Eating of the Green : Keep It Green
Loving green food is one thing. Keeping it green once it’s cooked is quite another.
Unfortunately, no matter how vivid the spinach might look in the market, it has a tendency to turn a disturbing shade of olive drab by the time it reaches the plate.
According to food science maven Harold McGee, green vegetables pass through two stages while being cooked. First, they turn a very vivid, bright green as gases trapped in spaces between the plant’s cells collapse, giving a clearer view of the chloroplasts that give the plants their color. Fine so far.
But we rarely let well enough alone. After that brief cooking, the chlorophyll in the plant begins going through chemical changes. In the presence of acid (water with lemon juice, for instance, or even the plant’s own natural acidity), it turns grayish green. The exception is if there is also zinc or copper in the water, in which case, it will turn an even brighter green.
So we readily see two possible cures for boiling green vegetables. First, cook it in alkaline water (either by adding baking soda or by using a naturally alkaline “Vichy” water), or by cooking the vegetables in an unlined copper pot or with a copper penny added to the water.
Both methods have been used for centuries, though both are somewhat out of favor these days. Cooking vegetables in alkaline water also breaks down the cell walls more quickly, making them mushy (though vividly green--at least until the chlorophyll begins leaking), and leaching vitamins. And--in large quantities anyway--copper is a poison.
McGee solves the problem by cooking green vegetables for moderate lengths of time (5 minutes to 7 minutes) in large amounts of rapidly boiling water. This does two things. First, it does not cook the vegetables long enough to destroy the cell walls. More interestingly, the large amount of water dilutes the plant’s natural acidity and color-destroying enzymes. (That is also why it is important to never cook green vegetables with the lid on--those acids and enzymes will condense on the underside of the lid and fall back onto the vegetables.)
Of course, it also helps to have a pan of iced water sitting beside the boiling water to dump the blanched vegetables into. This brings the cooking process to an immediate halt, “freezing” the color in place.