Slicing onions causes involuntary tearing. This common kitchen phenomenon is a natural defense mechanism of the onion, designed to deter pests and predators. The chemical reactions that occur when an onion’s cells are ruptured create a volatile compound that irritates the eyes, leading to a protective reflex.
The Chemical Culprit
Tears begin with chemical reactions triggered when onion cells are damaged, such as by a knife. Onions contain sulfur-containing compounds called amino acid sulfoxides. When the onion is cut, an enzyme known as alliinase, which is normally kept separate within the onion’s cells, is released. Alliinase then acts on the amino acid sulfoxides, converting them into sulfenic acids.
These sulfenic acids are highly unstable and quickly undergo a further transformation. Another specific enzyme, lachrymatory factor synthase (LFS), rearranges the sulfenic acids into a volatile chemical compound called syn-propanethial S-oxide. This compound is a gas that rapidly disperses into the air. Syn-propanethial S-oxide is the direct irritant responsible for the stinging sensation and subsequent tearing.
How Our Eyes React
When syn-propanethial S-oxide gas reaches the eyes, it dissolves in the tear film, the watery layer covering the eye’s surface. This reaction forms a mild sulfuric acid, which causes a burning and stinging sensation. The eye’s cornea, the outermost layer, is particularly sensitive to this compound.
This irritation stimulates nerve endings in the eye, specifically activating the trigeminal nerve, which is responsible for sensation in the face. The stimulation of these sensory neurons triggers a reflex action. In response, the lacrimal glands, located above each eye, produce a rush of “reflex tears.” These tears serve to dilute the irritant and wash it away from the eye’s surface to prevent potential damage.
Minimizing the Tears
Several practical methods can help reduce tearing when cutting onions. One effective strategy is to chill the onion in a refrigerator or freezer for about 30 minutes before cutting. Lower temperatures slow down the activity of the enzymes responsible for producing the tear-inducing compound, thus reducing the amount of gas released.
Cutting onions under running cold water or submerging them in water also helps, as the water absorbs the volatile gas before it can reach your eyes. Using a sharp knife is another beneficial technique. A sharp blade creates cleaner cuts, minimizing the damage to onion cells and reducing the release of the irritating enzymes and compounds. In contrast, a dull knife crushes more cells, leading to a greater release of the tear-inducing gas.
Additionally, ensuring good ventilation by cutting near an open window or under a range hood can help disperse the irritating compounds away from your face. Wearing protective eyewear, such as goggles, can also create a physical barrier to prevent the gas from reaching your eyes.