Why Onions Make You Cry ?

Onions, along with many other plants in the Allium species (garlic is another popular one), absorb sulfur from the soil.  When onions are chopped, it ends up breaking cells within the onion, which releases certain enzymes.  

These enzymes then react with the sulfur, creating amino acid sulfoxides.  These, in turn, create the highly unstable syn-propanethial-S-oxide, which is a combination of sulfuric acid, sulfur dioxide, and hydrogen sulfide.    When this substance, in a gaseous state, comes in contact with the moisture in your eye, it triggers a burning sensation via the ciliary nerve.

Tears in the eyes are regulated by the lachrymal gland, which is situated just above your eyelids.  When the brain gets a message that there is an irritant in the eye, such as the above syn-propanethial-S-oxide, which gives a burning sensation, it then kicks the lachrymal glands into overdrive, trying to flush the irritant out of your eye(s) with tears.

Cooked onions won’t produce this same effect because the process of cooking the onion inactivates the enzymes needed to make the syn-propanethial-S-oxide.  So you can safely chew the cooked onions without getting teary-eyed.

Best Ways to Stop Onions From Making Your Eyes Water

• Refrigerate the onion at least 30 minutes before cutting.  Alternatively, put the onion in ice-water for a few minutes before cutting.  By cooling the onion, you will slow the enzyme/sulfoxides reaction rate, minimizing the syn-propanethial-S-oxide production.
• Turn a fan on, blowing across the onions and away from you.
• Run water over the onions as you cut them.  The syn-propanethial-S-oxide that cause the eye irritation is water soluble, so can simply be washed away before having a chance to get in the air.
• Cook onions often.  Research has shown that the more you cut onions up, the less the sulfuric compounds will affect your eyes.