The green color in plants and algae is a miraculous molecule called chlorophyll that can absorb the energy of sunlight and use it to convert carbon dioxide and water into glucose and oxygen gas.
The plants then either use the glucose directly for their own growth energy or polymerize it (connect thousands of glucose molecules together) to form starches, which they store for future use. And because animals derive their vitality from eating those sugar and starch carbohydrates in plants, the chlorophyll molecule can be thought of as the source of most life on Earth.
But chlorophyll is a fickle friend to humans, the only species that cooks its plant foods to tenderize them. For when we do, the green color can become a dreary, unappetizing khaki. What happens is that the chlorophyll turns into chemicals called pheophytins.
A chlorophyll molecule consists of a conglomeration of carbon, hydrogen, oxygen, and nitrogen atoms called a porphyrin (POR-ferin), with a magnesium atom buried in the center. But chlorophyll isn’t a single chemical compound. There are two main types, which chemists, always eager to demonstrate their literacy, have named chlorophyll a and chlorophyll b. Chlorophyll a is blue-green, while chlorophyll b is a yellowish-green. Different ratios of a’s to b’s (most often two or three a’s to each b) determine the exact hues of various green plants.
When we cook our green beans, peas, Brussels sprouts, broccoli, or spinach, the heat first changes the shapes of the chlorophyll molecules (they isomerize), and if the vegetable is slightly acidic, as most vegetables are, the magnesium atoms may be ousted and replaced by a couple of the acid’s many hydrogen atoms.
This transforms the chlorophylls into chemicals called pheophytins. Chlorophyll a turns into a grayish-green pheophytin and chlorophyll b turns into an olive-green one. Because chlorophyll a is usually more prevalent and undergoes this change more rapidly than chlorophyll b, the grayish green color is what we get.
The fact that acids initiate the chlorophyll-conversion reactions has on occasion tempted people to add a pinch of baking soda (sodium bicarbonate) to the cooking water to make it alkaline. But alkalinity attacks the complex carbohydrates that cement the vegetables’ cells together, so one is merely trading ugliness for mushiness, with the dubious bonus of a soapy flavor from the bicarbonate.
Another chemical oddity in cooking green vegetables is that sodium, magnesium, and calcium salts inhibit the chlorophyllconversion reactions, presumably by making it more difficult for the hydrogen atoms to get through the cell membranes and oust the magnesium atoms. Thus, salted cooking water (containing sodium chloride) and hard water (containing magnesium and calcium salts) help to retain the green color.
The practical message in all this is that the quicker a green vegetable is cooked, the less of its chlorophyll can change to muddy-colored pheophytins. In one study, broccoli lost 17.5 percent of its chlorophyll after being cooked for five minutes and 41.1 percent after ten minutes.