The rowdy onion joins the aristocratic shallot, the gentle leek, the herbaceous chive, sharp scallion and assertive garlic among the 500 species of the genus Allium. Allium cepa is an ancient vegetable, known to Alexander the Great and eaten by the Israelites during their Egyptian bondage. Indeed, his charges chastened Moses for leading them away from the onions and other flavorful foods that they had come to relish while in captivity. And with good reason: onion is a rich source of nutrients, including vitamins B, C , protein, starch and a series of essential elements. The chemicals contained in onions are reported to be effective agents against fungal and bacterial growth; they protect against stomach, colon and skin cancers; they have anti-inflammatory, antiallergenic, antiasthmatic and antidiabetic actions; and they treat causes of cardiovascular disorders, including hypertension, hyperglycemia and hyperlipidemia while also inhibiting platelet aggregation.
The price of this goodness is tears. The volatile oils that help to give Allium vegetables their distinctive flavors contain a class of organic molecules known as amino acid sulfoxides. Peeling, cutting or crushing an onion’s tissue releases enzymes called allinases, which convert these molecules to sulfenic acids. The sulfenic acids, in turn, spontaneously rearrange to form syn-propanethial-S-oxide, the chemical that triggers the tears. syn-Propanethial S-oxide (C3H6OS) (IUPAC: 1-Sulfinylpropenea), member of a class of organosulfur compounds known as thiocarbonyl S-oxides (formerly “sulfines”),is a gas that acts as a lachrymatory agent (triggers tearing and stinging on contact with the eyes).They also condense to form odorous thiosulfinates, coincidentally evoking the pungent odor associated with chopping onions and eliciting the false accusation that it is the odor that causes the weepy eye. Incidentally, sulfenic acid in garlic takes a different chemical route, sparing the eyes. The formation of syn-propanethial-S-oxide peaks at about 30 seconds after mechanical damage to the onion and completes its cycle of chemical evolution over about five minutes.
Its effects on the eye are all too familiar. The front surface of the eye–the cornea–serves several purposes, among them protection against physical and chemical irritants. The cornea is densely populated with sensory fibers of the ciliary nerve, a branch of the massive trigeminal nerve that brings touch, temperature and pain sensations from the face and front of the head. The cornea also receives a smaller number of autonomic motor fibers that activate the lachrymal (tear) glands. Free nerve endings detect syn-propanethial-S-oxide on the cornea and drive activity in the ciliary nerve–which the central nervous system interprets as a burning sensation–in proportion to the compound’s concentration. This nerve activity reflexively activates the autonomic fibers, which then carry a signal back to the eye ordering the lachrymal glands to wash the irritant away.
There are several solutions to the problem of onion tears. You can heat onions before chopping to denature the enzymes. You might also try ways to limit contact with the vapors: chop onions on a breezy porch, under a steady stream of water or mechanically in a closed container.