On Food and Cooking

active between 150–170ºF/66–77ºC 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
140ºF/60ºC, 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