broccoli stem contains a lot of cellulose and will stay firm for a
long time even through the most vigorous cooking, but the cell walls
of the florets are not so strongly fortified and will soften,
eventually turning to mush if you cook the broccoli long enough.
other cruciferous vegetables, broccoli contains mustard oils (isothiocyanates),
natural chemicals that break down into a variety of smelly sulfur
compounds (including hydrogen sulfide and ammonia) when the broccoli
is heated. The reaction is more intense in aluminum pots. The longer
you cook broccoli, the more smelly compounds there will be, although
broccoli will never be as odorous as cabbage or cauliflower.
a lid on the pot will stop the smelly molecules from floating off into
the air but will also accelerate the chemical reaction that turns
green broccoli olive-drab.
Chlorophyll, the pigment that makes green vegetables green, is
sensitive to acids. When you heat broccoli, the chlorophyll in its
florets and stalk reacts chemically with acids in the broccoli or in
the cooking water to form pheophytin, which is brown. The pheophytin
turns cooked broccoli olive-drab or (since broccoli contains some
yellow carotenes) bronze.
broccoli green, you must reduce the interaction between the
chlorophyll and the acids. One way to do this is to cook the broccoli
in a large quantity of water, so the acids will be diluted, but this
increases the loss of vitamin C. Another alternative is to leave the
lid off the pot so that the hydrogen atoms can float off into the air,
but this allows the smelly sulfur compounds to escape, too. The best
way is probably to steam the broccoli quickly with very little water,
so it holds onto its vitamin C and cooks before there is time for
reaction between chlorophyll and hydrogen atoms to occur.
Freezing. Frozen broccoli usually contains less vitamin C than
fresh broccoli. The vitamin is lost when the broccoli is blanched to
inactivate catalase and peroxidase, enzymes that would otherwise
continue to ripen the broccoli in the freezer. On the other hand,
according to researchers at Cornell University, blanching broccoli in
a microwave oven—2 cups of broccoli in 3 tablespoons of water for
three minutes at 600-700 watts—nearly doubles the amount of vitamin C
retained. In experiments at Cornell, frozen broccoli blanched in a
microwave kept 90 percent of its vitamin C, compared to 56 percent for
broccoli blanched in a pot of boiling water on top of a stove.