Why do lobsters, shrimp, and crabs turn red when cooked?
The heat breaks up the proteins and releases the carotenoid pigments, whose real color is the aforementioned bright orange and red.
Crustaceans such as lobsters, shrimps, and crabs live at the bottom of the sea, the shell of these animals that inhabit seas, oceans, and rivers, is dark and their color varies between blue-green and reddish-brown. These colors are similar to the sea bottom and help them go unnoticed by predators.
These colors are not natural to them, (in fact they are transparent), they get them through the pigments ingested with the diet, based mainly on plankton. Plankton is rich in carotenoid pigments (astaxanthin, canthaxanthin, beta-carotene, among others) which are orange and red. After digestion these molecules bind with the proteins of the crustacean, changing their transparent color to those greenish and brown tones.
When we cook these animals the heat breaks the proteins and the carotenoid pigments are released, whose real color is the mentioned bright orange and red. The denaturalization of the protein, by heating, makes the color of the carotenoid appear without changes in its spectrum. Crustaceans thus become one of the most cheerful foods on the table.