The reason why onions make us cry-
When you cut an onion, your eyes may burn and tears may flow down your cheeks.
Onions contain syn-Propanethial-S-oxide, a chemical irritant that makes people cry while cutting the onion. lachrymal glands in the eyes is activated , causing them to produce tears in our eyes. The enzyme alliinase was once blamed by scientists for the instability of substances in a cut onion. However, recent Japanese research has shown that lachrymatory-factor synthase (a previously unknown enzyme) is to blame (Imani et al, 2002).
The process is-
1) When we cut an onion, lachrymatory-factor synthase is released into the air.
2) Sulfenic acid is generated by the synthase enzyme, which transforms the onion's amino acids sulfoxides into sulfenic acid.
3) Syn-Propanethial-S-oxide is formed when the unstable sulfenic acid rearranges.
4) Syn-Propanethial-S-oxide floats about in the air and lands in our eyes. The tears are produced when the lachrymal glands become irritated!