‘Mist opportunity’ reveals how onions make cooks cry

A new discovery about how cutting onions ejects pungent aerosols up to two-thirds of a meter into the air has led to practical advice for reducing the spray: Cut onions slowly with a sharpened blade or coat an onion in oil before cutting.

Past research had already identified that a chemical irritant called propanethial S-oxide is what pushes cooks to tears, but this study explains the underlying mechanics of how the sulfur compounds reach the eyes.

The cutting-edge research used high-speed cameras and a simple computer model to reveal how pressing a knife against an onion pressurizes the cells within its layers. Each layer of the onion is encased in a top and bottom layer of skin, and when the knife penetrates the top skin, the pent-up pressure within the layer forces out a mist of tiny droplets at estimated rates of 5 to 40 meters per second. That was a surprise since professional chefs - who were filmed and the speed of their cuts measured - slice at a rate of only 1 meter per second.

"We found out the speed of the mist coming out is much higher compared to the speed of the blade cutting through," said Sunghwan "Sunny" Jung, the paper's senior author and professor of biological and environmental engineering in the College of Agriculture and Life Sciences. Zixuan Wu, Ph.D. '23, is first author of the studypublished Oct. 7 in the Proceedings of the National Academies of Science.

Onions have been known to harbor pathogens, and suggestions for limiting sprays could help prevent bacteria from spreading around a kitchen, Jung said. In 2024, the Centers for Disease Control investigated an E. coli outbreaklinked to onions served at McDonald's that led to 104 cases of illness across 14 states and one death. Onion mists can eject from the skin at speeds of up to 40 meters per second, while human coughing can propel droplets at speeds of only up to 20 meters per second to spread diseases, Jung said.

"Suppose you have pathogens on the very top layer on the onion," Jung said. "By cutting this onion these pathogens can become encapsulated in droplets where they can then spread."