Cutting an onion often brings tears to the eyes, a familiar reaction for many home cooks. However, some individuals find their eyes remain dry, even when chopping pungent varieties. This difference raises questions about the specific reasons behind this common kitchen phenomenon.
The Science of Onion-Induced Tears
When an onion is cut, its cells are damaged, releasing sulfur-containing compounds and enzymes that were previously stored separately. One key enzyme, alliinase, acts on amino acid sulfoxides, such as isoalliin, to produce sulfenic acids. These sulfenic acids are highly unstable and quickly rearrange.
Another enzyme, lachrymatory factor synthase (LFS), then converts a specific sulfenic acid, 1-propenyl sulfenic acid, into syn-propanethial S-oxide. This volatile chemical, also known as the lachrymatory factor, is a gas that easily vaporizes into the air. When syn-propanethial S-oxide reaches the eyes, it dissolves in the moisture covering the eyeballs, forming a mild sulfuric acid. This acid irritates the sensory nerves in the eyes, signaling the brain to trigger a protective tear response to wash away the irritant.
Why Eyes May Not Water
Not all onions are equally potent; the concentration of lachrymatory compounds varies significantly by type. Sweet onions, like Vidalia, Maui, and Walla Walla, have lower levels of these tear-inducing chemicals compared to pungent yellow or red varieties. This genetic difference often results in a milder reaction, or none at all. The age of the onion also plays a role, with fresher onions sometimes causing less irritation than those that have been stored for extended periods.
The way an onion is cut directly impacts irritant release. A sharp knife makes clean cuts, minimizing cell damage and reducing the amount of enzymes and precursors released. A dull knife crushes cells, leading to greater expulsion of tear-inducing compounds. Cutting onions under running water, near a strong kitchen vent, or with a fan can help disperse or dissolve irritating gases before they reach the eyes.
People exhibit a range of sensitivities to airborne irritants, and the lachrymatory factor from onions is no exception. Some individuals naturally possess less reactive sensory nerves in their eyes, requiring a higher concentration of the irritant to trigger a tear response. Frequent exposure to onion fumes may also lead to slight desensitization over time, diminishing the intensity of the tearing reflex for some individuals.
Wearing glasses or contact lenses can inadvertently provide a physical shield against the volatile compounds. These barriers can reduce the direct contact of syn-propanethial S-oxide with the eye’s surface and the delicate nerve endings. This physical separation limits the amount of irritant reaching the lacrimal glands, thereby preventing or lessening tear production. Specialized onion goggles are also available to create a sealed barrier around the eyes.
Kitchen airflow affects how quickly irritants dissipate. A well-ventilated area, such as one with an extractor fan running or a window open, can draw away airborne compounds before they accumulate sufficiently to cause irritation. This rapid dispersion reduces the concentration of the lachrymatory factor around the face, mitigating its effect. Chilling onions in the refrigerator or freezer for a few minutes before cutting can also help, as colder temperatures slow the release of these volatile compounds.