First, drop the sprouts into salted ice water to flush out any
small bugs hiding inside. Next, trim them. Remove yellow leaves and
leaves with dark spots or tiny holes, but keep as many of the darker,
vitamin A—rich outer leaves as possible. Then, cut an X into the stem
end of the sprouts to allow heat and water in so that the sprouts cook
Easter.
Brussels sprouts contain mustard oils (isothiocyanates), natural
chemicals that break down into a variety of smelly sulfur compounds
(including hydrogen sulfide and ammonia) when the sprouts are heated,
a reaction that is intensified in aluminum pots. The longer you cook
the sprouts, the more smelly compounds there will be. Adding a slice
of bread to the cooking water may lessen the odor; keeping a lid on
the pot will stop the smelly molecules from floating off into the air.
But keeping the pot covered will also increase the chemical
reaction that turns cooked brussels sprouts drab. Chlorophyll, the
pigment that makes green vegetables green, is sensitive to acids. When
you heat brussels sprouts, the chlorophyll in their green leaves
reacts chemically with acids in the sprouts or in the cooking water to
form pheophytin, which is brown. The pheophytin turns cooked brussels
sprouts olive or, since they also contain yellow carotenes, bronze.
To keep cooked brussels sprouts green, you have to reduce the
interaction between chlorophyll and acids. One way to do this is to
cook the sprouts in a lot 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 solution is to steam the sprouts quickly in very little water, so
they retain their vitamin C and cook before there is time for reaction
between chlorophyll and hydrogen atoms to occur.