Like the onion family, the cabbage family is a group of formidable chemical warriors with strong flavors. It's also a uniquely protean family. From two weedy natives of the Mediterranean and central Asia, we have managed to develop more than a dozen major crops of very different kinds: some leaves, some flowers, some stems, some seeds. Then there are a dozen or more relatives, notably the radishes and mustards, and crosses between species: altogether a rich and ongoing collaboration between nature's inventiveness and our own. Beyond the cabbage family itself, some of its distant botanical relatives share elements of its biochemistry and therefore its flavors; these include capers and papayas.
The Flavor Chemistry of the Cabbage Family
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 enoughto have inspired a notorious man-made version, the mustard gas of World War I ...
The stockpiled, defensive precursors in the cabbage family are called glucosinolates... Some of the flavor precursors and products are very bitter, and some have significant effects on our metabolism. Particular [precursors] interfere with the proper function of the thyroid gland and can cause it to enlarge if the diet is poor in iodine. But others help protect against the development of cancer by finetuning our system for disposing of foreign chemicals. This is the case for substances in broccoli and broccoli sprouts.
A given vegetable will contain a number of different precursor glucosinolates, and the combinations are characteristic. this is why cabbage, brussels, sprouts, broccoli, and mustard greens have similar but distinctive flavors. the chemical defensive system is most active -- and the flavor strongest -- in young, actively growing tissues: the center of brussels sprouts, fr example, and portions of the cabbage core, which are twice as active as the outer leaves. Growing conditions have a strong influence on the amounts of flavor precursors the plant stockpiles. Summer temperatures and drought stress increase them, while the cold, wetness, and dim sunlight of autumn and winter reduce them. Autumn and winter vegetables are usually milder.On Food and Cooking by Harold McGee