Tearing up while chopping an onion is a common kitchen experience unrelated to emotion. This involuntary reaction is a specific biological response triggered by the rapid release of a chemical irritant. The onion plant evolved this mechanism as a defense against pests and animals. The process, from the first slice to the flush of tears, is a sequence of precise chemical steps that convert harmless internal compounds into a powerful, volatile gas.
The Onion’s Raw Materials
The onion bulb stores the ingredients for its defense mechanism in separate internal compartments. This separation is necessary because the compounds would react immediately if they were stored together. The primary precursors are sulfur-based compounds known as amino acid sulfoxides, which the onion absorbs from the soil as it grows.
These sulfur compounds are kept isolated from the enzymes that act upon them. The enzymatic machinery required includes alliinase and a second, specialized enzyme called lachrymatory-factor synthase (LFS). Only when the onion’s cell walls are breached—such as by a knife—do these isolated components come into contact to begin the chemical cascade.
The Chemical Reaction That Causes Tearing
When a knife damages the onion’s cellular structure, the stored amino acid sulfoxides and the alliinase enzyme mix freely. This contact initiates the first step: alliinase rapidly converts the amino acid sulfoxides into sulfenic acids. These sulfenic acids are unstable intermediates in the defense sequence.
The presence of the second enzyme, lachrymatory-factor synthase (LFS), makes the onion’s reaction unique. LFS acts immediately on the unstable sulfenic acids, rearranging their structure into the highly volatile gas known as syn-propanethial S-oxide. This lachrymatory factor is the compound responsible for the stinging sensation.
Because syn-propanethial S-oxide is a volatile gas, it quickly vaporizes and travels upward. When the gas reaches the eye, it dissolves in the thin, watery film covering the eyeball, converting the chemical into a dilute solution of sulfuric acid. This acid immediately irritates the sensory nerve endings in the cornea, signaling a threat to the brain.
In response to this chemical attack, the lacrimal glands are stimulated to flush the irritant away. The resulting flood of tears is a protective reflex intended to dilute and wash the sulfuric acid off the eye’s surface.
Controlling the Irritant Gas
The physical reaction of tearing is the eye’s attempt to rinse away the irritant. Several methods can interfere with the production or transmission of the syn-propanethial S-oxide gas before it reaches the eye. These techniques alter the conditions of the chemical reaction or create a physical barrier.
Chilling the Onion
One strategy involves lowering the temperature of the onion by chilling it in a refrigerator before cutting. Enzymes are biological catalysts whose activity slows down significantly at colder temperatures. A cold onion reduces the speed at which the alliinase and LFS can produce the lachrymatory factor gas.
Using Air Movement
Another method focuses on the volatility of the gas by using air movement to direct it away from the face. Cutting the onion near an open window, a running kitchen exhaust fan, or a small personal fan physically disperses the syn-propanethial S-oxide. This prevents the chemical from dissolving in the eye’s moisture and causing irritation.
Submerging in Water
Cutting the onion under running water or fully submerged in a basin of water is an option because the lachrymatory factor is highly water-soluble. The water captures the gas the moment it is released from the damaged cells. This prevents it from becoming airborne and eliminates the irritant before it can travel to the eyes.
Using a Sharp Knife
The sharpness of the knife blade also plays a role in the reaction’s severity. A dull knife crushes more cell walls during the cutting process, causing a more widespread release of the precursor compounds and enzymes. Conversely, a thin, sharp blade makes a cleaner slice, minimizing the cellular damage and resulting in a smaller expulsion of the tear-inducing chemicals.