Chemistry when onions are choped

Why do onions make us cry?

chemistry of onions

Have you ever wondered how stinging sensation will come over the surface of your eyes?

Why people will hesitate to cut or chop the onions?

What is the chemistry of crying over onions?

Mom: Go and chop some onions!

You:  Started to cry!

Mom: What happened dear?

You: The worst job in the kitchen is to peeling, cutting, slicing, or dicing of onions! It will make me cry!

Mom: Yes, of course! I know the reason. Let me explain you the details.

Onion is a good source of nutrients, vitamins B, C and G, protein, starch. It also contains amino acid sulfoxides that form sulfenic acids in the onion cells.As part of chemical defense onions release some gases which makes us cry.


As the onion plants grow, they absorb the sulphurous compounds in the soil; can then break down to form amino acid sulfoxides like S-propenyl-L-cysteine sulfoxide. The enzymes which are responsible for chemical mode of defense mechanism are Alliinase and Lachrymatory Factor synthase

Alliinase speeds up the conversion of the sulfoxide into two products

  • Alkyl sulfenicate
  • Aminoacrylate

This sulfenic acid is then acted upon by the second enzyme i.e. lachrymatory factor synthase which converts sulfenic acid into the propanethiol S-oxide compound.

Propanethiol-S-oxide is a volatile compound, meaning that it very readily becomes a gas. The eye, specifically the cornea, has a dense population of nerve endings that connect to the tear ducts. The gas diffuses through the air and, on contact with the eye, and free nerve endings detect propanethiol-S-oxide on the cornea. As a consequence of action the central nervous system will interpret these nerve impulses as a burning sensation.Then this nerve activity goes back to the effectors to stimulate response. This is the cause of the burning and itching sensations that accompany the tears released from the glands to dilute and flush out the irritant.


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