Why does a crabs shell turn red-orange when cooked?
The red pigment is the most stable component of the coloring in a crab shell. The greens, blues, and browns which darken the shell in a live crab are destroyed by cooking. The red pigment common to all shrimp, crab, and lobster shells is astaxanthin, a carotenoid (e.g., like Beta-carotene, the pigment that makes fruits red-orange). Astaxanthin was first identified in the exoskeletons of crayfish (Astacidea), hence its name. In crabs, as in many decapods, astaxanthin is not a free pigment, but is complexed with a protein called Alpha-crustacyanin, which alters the resonance of astaxanthin such that the complex acts as a blue-green pigment. As mentioned above, astaxanthin is heat stable, while the Alpha-crustacyanin protein is not, so boiling the crab shell denatures the blue-green Alpha-crustacyanin releasing the red-orange astaxanthin.
