The Cabbage Family: the Effects of Heat

Why does the whole house smell like cabbage (or broccoli or cauliflower) after you've cooked it for dinner? Here ya go...

Heating cabbages and their friends has two different effects. Initially the temperature rise within the tissue speeds the enzyme activity and flavor generation, with the maximum activity at around 140°F/60°C. The enzymes stop working altogether somewhere short of the boiling point. If the enzymes are quickly inactivated by plunging the vegetables into abundant boiling water, then many of the flavor precursor molecules will be left intact. This isn't always desirable: cooking some mustard greens quickly, for example, minimizes their hot pungency but preserves the intense bitterness of their pungency precursors. Boiling in a large excess of water leaches flavor molecules out into the water, and produces a milder flavor than does stir-frying or steaming. If the cooking period is prolonged, then the constant heat gradually transforms the flavor molecules. Eventually the sulfur compounds end up forming trisulfides, which accumulate and are mainly responsible for the strong and lingering smell of overcooked cabbage. Prolonged cooking makes members of the onion family more sweet and mellow, but the cabbage family gets more overbearing and unpleasant.

On Food and Cooking by Harold McGee