The familiar sting and sudden flow of tears when slicing an onion is a common kitchen experience. This powerful reaction is not a physical injury but a rapid and complex chemical defense mechanism unique to the Allium genus of plants. The tear-inducing effect is the result of a precise sequence of biological events that begins the moment the onion’s cellular structure is broken by a knife.
Storing the Necessary Ingredients
An intact onion bulb stores all the components required for its defense system in separate compartments within its cells. The precursors to the irritating gas are stable, sulfur-containing molecules known as S-alk(en)yl cysteine sulfoxides. These compounds are concentrated within the cytoplasm of the onion cells. A key enzyme, alliinase, is separately sequestered in the vacuole. This physical separation ensures that no chemical reaction occurs until the onion tissue is damaged.
The Enzymatic Chain Reaction
The process that generates the tear-inducing gas begins immediately upon cutting, which ruptures the cell membranes and allows the separated compounds to mix. The alliinase enzyme rapidly catalyzes the breakdown of the stored S-alk(en)yl cysteine sulfoxides. This initial conversion produces unstable intermediate molecules known as sulfenic acids. Specifically, the precursor compound is converted into 1-propenesulfenic acid.
This sulfenic acid intermediate is then acted upon by a second, specialized enzyme called lachrymatory factor synthase (LFS). The LFS enzyme converts the sulfenic acid into the final volatile irritant. This end product is a highly reactive chemical called syn-propanethial S-oxide, often referred to as the lachrymatory factor.
Syn-propanethial S-oxide is a volatile compound, meaning it readily evaporates and diffuses into the surrounding air. This chemical transformation is fast, allowing the onion to generate a cloud of irritant gas within seconds of being cut. The entire two-step enzymatic process is the onion’s self-defense strategy.
How Tears Are Triggered
Once the volatile syn-propanethial S-oxide is released, it travels through the air and contacts the surface of the eye. The chemical dissolves instantly in the thin layer of watery tear film protecting the eye. The dissolved irritant then stimulates the sensory nerve endings located in the cornea, which are branches of the trigeminal nerve.
The brain interprets this chemical stimulation as a warning signal. In response to this irritation, the central nervous system rapidly initiates a protective mechanism. This reflex triggers the lachrymal glands to produce a sudden and copious flow of tears. These “reflex tears” serve a direct, functional purpose to dilute and physically flush the irritating syn-propanethial S-oxide away from the eye.
Simple Ways to Reduce Tearing
Several simple techniques can interfere with the chemical reaction or the path of the irritant to the eye. One effective method is chilling the onion in the refrigerator or freezer for a short time before slicing. Lowering the temperature slows the kinetic rate of the enzymes (alliinase and LFS), which reduces the speed at which syn-propanethial S-oxide is produced.
Cutting the onion under running water or near a strong kitchen vent also helps to minimize the irritant’s effect. Water is highly effective at dissolving the water-soluble syn-propanethial S-oxide, capturing the molecules before they can reach your eyes. A draft from a fan or vent draws the volatile gas away from your face, preventing trigeminal nerve stimulation. Using a very sharp knife creates cleaner cuts, minimizing the rupture of cell walls and limiting the overall release of the reactive enzymes.