Onions possess a chemical defense mechanism in order to shield itself from the attack of microbes and insects when still planted in the soil. It has sulfur compounds that act as an effective repellent against pests, insects and microbes when the onion not uprooted. This substance is also responsible for the strong and pungent odor of onions. When the onions get chopped breakdown of these sulfur compounds occur forming gas that reacts with the human eye and causes irritation. As a reflex action to an acidic attack the tear ducts give out tears to negate the irritation and prevent any allergic reaction to the eyes.
Why onions make us cry? – See in detail
Onion is a very intriguing and interesting vegetable as cutting onions invariably makes people cry. So what exactly is there in onions that cause this to happen? When onions are chopped the cellular structure gets broken down. As a result the enzyme present in the onions called “alliinase" breakdown and gets converted into “amino acid sulfoxides". This chemical conversion gives out sulfenic acids. Amongst the many sulfenic acids formed on chopping of onions one particular sulfenic acid known as “1-propenesulfenic acid" gets chemically modified by lachrymatory factor synthase (LFS) and gives out a highly volatile gas called the “syn-propanethial-S-oxide". This gas immediately mixes with the air around and reaches the eye causing severe irritation and burning sensation. When this gas reaches the eyes the sensory neurons present in the eyes get activated and tears are formed. The tears are in fact the human body’s natural reaction to dilute the volatile gas irritating the eye to reduce the irritation.
Prevent crying when chopping onions
There are some ways that can help prevent excessive tears during cutting of onions. The most popular method is to chill the onion. Cooling onion ends up reducing the chemical activity of the enzyme and thus does not cause tears. Another way of preventing tears is to first peel the onions and then keep them dipped in water for a while before chopping it. This is effective because the water reacts with the sulfur compounds and prevents the formation of the gas that irritates the eyes.