By Harold McGee
Like onions, cabbages and their relatives stockpile two kinds of defensive chemicals in their tissues: flavor precursors, and enzymes that act on the precursors to liberate the reactive flavors. When the plant’s cells are damaged, the two stockpiles are mixed, and the enzymes start a chain of reactions that generates bitter, pungent, and strong-smelling compounds. The special cabbage-family system is effective enough to have inspired a notorious man-made version, the mustard gas of World War I. And the cabbage family turns out to have parts of the onion defensive system as well; these contribute some sulfur aromatics to the overall family flavor.