The Enemy of Green: Acids

Green chlorophyll is susceptible to two chemical changes during cooking. One is the loss of its long carbon-hydrogen tail, which leaves the pigment water-soluble—so that it leaks out into the cooking liquid—and more susceptible to further change. This loss is encouraged by both acid and alkaline conditions and by an enzyme called chlorophyllase, which is most active between 150–170°F/66–77°C and only destroyed near the boiling point. The second and more noticeable change in chlorophyll is the dulling of its color, which is caused when either heat or an enzyme nudge the magnesium atom from the center of the molecule. The replacement of magnesium by hydrogen is by far the most common cause of color change in cooked vegetables. In even slightly acidic water, the plentiful hydrogen ions displace the magnesium, a change that turns chlorophyll a into grayish-green pheophytin a, chlorophyll b into yellowish pheophytin b. Cooking vegetables without water—stir-frying, for example—will also cause a color change, because when the temperature of the plant tissue rises above 140°F/60°C, the organizing membranes in and around the chloroplast are damaged, and chlorophyll is exposed to the plant’s own natural acids. Freezing, pickling, dehydration, and simple aging also damage chloroplasts and chlorophyll. This is why dull, olive-green vegetables are so common.