WHY ONION MAKES US CRY ?

Onions produce a 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, is the culprit (Imani et al, 2002). It is not the odor from the onion that makes us cry, but the gas that the onion releases when cut. It is an irritant to both our noses and to our eyes. Cutting an onion arouses a gas contained within the onion. When this upwardly mobile gas encounters the water produced by the tear ducts in our eyelids, we cry !

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 sulfoxides (amino acids) of the onion into sulfenic acid.
3. The unstable sulfenic acid rearranges itself into syn-propanethial-S-oxide.
4. Syn-propanethial-S-oxide gets into the air and comes in contact with our eyes.
5. The lachrymal glands become irritated and produces the tears.

Cooking the onion inactivates the enzyme, so while the smell of cooked onions may be strong, it doesn't burn your eyes.