WHY CUTTING AN ONION MAKES US CRY??

Freshly cut onions often cause a stinging sensation in the eyes of people nearby, and often uncontrollable tears. This is caused by the release of a volatile liquid, syn-propanethial-S-oxide and its aerosol, which stimulates nerves in the eye.

This gas is produced by a chain of reactions which serve as a defence mechanism:

1.     Chopping an onion causes damage to cells which releases enzymes called alliinases.

2.     These break down amino acid sulfoxides and generate sulfenic acids.

3.     A specific sulfenic acid, 1-propenesulfenic acid, is rapidly acted on by a second enzyme, the lacrimatory factor synthase, producing the syn-propanethial-S-oxide.

Syn-Propanethial S-oxide

                                                      

4.     This gas diffuses through the air and soon reaches the eyes, where it activates sensory neurons.

5.     Lacrimal glands produce tears to dilute and flush out the irritant.

WAYS TO REDUCE TEARS WHILE CUTTING ONION

1.     Eye irritation can be avoided by cutting onions under running water or submerged in a basin of water.

2.     Leaving the root end intact also reduces irritation as the onion base has a higher concentration of sulphur compounds than the rest of the bulb.

3.     Refrigerating the onions before use reduces the enzyme reaction rate and using a fan can blow the gas away from the eyes.

4.     The more often one chops onions, the less one experiences eye irritation.

The amount of sulfenic acids and lacrimal factor released and the irritation effect differs among Allium species. In 2008, the New Zealand Institute for Crop and Food Research created "no tears" onions by genetic modification to prevent the synthesis of lachrymatory factor synthase in onions. In early 2018, Bayer released the first crop yield of commercially available LFS-silenced onions under the name "Sunions". They were the product of 30 years of cross-breeding; genetic modification was not employed.

Guinea hen weed and honey garlic contain a similar lachrymatory factor. 

COMMERCIAL USES

Synthetic onion lachrymatory factor has been used in a study related to tear production, and has been proposed as a nonlethal deterrent against thieves and intruders.