Chopping an onion? Sharp knives can keep its juice out of your eyes

Crying over chopped onions could be a thing of the past. Slicing with sharper blades and slower cuts can eliminate those painful tears, a new study finds.

A chemical formed from onion juice (propanethial S-oxide) triggers those tears. Slicing the onion ruptures its cells, triggering a chemical reaction that forms this compound. Chopping can fling tiny droplets of it into the air. If they bind to sensory nerves in a cook’s eyes, they’ll cause that well-known stinging — and tears. 

But slicing onions slowly with a sharpened knife cuts the number of tear-inducing droplets that were spewed. This technique could offer serious relief to everyday cooks. It also could shed light on how pathogens spread.

The researchers shared their findings on Oct. 21 in Proceedings of the National Academy of Sciences.

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

Slashing onion tears

Onions were the focus of Hooshanginejad’s research at Cornell University in Ithaca, N.Y. He sliced up onions for a salad, one day, while brainstorming about his project with his advisor. Drops from the juicy onions landed on his hand. He thought, “What if these drops flew high enough to make me cry?”

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The next day, he lugged a 10-pound (4.5-kilogram) bag of onions to the lab. Then he set up a high-speed camera, grabbed a knife and started slicing.

He and his coworkers wanted to find out what about the cutting action matters most in determining how far onion-juice droplets fly. So they built a miniature guillotine, which allowed them to insert blades of varying sharpness. Then they attached sensors to measure the force and speed of each cut. The high-speed camera captured the spray of flying droplets emitted with each slice of a blade.

Fluid-containing cells sit in each layer of an onion’s skin. Every pass of the knife slices open these cells. Released juices then pool and press against the onion’s skin. This pressure causes the juices to fly once the blade breaks through that skin.

A blunt blade requires more force to break an onion’s skin than a sharp one. That extra force squashes a greater number of cells, releasing more fluid. And it makes a difference: Rapid cuts made with dull knives can shoot onion droplets nearly 40 meters (130 feet) high, they found. That’s almost half the height of a giant sequoia!

Slow and steady chopping with a sharp blade, however, kept the droplets from even reaching eye level, Hooshanginejad now reports.

“It’s just like the idea of a water balloon,” says physicist Jim Wilking. He works at the Mayo Clinic in Rochester, Minn., and did not take part in the new study.

“If you pop a water balloon by using a needle, you don’t need to apply much pressure,” he says. But try using just your finger. Then, he says, “you’re essentially pressurizing the elastic shell in order to pop it.”

Additional pressure applied to the surface will spray the water (or juice, in the case of the onion) farther.

The team’s videos of spurting onion juice show how droplets break apart in the air and gain distance. Says Hooshanginejad, these new data may help scientists better understand how infectious microbes disperse through the air.