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When I looked on a couple of sites, I found that it is because of sulfuric acid produced by onions. But I could not find more details. What is the biochemical pathway by which onion causes tears? Also, which compound is responsible for it? If it is enzyme-catalyzed reaction, can we just stop the production of this enzyme without causing any side-effects?

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    $\begingroup$ Not what you're asking, and not biology-related, but as a practical cooking point, using a sharp knife definitely decreases the effect. $\endgroup$ – David Richerby May 17 '17 at 17:57
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    $\begingroup$ Your last sentence refers to the Vidalia Onion, IIRC. Lacking sufficient sulpher, it doesn’t produce significant quantities of the irritating sulfoxide. $\endgroup$ – JDługosz May 17 '17 at 23:38
  • $\begingroup$ @JDługosz thats a nice point, why don't you add that as an answer? :) $\endgroup$ – another 'Homo sapien' May 18 '17 at 1:38
  • $\begingroup$ I don't think it is an Answer. But it makes a good comment. $\endgroup$ – JDługosz May 18 '17 at 2:30
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    $\begingroup$ ...because they're so cute and defenseless... a..an..and I need t-to kill them... (sob) ... but I can't.. (sob) ... I just can't... (cries) $\endgroup$ – xDaizu May 18 '17 at 10:15
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Interesting question! The cause of tears and itching is the chemicals produced by onion (Allium cepa). Lets go into some details.

Onions, coming from the family Liliaceae (also containing garlic, chives, scallions and leeks) store compounds known as amino acid sulfoxides, and the one we are talking about here is S-1-propenyl-L-cysteine sulfoxide (abbreviated as PRENCSO), also calles isoalliin (due to its similarity with alliin found in garlic). When onion is damaged (cut, chewed, etc.), an enzyme alliinase converts PRENCSO into 1-propenyl sulfenic acid. This compound is then converted into propanethial-S-oxide by an enzyme lachrymatory factor synthase (earlier this reaction was considered spontaneous). The reaction looks like this:

pathway

Propanethial-S-oxide is the major cause of the flavor and aroma of onion. However, it is a volatile compound i.e. vaporizes very quickly. When its vapors reach the eye, it causes tears because of being a lachrymator (aka tear gas) i.e. as soon as it comes in contact with cornea, it triggers a nervous response which leads to activation of lachrymal (tear) glands.

PS: when propanethial-S-oxide comes in contact with cornea, a small amount of it reacts with water to form sulfuric acid. This sulfuric acid is the cause of itching and irritation in eyes due to onion. Also, scientists are now trying to genetically either modify or stop the production of lachrymatory factor synthase enzyme to produce tearless onions. This (modification) has even been achieved to a high efficiency, as another answer discusses. However, making tearless onions could prove harmful to the crop in several ways, as discussed here.

EDIT: As asked in comments, I will add some details about how the sulfuric acid is produced from the reaction between propanethial-S-oxide and water.

The only resource I could find giving some details about this was Marta Corzo-Martínez, 2014. They summarize the complete pathway in the following diagram:

diagram

After applying some common chemistry principles, the concerned reaction turns out to be:

$\ce{4~C_3H_6SO~+~4~H_2O \rightarrow 4~C_3H_6O~+~H_2SO_4~+~3~H_2S}$

As you see, one of the products of hydrolysis of propanethial-S-oxide is hydrogen sulfide ($\ce{H_2S}$). Just like $\ce{H_2SO_4}$, $\ce{H_2S}$ also causes irritation in the eyes (its effect on eyes has been well documented, see Lambert et al, 2006 as an example). Thus, the produced $\ce{H_2S}$ only increases the irritation and itching in the eyes caused due to $\ce{H_2SO_4}$.

BONUS: Another interesting point here is runny nose. propanethial-S-oxide is actually the compound responsible for the smell and flavor of onions. But, it causes tears by exciting the lachrymal glands i.e. reflexive lachrymation. propanethial-S-oxide excites the trigeminal nerve (the fifth cranial nerve) causing activation of lachrymal glands. Interestingly, the nerve endings of trigeminal nerve are also present in the nose, along with the eyes. So, this compound can also activate the lachrymal glands from your nose, and since the lachrymal duct is joined from eyes to nose, you can also experience runny nose along with tears and irritation in eyes.

References:

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    $\begingroup$ I'm curious about the sulfuric acid. The oxidation number of S is rather low in propanethial S-oxide, are you certain that sulfuric and not sulfurous acid is produced? I know about the relatively low stability of the latter. $\endgroup$ – TAR86 May 17 '17 at 15:14
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    $\begingroup$ @another homo sapien Thank you for a wonderful answer :-)​​​ $\endgroup$ – superbug May 17 '17 at 17:44
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This is due to a lachrymatory agent called as syn-propanethial-S-oxide.

The process goes as follows:

  • Lachrymatory-factor synthase is released into the air when we cut an onion.

  • The synthase enzyme converts the amino acids sulfoxides of the onion into sulfenic acid.

  • The unstable sulfenic acid rearranges itself into syn-ropanethial-S-oxide.

  • Syn-propanethial-S-oxide gets into the air and comes in contact with our eyes. The gas diffuses through the air and, on contact with the eye, it stimulates sensory neurons creating a stinging, painful sensation.

  • The lachrymal glands become irritated and produces the tears.

    Tears are released from the tear glands to dilute and flush out the irritant. In this case it is to reduce the effects of sulfuric acid,a chemical that could damage the eye if it was not for the tear reflex that renders the sulphuric acid largely harmless.

So, we can stop tear production by not irritating our eyes by using the onions which produce less irritants.

Through a single genetic transformation in onion (Allium cepa), a crop recalcitrant to genetic transformation, we suppressed the lachrymatory factor synthase gene using RNA interference silencing in six plants. This reduced lachrymatory synthase activity by up to 1,544-fold, so that when wounded the onions produced significantly reduced levels of tear-inducing lachrymatory factor.


Sources: https://www.loc.gov/rr/scitech/mysteries/onion.html

https://www.ncbi.nlm.nih.gov/pubmed/18583530

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    $\begingroup$ Woah, interestingly, I got a few seconds late due to captcha verification... $\endgroup$ – another 'Homo sapien' May 17 '17 at 9:50
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    $\begingroup$ Interesting. I was wondering, though, if it has to come in contact specifically with the eyes? As it seems for myself that the smell alone (well, the irritating gas actually) starts the tears, and the painful feeling comes from my sinuses rather than the eyes? I also noticed I sustain the feeling much longer if I pinch my nose. Yet that might be confirmation bias ^^ $\endgroup$ – spectras May 17 '17 at 15:06
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    $\begingroup$ @spectras I believe so; I have a pair of goggles that I wear when cutting onions and it seems to prevent the effect. $\endgroup$ – JAB May 17 '17 at 15:36
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    $\begingroup$ Just because it's a random thing I learned working in fast-food for 3 months at the ripe age of 16 before being fired, soaking onions in ice water dramatically reduces irritation. Maybe the ice isn't needed, can't recall. $\endgroup$ – Chris Schneider May 17 '17 at 16:14
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    $\begingroup$ @ChrisSchneider As temperature decreases enzyme activity and diffusion decreases. $\endgroup$ – JM97 May 17 '17 at 16:15

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