The experience of tearing up while chopping an onion is a common kitchen struggle, often leading people to wonder about the cause of the sudden, stinging sensation. This involuntary reaction is not a sign of emotional distress but rather a natural physiological response to a volatile chemical compound released from the vegetable’s damaged cells. The mechanism behind the burning involves a precise, multi-step chemical reaction that is actually an evolutionary defense system developed by the plant. Understanding this process, from the initial cellular structure to the final airborne irritant, reveals why this particular member of the Allium family causes such a dramatic physical effect.
The Onion’s Chemical Defense System
The onion’s structure contains the building blocks for the tear-inducing reaction, but these components are kept safely separated within the plant’s cells. Inside the cellular compartments are sulfur-containing molecules known as amino acid sulfoxides, specifically S-1-propenyl-L-cysteine sulfoxide. This precursor chemical is harmless and provides the sulfur base for the subsequent chain reaction.
The other necessary ingredient is a specialized enzyme called Lachrymatory Factor Synthase, or LFS. This enzyme is held in a different part of the cell, ensuring no reaction occurs while the onion remains intact. This compartmentalization only activates when the integrity of the plant tissue is breached. The design ensures that the irritant is produced rapidly and on-demand to deter herbivores from consuming the bulb.
The Creation of the Burning Compound
The moment a knife slices into the onion, the cellular walls rupture, allowing the previously separated compounds to mix and trigger the reaction. An enzyme called alliinase immediately acts upon the amino acid sulfoxides, converting them into highly unstable intermediates known as sulfenic acids. This first step of the breakdown is rapid and forms the immediate precursor to the volatile gas.
The second and most significant step involves the Lachrymatory Factor Synthase (LFS) enzyme. The LFS enzyme intercepts the sulfenic acid and swiftly converts it into a small, airborne molecule called syn-propanethial S-oxide. This compound is the true culprit, a volatile liquid that quickly vaporizes and wafts into the air, seeking moisture to react with. The name syn-propanethial S-oxide refers to the lachrymatory factor. This two-step enzymatic process is highly efficient, transforming the stable sulfur compounds into a tear-inducing gas within seconds of cutting.
Why Your Eyes React
The volatile syn-propanethial S-oxide gas travels from the cutting board and makes contact with the surface of the eye. The gas readily dissolves in the tear film, which is a thin layer of water covering the cornea. This chemical reaction immediately forms a small amount of a mild acid, specifically sulfuric acid, right on the surface of the eye.
The presence of this acid stimulates the delicate sensory nerve endings in the cornea, which are part of the trigeminal nerve system. The body’s protective reflex is instantly activated, signaling the lachrymal glands to produce a flood of tears. This rush of watery tears is an involuntary attempt to dilute the acid and physically flush the irritant away from the sensitive eye tissue.
Practical Ways to Stop the Burn
Since the chemical reaction relies on an enzyme, one effective strategy is to reduce the enzyme’s activity by lowering the temperature. Chilling the onion in the refrigerator or freezer for 10 to 15 minutes before cutting can significantly slow the rate at which the LFS enzyme produces the irritating gas. Another simple method is to maximize the speed and minimize the cell damage by using a very sharp knife. A clean, swift cut breaks fewer cells than a dull blade that crushes the tissue, resulting in less rapid release of the precursor chemicals.
Manipulating the environment is also a powerful way to mitigate the effect of the gas once it is released. The irritating gas is attracted to water, so several methods focus on diverting the compound away from your tear film:
- Chill the onion in the refrigerator or freezer for 10 to 15 minutes before cutting to slow the LFS enzyme activity.
- Cut the onion under an extractor fan or near an open window to pull the volatile compound away from your face.
- Cut the onion under a slow stream of running water or next to a damp paper towel to divert the compound.
- Use specialized onion goggles or regular swimming goggles to prevent the vapor from reaching the eye’s surface entirely.
- Cut the root end last or remove it early, as this area holds the highest concentration of the sulfur compounds.