WHY DO ONIONS MAKE YOU CRY? … STOP GETTING SO EMOTIONALLY ATTACHED! | SARA
- hederahelixscience
- Feb 1, 2021
- 2 min read
Onions are the most popular vegetable in the world with every year over 93.17 metric tonnes grown.[1] With so many onions being prepared in kitchens all over the world, why do onions create a burning sensation when it is sliced open?
I wanted to peel the layers of an onion to find out the reason, so I listened to the podcast, ‘Why do onions make you cry? - Chemistry for your life’, created by Jam and Mellissa. Mellissa, a scientist, describes the process to Jam, a non-scientist. Jam then reviews the process, while interpreting the chemistry to help everyone, from any background, easily understand. Mellissa’s calming voice made the podcast delightful to kick back and relax, while learning the fascinating science within the layers of an onion!
Simply put, tears are produced to dilute a compound, released by the onions, which irritates your eyes. I’m sure you guessed this, but did you know this compound is not actually present in onions. The compound is only created when the root of the onion becomes damaged by the knife; this is the onion’s defence mechanism. This act of defence prevents animals eating it while also killing bacteria and fungi which may destroy the health of the onion. Although the actual process is far more complex, the reaction can be simplified as follows. The initial reaction is between a precursor (a substance which produces another compound)[2], S-1-propenyl-L-cysteine sulfoxide, and an enzyme (also present in garlic). This initial compound then produces propanol sulphenic acid which then reacts with another enzyme, creating the airborne irritant propanethial-S-oxide or the lachrymatory factor. Propanethial-S-oxide then reacts with water, from the eyes, or oxygen, from the air, (it is not yet clear which) forming tiny amounts of sulphur-containing acid in the eye. This is what creates the burning sensation. Propanethial-S-oxide is only produced when the onion is cut as the cell membranes are broken, damaging the barrier and allowing the enzymes to react with the precursor.
So, what can you do to prevent the burning phenomenon? People have suggested using goggles, microwaving the onion or cutting it with a piece of bread in your mouth![3] Making the onion cold (by keeping it in the fridge) before cutting it, reduces the energy available for the reaction to occur. Therefore, slowing the rate of reaction for the acid to be produced. It has also been suggested to run water over the onion while it is cut. Although, this may avoid the burning sensation as the water reacts with the irritant before it reacts with the eye, it may result in an accident with the knife.
These suggestions may not be sufficiently effective or practical, however, a solution may not be necessary in the future due to onions being genetically modified to not produce the compound as a defence mechanism.
Next time you cut open an onion to put in your favourite dish or accompany your burger, you’ll know what causes the momentary stinging sensation and, remember, you may soon never feel it again!
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