Onion makes us cry, why?

Ever cut an onion and started tearing up? Most people do. It turns out when you cut an onion, it releases a gas called, ready for this, Propanethiol S-oxide.It stimulates the eyes’ lachrymal glands so they release tears. Scientists used to blame the enzyme allinase for the instability of substances in a cut onion. Recent studies from Japan, however, proved that lachrymatory-factor synthase, (a previously undiscovered enzyme) is the culprit (Imani et al, 2002).

The process goes 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!

Normally, your body would signal you to close your eyes. This, of course, is not a good idea if you are cutting an onion. The next best thing your eyes can do is to make tears to protect the eyes. Rubbing your eyes is a bad idea since your hands are likely full of the tear making onion juice. So how do you keep from tearing up? There are many theories out there.

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