The burning sensation and tears when cutting onions are a common kitchen experience. The tears are not a result of emotion, but rather a chemical reaction designed to deter predators. This phenomenon is a plant’s natural defense mechanism. Understanding the science behind this reaction demystifies why onions cause such a strong, involuntary response.
The Onion’s Chemical Defense
Onions possess a chemical defense system that activates when their cells are damaged. Inside the onion’s cells, sulfur-containing compounds exist separately from enzymes. When the onion’s cellular structure is broken, these components mix, initiating a cascade of chemical reactions.
The enzyme alliinase first breaks down amino acid sulfoxides within the onion, generating sulfenic acids. A specific sulfenic acid, 1-propenesulfenic acid, is then rearranged by lachrymatory factor synthase (LFS) to produce syn-propanethial S-oxide. This syn-propanethial S-oxide is a volatile liquid that quickly turns into a gas, triggering tearing and stinging.
Your Eyes’ Response
Once syn-propanethial S-oxide becomes airborne, it floats towards the eyes. When this gaseous compound contacts the watery layer of the eyes, it reacts, forming a mild sulfuric acid. This newly formed acid then irritates the delicate nerve endings in the cornea, the outermost layer of the eye.
The irritation triggers pain receptors in the eyes, sending signals to the brain. This activates the trigeminal nerve, which in turn stimulates the lacrimal glands, located under the upper eyelids, to produce reflex tears. These tears serve as a protective mechanism, working to dilute the irritant and flush it away from the eye’s surface.
Practical Prevention Methods
Several practical approaches can help minimize eye irritation when preparing onions. Chilling onions in the refrigerator for about 30 minutes before cutting can slow down the enzymatic reactions that produce syn-propanethial S-oxide, thus reducing the amount of irritant released. Cutting onions under cold running water also helps, as the water dissolves the volatile irritant before it can reach the eyes.
Using a very sharp knife can reduce irritation. A sharp blade creates cleaner cuts, minimizing the damage to onion cells and, consequently, releasing fewer tear-inducing compounds into the air. Conversely, a dull knife crushes more cells, leading to a greater release of the irritating gas.
Adequate ventilation, such as cutting near an open window or directly under a kitchen exhaust fan, can also help by drawing the irritating fumes away from your face. Wearing protective eyewear like kitchen goggles or safety glasses can create a physical barrier, preventing the gaseous irritant from reaching your eyes.