Cutting an onion often leads to watery eyes. This common kitchen phenomenon often leaves us wondering about the underlying reasons for our watery eyes. What might seem like a simple inconvenience is a complex biological defense mechanism within the onion itself, triggering a specific response in our bodies.
The Onion’s Chemical Secret
Onions absorb sulfur from the soil, which they then convert into various sulfur-containing compounds. When an onion’s cells are ruptured, such as during slicing, these compounds are released. Among them is a precursor molecule, S-1-propenyl-L-cysteine sulfoxide, which is crucial to the tear-inducing process. This compound then transforms into sulfenic acids, which are volatile molecules. The primary chemical irritant responsible for causing tears is known as syn-propanethial-S-oxide. This compound is not present in an intact onion but is rapidly formed once the onion is damaged. It is a potent airborne molecule that quickly diffuses from the cut surface.
The Enzyme’s Role
The formation of syn-propanethial-S-oxide is initiated by an enzymatic reaction triggered when an onion’s cells are broken. When the onion is cut, the enzyme alliinase is released from the damaged cells. This enzyme acts upon the amino acid sulfoxides, including S-1-propenyl-L-cysteine sulfoxide, converting them into sulfenic acids. Following this initial step, a second, more specific enzyme comes into play: lachrymatory-factor synthase (LFS). Lachrymatory-factor synthase then catalyzes the conversion of the unstable sulfenic acids into the volatile syn-propanethial-S-oxide gas. This gas quickly disperses into the surrounding air, making its way toward our eyes.
Why Our Eyes React
When the airborne syn-propanethial-S-oxide reaches the eyes, it acts as an irritant. This volatile compound comes into contact with the thin layer of water covering the surface of the eyeballs. Upon contact, a chemical reaction occurs, and the syn-propanethial-S-oxide transforms into a dilute form of sulfuric acid. This acid irritates the delicate nerve endings within the cornea, triggering a protective reflex mechanism. The lacrimal glands are stimulated to produce tears. These tears serve to dilute the irritant and wash it away from the eye’s surface.
Simple Ways to Cut Onions Without Tears
Several practical methods can help reduce or prevent the tear-inducing effects of cutting onions:
- Chill the onion for 15-30 minutes to slow enzyme activity and reduce gas release.
- Use a very sharp knife for cleaner cuts, minimizing cell damage and irritant release.
- Cut under running water or soak the onion to dissolve or trap irritating compounds.
- Improve air circulation by cutting near an open window, under a hood, or with a fan.
- Wear protective eyewear like goggles to prevent compounds from reaching your eyes.
- Leave the root end intact until last, as it contains a higher concentration of tear-producing compounds.