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Cutting Onion slowly and using the sharpest knife available reduce eye irritationPhysicists have found in a new study.
A familiar and painful experience for home cooks, crying while chopping onions is the result of a chemical reaction in the eye. A volatile liquid compound found in onions – syn-propanethyl-S-oxide – is released into the air when the vegetable is cut, and triggers nerve endings in the cornea.
The new study, published in the journal PNAS, assessed the factors that cause different amounts of liquid to be released as aerosols during onion cutting.
In the research, scientists cut onions using a special guillotine that could be fitted with different types of blades, as well as coating onion pieces with paint to better observe the cutting process.
In each test, scientists varied the knife’s size, sharpness, and cutting speed, and used a microscope to closely measure the knives before use.
Videos recorded during the study showed that there were differences in the amount of onion juice released – a measure of the potential for eye irritation from each bite.
The researchers found that the sprays were directly related to the sharpness of the knife and the speed at which each stroke of the knife cut the onion.
The study showed that the sharper the knife and the slower the cut, the less spray.
“By systematically varying the sharpness and speed of the blades, we found that sharp or blunt blades significantly increase both the number and energy of droplets emitted,” the researchers wrote.
It seemed as if dull knives were pushing downwards on the onion, forcing its layers to fold inward, and as the cut continued, these layers bounced back and expelled the juices into the air.
The researchers observed that as the juice droplets were thrown into the air, they broke up into smaller droplets, allowing them to last longer.
They also found that cutting faster produces more juice and thus more haze, causing eye irritation.
“Keep the knife sharp and cut gently to produce less spray,” the study authors advised.
During their study the scientists also decided to test the common belief that freezing onions before cutting them would reduce eye irritation.
They refrigerated the onions at 1 °C for 12 hours and compared the results to room temperature samples.
The researchers found that “there was no significant difference between the two conditions”.
They suggested further experiments to more rigorously evaluate the effects of temperature on onion cutting and droplet formation.
While their work was limited to onions, the scientists noted that their findings could have important implications for reducing other types of aerosols and droplets in the kitchen, including splatter that can spread foodborne illnesses.
“This work highlights the importance of routine sharpening of blades in the kitchen to limit droplets infected with pathogens, which pack additional blast energy due to the tough outer coverings of vegetables,” they wrote in the study.