It’s well known that onion is one of the highly potential healthy flavoring food, since it gives us few phytochemicals, nutrients, etc. for eg; one of the major constituent is Quercetin, which is an anti-inflamatory as well as antioxidant.
But, why it irritates our eyes while slicing them? here comes the reason
When we slice the onion, cells get break down which releases the enzyme called “Alliinase” (look at the picture) and it converts the more abundant sulphoxide of onion (a aminoacid called “1-propenyl L-cysteine sulphoxide”) into sulphenic acid (1-propenylsuphenicacid), which is unstable (never isolated) and it ultimately rearranges to the more volatile Propanthial s-oxide. Since this s-oxide is more volatile it can easily spread over the air and it reaches our eyes, where it reacts with water and converted into diluted sulphuricacid, this acid stains our never system and triggers the tear glands to produce tear for the purpose of diluting this sulphuric acid.
Some tips to reduce your tear while chopping onions,
1. Usually root of the onion releases maximum amount of sulfuric compounds, so it’s better to cut the stem (top) of onion and peel downwards to the root, chop it without cutting off the root until the last.
2. Cooling in refrigerator (enzyme will be less active) or washing the peeled onion in water or slicing under water flow also suppress the s-oxide to reach our eyes.
3. Chopping near to the flame (for eg; near to gar burner) also helps.
4. Use sharp knife for slicing, which could reduces the amount broken cells.
1. Imai, S.; Tsuge, N.; Tomotake, M.; Nagatome, Y.; Sawada, H.; Nagata, T.; Kumagai, H., Plant biochemistry: An onion enzyme that makes the eyes water. Nature 2002, 419, (6908), 685 doi:10.1038/419685a.