The experience of cutting an onion and suddenly shedding tears is a universal kitchen phenomenon. This involuntary reaction is not an emotional response but a direct chemical defense mechanism evolved by the plant. The moment a knife slices through the onion’s layers, a cascade of chemical reactions begins, releasing a volatile gas that rapidly irritates the eyes. This process involves specific enzymes and compounds that the onion keeps segregated until its cellular structure is damaged.
The Specific Chemical Irritant
The culprit behind the tears is a volatile sulfur compound identified as Syn-propanethial S-oxide. This airborne molecule is a potent lachrymatory factor, meaning it is designed to induce tear production. When the onion is cut, this compound vaporizes and wafts into the air, quickly reaching the eyes. The compound’s high volatility allows it to travel easily from the cutting board to the sensitive membranes of the eye’s surface. The irritant is not naturally present in the intact onion but is created instantly when the cell walls are ruptured.
The Enzymatic Process That Creates the Irritant
The creation of Syn-propanethial S-oxide is a two-step enzymatic reaction that begins when an onion’s cells are broken. Onions store sulfur-containing precursor molecules, such as S-1-propenyl-L-cysteine sulfoxide, separately from the enzymes that act upon them. The moment a knife cuts into the bulb, the precursor molecules mix with the enzyme alliinase, which has been sequestered in different cell compartments.
The alliinase enzyme rapidly cleaves the precursor molecules, transforming them into various sulfenic acids, including 1-propenyl sulfenic acid. This intermediate compound is then acted upon by a second enzyme: Lachrymatory Factor Synthase (LFS). LFS is the specific catalyst that converts 1-propenyl sulfenic acid into the final tear-inducing gas, Syn-propanethial S-oxide. This chain reaction is the onion’s evolutionary defense, designed to deter herbivores.
The Biological Defense Mechanism
Once the volatile Syn-propanethial S-oxide enters the eye, it dissolves in the eye’s natural moisture, forming a small amount of sulfuric acid. This mild acid immediately triggers pain receptors on the surface of the cornea. These sensory neurons are part of the trigeminal nerve system, which sends a rapid signal to the central nervous system.
The brain interprets this signal as a chemical attack and initiates an immediate, involuntary defense reflex. The response involves sending a command to the lacrimal glands, which are responsible for tear production. Tears are then quickly released onto the eye’s surface to dilute and wash away the irritating compound, flushing the eye.
Practical Strategies for Tear Prevention
Mitigating the tear response involves either slowing the enzymatic creation of the irritant or preventing the gas from reaching the eye. Lower temperatures slow the reaction rate of the Lachrymatory Factor Synthase enzyme, reducing the amount of gas released into the air.
Using a sharp knife is also beneficial because it minimizes damage to the onion’s cell walls. A dull blade crushes the cells, releasing significantly more precursor chemicals and enzymes, which results in a greater burst of the irritating compound.
Here are practical strategies for tear prevention:
- Chill the onion in the refrigerator before cutting.
- Use a sharp knife to minimize cell damage.
- Cut the onion near a gentle stream of running water, which absorbs the volatile gas.
- Use a kitchen fan or vent hood to disperse the gas before it reaches the eyes.