The familiar sting and sudden rush of tears while chopping an onion result from a precise and rapid chemical chain reaction. This lachrymatory response is an effective defensive mechanism developed by the plant to deter pests and microbes. The involuntary watering of the eyes is a biological attempt to flush away a potent airborne irritant. This irritant is created only when the onion’s cellular structure is damaged, initiating a complex biochemical sequence.
The Onion’s Stored Precursors
The potential for irritation is contained within the onion’s cells as odorless, sulfur-rich molecules. These molecules are amino acid derivatives known as S-alk(en)yl cysteine sulfoxides, primarily isoalliin. These precursor compounds are stable and non-volatile as long as the onion remains whole. The onion keeps these precursors and the necessary enzymes physically separated within different cellular compartments. When the cell is ruptured by cutting, this protective separation is instantly lost, initiating the chemical cascade.
Enzymatic Transformation
The first step begins immediately after the knife breaks the cell walls. A stored enzyme, alliinase (cysteine sulfoxide lyase), is released and quickly encounters the S-alk(en)yl cysteine sulfoxides. Alliinase acts as a catalyst to cleave the precursor molecules, resulting in intermediate compounds called sulfenic acids, specifically 1-propenesulfenic acid. These sulfenic acids are unstable and would normally condense to form thiosulfinates, which contribute to the onion’s flavor and aroma. However, a second, specialized enzyme interrupts this process to create the tear-inducing chemical.
The Crying Culprit and Nerve Response
The second enzyme is Lachrymatory Factor Synthase (LFS), the direct cause of eye irritation. LFS quickly intercepts the unstable sulfenic acids before they decompose into flavor compounds. This enzyme acts as a biological switch, redirecting the chemical pathway toward defense. LFS converts the 1-propenesulfenic acid intermediate into the final volatile irritant: Syn-propanethial S-oxide.
The Protective Reflex
Syn-propanethial S-oxide is a highly volatile gas that diffuses out of the damaged onion tissue and into the surrounding air. When the gas reaches the eyes, it dissolves in the moist surface of the cornea and conjunctiva. This creates a mild form of sulfuric acid that stimulates the sensory nerves. The brain registers this irritation, triggering a protective reflex that causes the lacrimal glands to produce tears to wash away the chemical irritant.
Practical Mitigation Techniques
Reducing tears relies on techniques that either slow the chemical reaction or prevent the volatile gas from reaching the eyes. Using a very sharp knife is effective, as it causes less cellular damage and a cleaner cut, minimizing the release of enzymes and precursors. Sharp blades reduce the physical force needed, which lessens the outburst of irritant-laden mist into the air.
Another successful strategy involves introducing a physical barrier or a means of dispersion. Cutting the onion under running water or submerging it allows the water to absorb the Syn-propanethial S-oxide before it becomes airborne. Alternatively, utilizing a kitchen exhaust fan or cutting near an open window can pull the gas away from the face.
Traditional advice to chill the onion is based on slowing the enzyme activity. However, controlling the physical spray of droplets with a sharp, slow cut is generally more directly effective.