Onions are mild unless attacked. Most do not even have a strong smell. But bitten or cut, they retaliate. The vengeful onion sends a toxic spittle into your eyes—it is a low-molecular-weight substance with sulfur atoms, which is an extremely rare way for chemicals to present themselves in nature. It is said that they are spewing sulfur. But to say that is to ignore the unusual complexity of the operation. The molecules are “highly reactive”; in other words, they change very easily. The most famous change, the one we care about, is that they become lacrimatory; they induce tears, make you cry.
Onions are designed to fight against mammals. The molecules dissolve into the water of the eyes and turn into sulfuric acid, a nasty little trick designed for defense. The compound activates nerve endings in the cornea that send a message to the brain translated as pain. The purpose of all pain messages is to tell you to stop whatever is causing it. It teaches most animals to stay away—but humans are undeterred.
This sulfur compound is completely different from the compound that gives the onion its smell and strong taste. You smell the onion as you feel the sting so it seems to be the same thing, but it is not. The smell of onions does not hurt your eyes. The pain comes from that odorless lacrimator, that tear inducer. An entirely different set of compounds—combining unstable sulfonic acids and ammonia and pyruvic acid—produce the pungent flavor and smell. Once it has been ingested, this sulfur compound will try to escape through expiration and perspiration. That is, unpleasant breath and sometimes body odor.
But because all these sulfur compounds are so unstable, they do not hang out for long. They are easily transformed. Heat, for example, completely changes them, which is why a cooked onion does not taste or smell or act anything like a raw onion. The new compound can be more than fifty times sweeter than table sugar. This transformation from the harsh raw onion is probably why there have always been so many recipes for baking, stewing, and roasting whole onions throughout the centuries. There has been a sense that onions are a tough and harsh thing that needs to be tamed, that they cause pain but are worth it, that, in the words of Pablo Neruda, they make you “weep without suffering.”
The compounds that cause pain or give off strong odors or flavor, the traits for which the onion is famous, are not vital to the life of an onion. They are extravagant extras, what is known in botany as a secondary metabolite. Primary metabolites are ones that are necessary for growth, development, or reproduction. But this secondary chemistry is only useful for defense.
In botany an onion is known as genus Allium, a name first given in 1753 by Carl Linnaeus, who established the naming system for the natural order. One of the most numerous plant genera, there are between 600 and 750 different species of Allium. The word allium may come from a Celtic word meaning “strong flavored.” Others say it is from the Greek aleo, “to avoid,” because of the pungency. The Celtic version is more pleasant but the Greek more likely.
Onion recipes throughout the centuries have offered a variety of strategies to ease the pain or even avoid tearing. A Chinese text from the Song dynasty (960–1279) recommended ginger and jujubes, sometimes called Chinese dates. The parsley cure for both breath and tearing eyes was suggested in 1629 by London apothecary John Parkinson in his Paradisi in Sole, which also prescribed onion juice to heal burns.
Florence Irwin, a traveling Irish “domestic science” instructor in the early 1900s, taught her students to remove the sting from onions by a technique called scalding. This was her technique, which actually works: “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 suggest that the pain to the eyes can be reduced by running water near them. This seems a folk cure, but it has a scientific underpinning: Just as the sulfur compound is drawn to the water of the eyes, it will be drawn away to the tap water. Keeping onions cold in the refrigerator can also help, because cold reduces the onion’s ability to release its gasses.
Using a sharp knife is a good idea too, because the sharper the knife, the fewer cells are disturbed, but the onion can still make you cry. It is sometimes suggested that lighting a match, holding an unlit match in your teeth, holding a crust of bread in the mouth, or biting on the handle of a wooden spoon can help. There is not much science behind those solutions.
I myself have found some success with the running water or refrigeration, and none with the bread, match, or wooden spoon. Another thing that I have found helpful that, oddly, is rarely mentioned, is to protect your eyes by wearing glasses. Onion goggles are available too. But remember, the onion’s defense system is powerful: If the compound can get to your nose, it has a pathway to your eyes.
From The Core of an Onion: Peeling the Rarest Common Food – Featuring More Than 100 Historical Recipes by Mark Kurlansky, releasing November 7 from Bloomsbury Publishing. Copyright © Mark Kurlansky, 2023. All rights reserved.