These simple knife tricks stop onion tears instantly

It’s your kitchen. You can cry if you want to. But with sharper knives, you might not need to. 

Cutting onions slowly with sharper knives slashes the number of tear-inducing droplets the vegetables eject into the air, researchers report in the Oct. 21 Proceedings of the National Academy of Sciences. This technique could not only improve an everyday cook’s culinary experience but also inform how pathogens spread. 

The culprit of kitchen crying is a chemical compound called propanethial S-oxide. When a knife pierces an onion, the cells rupture and trigger a chemical reaction that forms the compound. Propanethial S-oxide rockets into the air in a shower of tiny droplets, which bind to sensory nerves in the eyes and produce a tear-jerking stinging sensation. 

“This is something everybody’s dealing with,” says Navid Hooshanginejad, a physicist at SharkNinja, a product design company in Needham, Mass. “Now we can also explain and understand it better fundamentally.” 

Hooshanginejad studied onions while at Cornell University. One day, while preparing the vegetable for a salad and brainstorming with his advisor about his research project, drops from the onions’ pungent spray settled on Hooshanginejad’s hand. He then wondered, “What if these drops flew high enough to make me cry?” The next day, he hauled a 10-pound bag of onions into the lab, set up a high-speed camera and started slicing with a knife.

He and his colleagues then built a miniature guillotine to halve the fragrant vegetable and further investigate the spray’s airborne journey. The researchers used blades with varying sharpness and attached sensors to measure the force and speed of each cut. High-speed cameras continued to capture the fine mist and flying droplets that spewed forth after each slice.

Beneath each layer of an onion are fluid-containing cells that create a unique pressurized system. When a knife presses down on the onion’s skin, it crushes the cells. The juices pool and push against the skin, poised to fly once it breaks.

Blunt blades require more force to break the skin, squashing more cells in the process. Quick cuts made with dull knives caused onions to spurt liquid beads nearly 40 meters high, almost half the height of a giant sequoia. Unhurried chopping with sharper blades kept the deluge of soaring drops from reaching eye level, Hooshanginejad says.

“It’s just like the idea of a water balloon,” says Jim Wilking, a physicist at Mayo Clinic in Rochester, Minn., who was not involved with the study. “If you pop a water balloon by using a needle, you don’t need to apply much pressure. But if you come in with your finger, you’re essentially pressurizing the elastic shell in order to pop it.” The added pressure shoots the spritz of water — or onion juice — farther.

The onion’s pressurized ejection shows how droplets break apart in flight and gain distance. The finding may help scientists understand how pathogens disperse in other biological systems, Hooshanginejad says.