Onions produce the chemical irritant known as syn-propanethial-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:
- Lachrymatory-factor synthase is released into the air when we cut an onion.
- The synthase enzyme converts the sulfoxides (amino acids) 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 lachrymal glands become irritated and produces the tears!