For many home cooks, the familiar sting and flow of tears while cutting an onion are an unavoidable part of meal preparation. This common kitchen challenge often leaves people wondering why this vegetable causes a strong, watery reaction. Understanding the science behind this phenomenon can transform a frustrating experience into an interesting lesson in biochemistry and human physiology.
The Irritating Compound
Cutting an onion initiates a rapid chemical process that produces an eye-irritating compound. Onions absorb sulfur from the soil, incorporating it into sulfur-containing amino acids. When an onion is sliced, its cells are ruptured, allowing previously separated enzymes to mix with these sulfur-rich amino acids. This interaction leads to the formation of unstable sulfenic acids.
These sulfenic acids then quickly rearrange into a highly volatile gas known as syn-propanethial S-oxide. This chemical is the primary irritant responsible for the burning sensation in your eyes. Because it is volatile, syn-propanethial S-oxide readily vaporizes and floats up from the cutting board, making contact with your eyes.
How Tears Form
When syn-propanethial S-oxide reaches your eyes, it dissolves in the tear film. This dissolution creates a mild form of sulfuric acid. This acid irritates delicate nerve endings in the cornea, triggering a rapid reflex response.
Pain receptors send signals to the brain, indicating a harmful substance. In response, the brain activates the lacrimal glands, which are responsible for tear production. A flood of tears is released, serving as the body’s natural defense to dilute and wash away the irritating sulfuric acid and syn-propanethial S-oxide.
Simple Solutions for Cutting Onions
Several methods can minimize eye irritation when cutting onions, by disrupting the chemical reaction or preventing the irritant from reaching your eyes. Chilling the onion before cutting can slow down the enzymatic reactions that produce the lachrymatory factor, reducing the amount of gas released. A sharp knife is also beneficial because it creates cleaner cuts, damaging fewer cells and releasing less of the irritating compounds into the air.
Cutting onions near a source of ventilation, such as an open window, an exhaust fan, or even a small personal fan, can help disperse the volatile gas before it reaches your eyes. Another effective strategy is to cut the onion under running water or submerged in a bowl of water, as the water helps to dissolve the irritating compounds before they can become airborne. For a physical barrier, wearing protective eyewear like swimming goggles can completely prevent the irritant from making contact with your eyes.