A Simple Way to Cut Onions Without Crying
The Mist That Makes Us Weep
When an onion is cut, cells fracture and release an enzyme‑driven chemical that becomes propanethial S‑oxide, the tear‑inducing gas. In kitchens, it emerges as a fine aerosol—a mist of droplets that reaches our eyes and stings. Scientists have now shown there’s a surprisingly simple way to keep those tears to a minimum: use a sharper blade and slower cuts, confirmed by careful experiments tracking the onion mist released when onions are chopped.
What the Researchers Did
A Cornell University team built a controlled lab setup—a mini guillotine, high‑resolution imaging, and sensors—to watch onions shed droplets in real time. They tested how knife sharpness, cutting speed, and force shaped the spray. The imagery revealed that every cut triggers two phases: an almost instantaneous burst of fine droplets, then a slower seep that follows the grain of the onion’s layered skins, as documented in their paper in PNAS.
Sharp And Slow Beat Dull And Fast
Blunter knives demanded more force to rupture skins, building pressure in the juices and launching larger, faster sprays. Quick, forceful chops with a dull blade sent droplets even farther. Sharper blades and gentler, slower strokes reduced the amount of aerosol released and tended to keep the mist below eye level—exactly where you want it if you’d rather not cry, according to the cutting‑dynamics experiments. High‑speed imaging clocked the initial ejection of droplets at up to roughly 40 meters per second, making those first milliseconds the riskiest for your eyes, the Cornell group reports.
A Popular Kitchen Myth, Tested
Chilling onions has long been floated as a fix. The Cornell experiments found no meaningful benefit from starting with cold bulbs; if anything, chilling could make things worse. The key variable wasn’t temperature—it was how sharp the knife was and how you used it, as the team’s analysis of chilled vs. room‑temperature onions shows.
Inside The Physics
The team observed that the mist’s velocity exceeds the blade’s cutting speed, a sign that stored elastic energy in cell layers is released explosively as skins fracture. Their theoretical models matched independently measured fracture forces, lending confidence that these dynamics—not kitchen lore—govern the spray, as detailed in their news summary and in the peer‑reviewed PNAS paper.
Not Just Tears: Food Safety Stakes
Aerosolized droplets don’t only carry irritants; in the wrong circumstances, they can ferry microbes. Style of cutting can influence where droplets land and how potential pathogens spread—an important consideration after an onion‑linked E. coli outbreak at McDonald’s in the US last year. The takeaway is the same: sharp blades and controlled, slower cuts reduce both eye irritation and the radius of any possible contamination, the Cornell team notes.
How to Put It To Work Tonight
- Hone or sharpen your knife before you start. A few passes on a honing steel helps, and periodic sharpening matters most.
- Use steady, deliberate strokes. Slice, don’t smash. Avoid fast, forceful chops—consistent with the “sharp and slow” evidence.
- Work with the cut face down and keep your eyes above the board, not over the plume.
- Ventilate if you can. A gentle cross‑breeze helps move aerosols away from your face.
- Keep surfaces clean. Wipe the board between batches to limit residue.
A Long Line of Onion Tears
We have been cooking with onions for millennia, and we’ve been writing about their tears nearly as long—Shakespeare even nods to them in Antony and Cleopatra. The Cornell work adds a modern, high‑speed camera view to that old kitchen experience, showing exactly how and when the droplets escape, as summarized by the Cornell team.
The Bottom Line
There’s no gimmick here. If you want fewer tears, sharpen the knife and take your time. The physics says those two changes meaningfully cut down the mist at the moment it matters most—and they make your kitchen a little safer, too, as shown in both the experiment write‑up and the PNAS study.
