A team of researchers has uncovered the precise physics explaining why slicing an onion often brings a cook to tears, revealing a two-stage process of fluid ejection that blasts irritating chemicals toward the eyes. Using high-speed cameras and a custom-built apparatus, the scientists meticulously analyzed the spray of droplets released during the cut, discovering that the speed of this mist is far greater than the speed of the blade itself. The findings, published in the *Proceedings of the National Academies of Science*, not only offer a deeper understanding of this common kitchen phenomenon but also provide practical advice for minimizing the tearful effects.
The study moves beyond the well-known chemical explanation—the release of the irritant propanethial S-oxide—to detail the mechanical process that turns the chemical into an airborne assault. Researchers found that the pressure from a knife blade builds up within the onion’s layers, and upon piercing the outer skin, this pressure is violently released, expelling a fine, high-velocity mist followed by slower-moving droplets. Counterintuitively, faster cutting with a duller blade produces a much more significant and energetic spray of these tear-inducing particles, while a slower, sharper cut can dramatically reduce the effect.
Visualizing the Tear-Jerking Process
To investigate the fluid dynamics of a sliced onion, researchers at Cornell University designed a guillotine-like device to make precise, controlled cuts while recording the action with high-speed video. This setup allowed them to observe the release of droplets from the onion’s cells with unprecedented detail. The team, led by Professor Sunghwan Jung, wanted to quantify a process that had never been visualized before, driven by curiosity about the mechanics of droplet ejection during cutting. Their method involved treating the onions to make the droplets more visible and then analyzing the footage to measure the speed, number, and trajectory of the ejected particles.
The high-speed imaging revealed a two-stage release of fluids. The first is a rapid, mist-like burst that occurs the moment the blade breaches the onion’s skin. This initial spray contains the most energetic droplets, which can travel up to 60 cm. Following this initial mist, threads of liquid are pulled from the cut, which then break apart into a larger volume of slower-moving drops. This detailed visualization confirmed that the irritant doesn’t just passively leak out; it is actively propelled in a powerful, directed spray.
The Surprising Role of Blade and Speed
One of the most unexpected findings from the study was the relationship between the cutting tool, its speed, and the volume of irritating mist produced. The researchers experimented with blades of varying sharpness and cutting speeds up to 40 meters per second. They discovered that a blunter blade generates a significantly greater number of droplets compared to a sharp one—in some cases, as much as 40 times more. A dull blade tends to crush the onion’s cells and bend the skin before cutting through it, which builds up substantial pressure. When the blade finally pierces the skin, this pent-up energy is released explosively.
Similarly, increasing the cutting speed also led to a more dramatic release of droplets, with faster cuts producing up to four times as many particles as slower ones. The combination of a dull blade and a high cutting speed proved to be the most potent formula for generating a tear-inducing cloud. This finding contradicts the common kitchen wisdom that a quick chop is best, showing instead that a slow, deliberate motion with a well-honed knife is the superior method for minimizing eye irritation.
From the Lab to the Kitchen
The research offers clear, science-backed advice for home cooks looking to avoid crying while preparing onions. The primary recommendation is to use a very sharp knife. A sharp blade slices cleanly through the onion’s cells with minimal crushing, which in turn minimizes the pressure buildup that leads to the high-speed mist. Paired with this, the researchers advise cutting the onion slowly and deliberately. This controlled motion further reduces the energy transferred to the onion, limiting the force with which the irritant-laden droplets are expelled.
Interestingly, the study also debunked a common folk remedy: refrigerating the onion before cutting. The experiments showed that chilled onions actually produced an increased number of droplets when cut compared to their room-temperature counterparts, although the velocity was similar. Therefore, for the least tearful experience, the evidence points squarely toward technique and tool maintenance: sharpen your knives and take your time.
Broader Implications for Food Safety
Beyond the inconvenience of watery eyes, the study highlights a potential concern for kitchen hygiene and food safety. The aerosolized mist generated by cutting an onion can travel a considerable distance, and these droplets can carry more than just tear-inducing chemicals. If pathogens such as *E. coli* or *Salmonella* are present on the onion’s surface, the cutting process can effectively encapsulate them in the airborne mist, spreading them around the kitchen environment.
Professor Jung noted that the velocity of the onion mist can reach up to 40 meters per second, which is significantly faster than the droplets expelled by a human cough, known to be a vector for disease transmission. This suggests that the same techniques that reduce tears—using a sharp blade and cutting slowly—could also play a role in reducing the cross-contamination of foodborne pathogens in both home and commercial kitchens. The research, therefore, provides another compelling reason to maintain sharp kitchen tools, linking the physics of a simple cut to the broader practice of safe food preparation.
