When we sliced or cut an onion, we break cells, allowing enzymes called alliinases to break down amino acid sulphoxides and generate sulfinic acids. Amino acid sulfoxides form sulfenic acids.
Sulphenic acids are unstable and spontaneously rearrange into a volatile gas called syn-propanediol-S-oxide. The gas diffuses through the air and eventually reaches the eye, where it reacts with the water to form a diluted solution of sulphuric acid. This acid irritates the nerve endings in the eye, making them sting. Tear glands produce tears to dilute and flush out the irritant.
Cooking the onion inactivates the enzyme, so while the smell of cooked onions may be strong, it doesn't burn our eyes.Aside from wearing safety goggles or running a fan, we can keep from crying by refrigerating our onion or soak in water for 5 to 10 minutes before cutting them, we can also cut onions under water, so slows reactions and changes the chemistry inside the onion.
The sulfur-containing compounds also leave a characteristic odor on your fingers. You may be able to remove or reduce some of the smell by wiping your fingers on a stainless steel odor soap.
