Simply peeling an onion won’t make your eyes water. But if you chop, cut, crush or smash one — boohoo. The onion’s cells break open, allowing two normally separated substances to combine. Linked together like pieces of a puzzle, they become a potent chemical weapon.
Lachrymatory factor evolved as a defense mechanism, protecting onions against microbes and animals like us, even if we’ve learned to bear tears for the sake of flavor. Damaging an onion basically causes it to ramp up its defenses: as cells break, the chemical reaction is unlocked. Inside the intact cells of an onion, a molecule called sulfenic acid precursor floats around the watery filler like a napping human in a lazy river. Also floating in that cytoplasm are little sacs called vacuoles, containing a protein called alliinase, which is like a little drill sergeant of the process.
“One has not seen the other, but if you damage the cells, they can now meet and make these reactions,” said Marcin Golczak, a biochemist at Case Western Reserve and principal investigator of the latest study. The molecule and the protein fit together perfectly, the chemical structure of the molecules change, and that lazy floater becomes a smelly soldier armed with tear gas.
