Preparing Broccoli

First, rinse the broccoli under cool running water to wash off any dirt and debris clinging to the florets. Then put the broccoli, florets down, into a pan of salt water (1 tsp. salt to 1 qt. water) and soak for 15 to 30 minutes to drive out insects hiding in the florets. Then cut off the leaves and trim away woody section of stalks. For fast cooking, divide the broccoli up into small florets and cut the stalk into thin slices.

What Happens When You Cook Broccoli

The 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.

Like 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.

Keeping 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.

To keep 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.

How Other Kinds of Processing Affect Broccoli

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.