., Kumagai, H. (2002). An onion enzyme that makes the eyes water. Nature, 419, 685.
  4. American Chemical Society. (2013). Syn-Propanethial S-oxide. Retrieved from https://www.acs.org/content/acs/en/molecule-of-the-week/archive/s/molecule-of-the-week-syn-propanethial-s-oxide.html
  5. Burnham, P.M. (1996). Propanethial s-oxide, the lachrymatory factor in onions. J. American Chemical Society, 118(32), 7492-7501.
  6. Kato, M., Masamura, N., Shono, J., Okamoto, D., Abe, T., & Imai, S. (2016). Production and characterization of tearless and non-pungent onion. Scientific Reports, 6, 23779. doi:10.1038/srep23779.
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  • http://www.mazepath.com/uncleal/qz4.htm Uncle Al

    Understanding plus rational progress spawns Franken-onions! Gene-gineering must disgorge expensive, tasteless, and toxic things to achieve social intent – and be declared mandatory re Mr. Michelle’s “healthy” school lunches and celery handler’s disease.

  • Людмил Трифонов

    Фитонцидните аерозоли са изключително мощно средство за борба с заразни заболявания на очите, независимо от вида на причинителя. Терапията: предварително почистените от секрети очи се приближават на 10-15 см. до мястото на рязане на лука и така процедурата продължава бавно до нарязване на цялата глава лук.

  • Robert

    The problem is that a GOOD onion is hard to find; those sold today are as sweet as a candy bar and have none of the good old onion taste or sensation of heat. I would like to find the old onions and grow them here in my vegetable garden; but……..no luck.

    • Maia

      Keep trying. It can be done! Organic gardeners and/or organic suppliers of starts are the likeliest source.

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  • ron-moore1

    For instant relief of onion tears, sniff vanilla extract. Don’t know why it works but it does, immediately.

  • P.Mathivanan

    No more tears and more flavor – will that weaken the defense system of the plant against the predators? Terminator Onions!

    • Maia

      Since the tear-factor is caused by an aspect of the onion’s “immune” response (re: insects, not sure about other organisms), your questions is an important one. I wish I knew the answer. But it makes sense that lowering that defensive factor WOULD make onions more vulnerable…and lead farmers to use poisons. Onions are one of the least sprayed (with poison) foods, even when raised non-organically. This is because few insects/diseases seem to get past this potent vegetable’s chemical “walls”. I’d rather eat the old fashioned Power Onions and deal with a few tears!

  • M.Mitchell Marmel

    “I Love Onions” by Susan Christie:
    https://www.youtube.com/watch?v=KM-lEhhsLQw

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