Before harvesting, onions absorb sulphur from the earth. Onions contain amino acid sulfoxides that form sulfenic acids in onion cells. Enzymes and sulfenic acids of the onion cells are kept separately. When cutting an onion, it separates the enzymes. The enzymes then start mixing with each other and produce a propanethiol S-oxide, which is a volatile sulphur compound. The gas emitted reacts with the water of one’s eyes and form sulphuric acid. Sulphuric acid causes burning sensation in eyes. The lachrymal glands in eyes then become irritated and produces tears.
Enzymes : alliinases – found in onion & garlic
Propanethiol S-oxide (C3H6SO) : main chemical compound that causes tears when chopping an onion
Lachrymal glands : an almond-shaped exocrine glands (produce and secrete substances onto an epithelial surface by the way of a channel leading from exocrine glands or organs) , one per eye, that secretes the aqueous layer of the tear film
Ways to avoid having to tear up when cutting an onion
1.Refrigerate the onion before cutting
– by doing so, it freezes the enzymes from mixing with the sulfenic acids & the production of propanethiol S-oxide is stopped.
2.Cook the onion before cutting
– it inactivates the enzyme so there will be no chemical reaction occurring
3.Soak the onion in water before cutting
– the water will absorb the gas of the onion and therefore there will not be any propanethiol S-oxide formed
Fun fact: allium cepa is another name for onions.
To learn more, here’s a short video that summarises the above topic