When an onion is cut, the resulting stinging sensation and immediate tears are caused by a precise chemical defense mechanism. This watery reaction is not a sign of emotional distress but a biological reflex triggered by physical damage to the plant cells. The irritation is the onion’s way of protecting itself, prompting the eyes to flush out a volatile irritant.
Storing the Necessary Compounds
The onion bulb contains sulfur-containing molecules, which are the precursors to the tear-inducing compound. These precursors are a class of amino acids called S-alk(en)yl cysteine sulfoxides. The onion keeps these chemical components safely separated from the necessary enzymes within its cells.
This structural separation ensures the onion remains harmless and odorless while whole. The chemical reaction only begins immediately after the cell walls are ruptured by cutting or crushing the tissue, allowing the components to mix.
The Enzyme That Starts the Reaction
Breaching the cell walls allows the stored sulfur compounds to mix with the first enzyme, alliinase (cysteine sulfoxide lyase). Alliinase cleaves the sulfoxides into unstable sulfenic acids, such as 1-propenyl sulfenic acid, which is a highly reactive intermediate molecule.
The reaction then moves to a second step involving the enzyme lachrymatory factor synthase (LFS). The LFS enzyme quickly converts the unstable sulfenic acids into the final volatile chemical. This two-step process differentiates the onion from related plants like garlic, which lacks the LFS enzyme.
The Chemical Irritant That Causes Tears
The final product of this enzymatic cascade is a volatile gas known as syn-propanethial S-oxide, which is the actual lachrymatory factor. This gas rapidly evaporates from the cut surface and travels into the air. When the gas reaches the eye, it dissolves instantly into the tear film, the thin layer of moisture covering the eyeball.
This dissolution initiates a secondary chemical reaction where the syn-propanethial S-oxide reacts with the water to form a mild sulfuric acid. The presence of this acid on the eye’s surface causes the burning and stinging sensation. This chemical stimulus activates sensory nerves, triggering a protective reflex.
The lachrymal glands respond by producing a flood of tears, a process called lacrimation. The biological purpose of this sudden watering is to dilute the mild sulfuric acid and physically flush the irritating syn-propanethial S-oxide away.
Simple Ways to Stop the Burning
Understanding the enzyme-driven process provides several methods to prevent the burning sensation. These strategies focus on slowing the reaction or preventing the volatile gas from reaching the eyes.
- Cooling the onion: Placing the onion in the refrigerator or freezer for 10 to 15 minutes before cutting slows the activity of the enzymes, significantly reducing the chemical reaction rate.
- Dispersing the gas: Since the irritant must reach the eye, cutting the onion near an open window, a kitchen exhaust fan, or a small directed fan pulls the gas away from your face.
- Using a physical barrier: Wearing kitchen goggles or contact lenses prevents the gas from contacting the tear film entirely.
- Cutting under water: Slicing the onion under running water or in a bowl of water prevents the irritant from becoming airborne, as the syn-propanethial S-oxide dissolves into the surrounding liquid.
- Using a sharp knife: A sharp knife causes less damage to the cell walls, minimizing the initial release of the precursor compounds.