Who doesn’t know it: the onion is peeled and half cut into cubes, and the first tears are already falling. Onions make us cry. But why actually? In his new book, “The Core of an Onion,” Mark Kurlansky explores the reasons for this.
Onions are actually harmless unless you attack them, Kurlansky explains. Most don’t even smell intense. But if you cut them, they fight back. The vengeful onion throws a poisonous substance into the eyes – it is a low-molecular substance with sulfur atoms, which are extremely rare in nature. So is it just sulfur compounds that make us cry? Kurlansky says it’s more complicated: The molecules are “highly reactive,” meaning they change very easily. The most famous and probably most unpleasant change is that it causes tears and makes us cry.
The onion actually does nothing other than defend itself. The molecules dissolve in tears and become sulfuric acid, a nasty little trick designed for defense. The compound activates nerve endings in the cornea, which signals pain to the brain. The purpose of this signal is to tell us to stop doing what is causing the pain. Animals are deterred by this, but we humans are not deterred by it.
However, the sulfur compound that causes tears is different than the one that gives it the smell and strong taste. The pain comes from the odorless lacrimator, the tear producer. The pungent taste and smell is created by a group of compounds that combine unstable sulfonic acids, ammonia and pyruvic acid. After consumption, these sulfur compounds escape through exhalation and sweating. This means bad breath and sometimes unpleasant body odor.
However, all of these sulfur compounds are so unstable that they can be easily converted. Heat, for example, changes it completely, which is why a cooked onion tastes and smells nothing like a raw onion. The new compound can be fifty times sweeter than sugar. This transformation from the spicy, raw onion is probably why there have been so many recipes for baking, braising, and roasting whole onions for centuries.
