Syn-propanethial-S-oxide gets into the air and comes in contact with our eyes. The lachrymal glands become irritated and produces the tears!
It can be an irritating and even painful process to slice and dice onions, so here are some ideas you can try without getting weepy:
Use a sharp knife to cut the onion—you'll release less of the enzymes into the air.
Cut onions in cold water.
Cut the root last—it has a higher concentration of enzymes.
Chill or freeze onions to minimize the amount of gas released into the air.
Light a match before you peel or slice the onion. The sulfur disables the compounds in onions that make your eyes water.
Use a small manual or electric food chopper or food processor.
Wear kitchen goggles to protect your eyes.
Remember not to touch your eyes when you chop onions. And make sure to wash your hands thoroughly after you're finished. If your eyes are especially irritated, flush them with cool water or a soothing eye rinse. If your symptoms don't disappear within a few hours…..
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 amino acids sulfoxides of the onion into sulfenic acid.
The unstable sulfenic acid rearranges itself into syn-ropanethial-S-oxide.
