If vegetables had personalities, then onions would probably be one of the more ambiguous examples in your kitchens. Sure, mushrooms may be social climbers, while carrots and peas are just a bit basic, but nothing compares to onions’ potential for treachery. It’s so versatile, goes well with so many meals, but preparing it can lead to a tear-stained problem for many people. The inherent lacrimatory effects produced by onions was even described by Shakespeare when, in Antony and Cleopatra, he wrote “Indeed the tears live in an onion that should water this sorrow”.
But if you are someone who struggles with tears for this cruel root veg, then a new physics paper may offer insights into how to avoid shedding more.
When onions are cut, they release a spray of tear-inducing aerosols into the air that contain a volatile liquid called syn-propanethial-S-oxide, which stimulates nerves in the eyes. This is where the tears come from. But while researchers may know what triggers the stinging pain in our eyes, they have not been completely clear on the mechanics that dictate how the tiny aerosol droplets behave when they’re released from the vegetable’s flesh.
In their new study, Sunghwan Jung at Cornell University in Ithaca, New York and colleagues combined custom-developed highspeed particle tracking velocimetry (PTV) and digital image correlation (DIC) – which is used to measure surface deformation, displacement, and strain in materials and structures – to visualize droplets as they were ejected from a cut onion and to quantify them.
The researchers mounted a guillotine-style setup that used a vertical slider with a blade that could be released from above to cut a quarter of an onion. The onion itself had been coated in black spray paint (Rust-Oleum commercial brand) and allowed to dry for 30 minutes before the experiment. This allowed the team to track how the vegetable deformed when cut and to see how the particle spray emerged.
In addition to monitoring the onion, the team also used an electron microscope to assess the sharpness of the blades they used (the width of the blades’ tips varied between 5 and 200 millimeters). The height of the blades was also adjusted so that the cutting speed could be varied.
The team found that sharper blades produced fewer droplets which were also slower, as they had less energy. In contrast, when onions were cut with blunter knives, the onion’s skin bent more, producing more pressure which resulted in more spray.
“Using our custom-developed particle visualization and tracking techniques, we revealed that onion cutting with unsharpened blades generates liquid droplets through a two-step process: an initial outburst driven by internal pressurization, followed by a slower fragmentation of ligaments in air”, the team explains in the paper.
In some instances, the spray ejected by dull blades reached speeds of up to 40 meters per second.
It seems then, that if you want to have fewer tears in the kitchen, you need to ensure that you are using sharper knives with slower, controlled cuts. But what role does temperature play?
Although the researchers suggest more work needs to be done on this particular factor in relation to ejection mechanics involved in onion cutting, they did test onions that had been refrigerated. This is because there is a common belief that chilling onions before you cut them can help prevent them from making you cry. The team, however, did not find this to be true. In fact, the chilled onions appeared to release a “noticeably larger volume” of droplets when cut.
This may seem like a trivial topic, but it has practical implications for minimizing the spread of airborne pathogens in the kitchen.
The paper is published in arXiv.
