Onions are grown and used worldwide; anyone who has cut into one knows it can make you cry. This happens because onions release an irritating chemical that makes your eyes sting.

The Truth Behind Onions

Onions, typically mild and unassuming, reveal their potent side when provoked. In their undisturbed state, they lack a robust aroma. However, when bitten or cut, they unleash a vengeful response, sending a toxic spittle into your eyes.

This spittle, a low-molecular-weight substance with sulfur atoms, presents a rare chemical occurrence in nature. Though described as “spewing sulfur,” the complexity of this operation goes beyond a simple label.

The molecules involved are highly reactive, easily undergoing changes. The most noteworthy transformation, and the one of concern, is their shift to a lacrimatory state, inducing tears and causing distress.

Onions are strategically designed to fend off mammals. Their molecules dissolve in eye water, transforming into sulfuric acid—a cunning defense mechanism.

This compound activates nerve endings in the cornea, sending pain signals to the brain. The purpose of such pain messages is to prompt the cessation of whatever is causing discomfort. While effective in deterring most animals, humans, undeterred, persist.

The sulfur compound responsible for the eye irritation differs significantly from the one contributing to the onion’s distinctive smell and taste.

Although the smell accompanies the sting, it originates from a different, odorless lacrimator—a tear-inducing substance.

Another set of compounds, combining unstable sulfonic acids, ammonia, and pyruvic acid, creates the pungent flavor and smell. Once ingested, the sulfur compound attempts to escape through breath and perspiration, resulting in unpleasant breath and occasional body odor.

Due to the inherent instability of these sulfur compounds, they undergo rapid transformations. Heat, for example, entirely alters them, explaining why a cooked onion differs vastly from its raw counterpart. The newly formed compound can be more than fifty times sweeter than table sugar.

This transformation, turning a harsh raw onion into a more palatable version, is likely why various recipes throughout history involve baking, stewing, and roasting whole onions.

The compounds responsible for pain, strong odors, and flavor—the traits that make onions famous—are not essential to the onion’s survival. They are extravagant extras, termed secondary metabolites in botany.

Primary metabolites are necessary for growth, development, or reproduction, whereas these secondary compounds serve only defensive purposes.

In the realm of botany, onions belong to the genus Allium, a term bestowed by Carl Linnaeus in 1753 when establishing the naming system for the natural order. With between 600 and 750 different species, Allium may derive from a Celtic word meaning “strong-flavored” or from the Greek “aleo,” meaning “to avoid,” reflecting its pungency.

Throughout history, various onion recipes have proposed strategies to alleviate pain or prevent tearing. A Chinese text from the Song dynasty recommended ginger and jujubes, while a 1629 suggestion by London apothecary John Parkinson involved parsley for both breath and tearing eyes, alongside onion juice for burns.

Florence Irwin, a traveling Irish “domestic science” instructor in the early 1900s, advocated a scalding technique to remove the sting from onions: “Peel the onions, place in a basin, add a pinch of salt. Cover with fast boiling water. Leave about one minute. Strain off the water.”

Some propose that running water near the eyes reduces eye pain—a folk cure with a scientific basis. Cold storage in the refrigerator can also help by diminishing the onion’s gas-releasing ability.

Using a sharp knife minimizes cell disturbance, although it doesn’t entirely prevent tears. Other suggestions, like lighting a match, holding bread in the mouth, or biting a wooden spoon handle, lack substantial scientific backing.

You may also succeed with running water or refrigeration, but none with bread, matches, or wooden spoons. Interestingly, wearing glasses—rarely mentioned—provides additional eye protection. However, it’s crucial to remember that the onion’s defense system is potent; if the compound reaches your nose, a pathway to your eyes exists.

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