Slicing an onion often causes involuntary tears, a universal kitchen phenomenon. This watery reaction is not emotional distress but a complex biochemical defense mechanism within the plant. When the onion’s structural integrity is compromised, it releases a highly volatile chemical compound into the air. Understanding the specific molecules and enzymes involved reveals the scientific reason why this common vegetable makes us cry.
The Specific Irritant: The Lachrymatory Factor
The stinging sensation in the eyes is caused by the chemical compound syn-Propanethial S-oxide, often called the Lachrymatory Factor (LF). This sulfur-containing gas is highly volatile, quickly vaporizing and traveling through the air.
When the gas contacts the eye’s surface, it dissolves into the thin layer of water, or tear film, covering the cornea. This reaction forms a dilute solution of sulfuric acid, which acts as the irritant. The acid stimulates sensory nerve endings, prompting the lachrymal glands to produce tears to dilute and flush the irritant away.
The Enzymatic Process That Releases the Tears
The irritant is not present in the intact onion but is generated only when the plant’s cells are damaged. Onions store sulfur-containing precursor molecules, specifically S-alk(en)yl-L-cysteine sulfoxides, in their vacuoles. These precursors are kept separate from the enzymes that act upon them.
When a knife slices through the layers, cellular compartments rupture, allowing the precursors to mix with the enzyme alliinase (cysteine sulfoxide lyase). The alliinase rapidly breaks down the sulfur compounds into unstable sulfenic acids, notably (E)-1-propenesulfenic acid. This intermediate is necessary for the tear agent’s formation.
A second enzyme, lachrymatory factor synthase (LFS), then acts on the sulfenic acid intermediate. LFS catalyzes a rearrangement reaction, transforming the sulfenic acid into the volatile syn-Propanethial S-oxide gas. This final product is the tear-inducing chemical released into the atmosphere, triggering reflexive tears.
Practical Methods to Stop Onion-Induced Crying
Mitigating the tear response requires interfering with either the enzymatic reaction or the path of the volatile gas. One method is to chill the onion in the refrigerator for about 30 minutes before cutting. Lower temperatures slow the activity of the enzymes, reducing the rate at which the irritant gas is produced.
Another technique is to minimize mechanical damage to the onion’s cell walls. Using a very sharp knife creates cleaner cuts that rupture fewer cells than a dull blade, which tends to crush the tissue. A sharp knife limits the “droplet outburst” of irritant-laden aerosols propelled toward the eyes.
Controlling the gas after its release is also possible. Since the irritant is airborne, cutting the onion near an exhaust fan or under a gentle stream of running water can divert or absorb the volatile compound. The moving air or water surface helps dissipate the gas before it reaches the eye’s moisture layer and causes irritation.