Why Onions Make Us Cry and How to Prevent Tears With This Chopping Technique
Anyone who spends a lot of time in the kitchen knows what it’s like to be driven to tears when chopping an onion. A handful of creative kitchen hacks have been explored to avoid this stinging sensation, from cutting an onion under running water to chilling it in the fridge. But there may be an even simpler answer to this culinary inconvenience, according to science.
A new study published in the Proceedings of the National Academies of Science has proposed that the key to keeping your eyes dry while working with an onion is cutting it slowly with a sharpened knife. This technique, which researchers found to subdue the chemical chain reaction that normally causes us to cry, might make cooking with onions less of a tear-jerking nightmare.
Why Onions Make Us Cry
When an onion makes our eyes well up with tears, that’s simply it’s way of telling us that it really doesn’t want to be eaten. This biochemical defense mechanism starts when onions absorb sulfur from soil as they grow, storing it as a compound called 1-propenyl-L-cysteine sulfoxide.
When an onion is cut with a knife, this compound reacts with enzymes to produce propanethial S-oxide, an irritating chemical also known as the lachrymatory factor. The chemical is unleashed in a high-speed spray of tiny droplets that bombards our eyes, reacting with their moisture to form sulfuric acid that upsets our tear glands.
At this point, tears start to flow in an attempt to purge the acid from our eyes. The amount of acid, though, is so small that it isn’t harmful, just bothersome.
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Spraying Under Pressure
The researchers involved with the new study dove deeper into the mechanism that propels droplets from onions, aiming to see what factors influence their assault on our eyes.
To test this, they set up a guillotine-style tool that they used to cut onions, filming the process with high-speed cameras. Using a simple computer model, the researchers were able to observe what occurs as a knife presses into an onion; in this moment, pressure builds up within the onion’s layers, each of which contains a tougher outer skin (the epidermis) and a softer interior (the mesophyll).
A stress zone then develops within the interior of an onion layer, where droplets start to form. Once a knife finally pierces through the outer skin, these droplets are forced out at speeds of 5 to 40 meters per second. The researchers were surprised to see how fast the spray shot out during their experiments.
“We found out the speed of the mist coming out is much higher compared to the speed of the blade cutting through,” said senior author Sunghwan Jung, a professor of biological and environmental engineering at Cornell University, in a press release.
Preventing Pathogens in the Kitchen
Based on their tests, the researchers confirmed that sharper blades minimize the number of droplets released by an onion and reduce their speed and kinetic energy.
Blade sharpness and cutting technique are crucial considerations when working safely with fruits and vegetables, which can harbor food-borne pathogens like Salmonella. This is relevant for onions in particular, since their mists can easily spread bacteria around a kitchen.
“Suppose you have pathogens on the very top layer on the onion,” said Jung. “By cutting this onion, these pathogens can become encapsulated in droplets where they can then spread.”
There are a few other solutions that may reduce or even eliminate onions’ tear-inducing droplets. The researchers suggest that coating an onion in cooking oil may help prevent the mist from reaching your eyes. Cooking with sweeter onions, which have lower sulfur content, may also be the way to go if you don’t want to cry.
Some scientists have even thrown out the idea of genetically modifying an onion to suppress an enzyme called lachrymatory-factor synthase, potentially allowing for “tearless” onions.
This article is not offering medical advice and should be used for informational purposes only.
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