Why do lobsters change colour on cooking?
Believe it or not, shrimp and lobster have something in common with carrots. They have a good dose of carotenoids in their body. Carotenoids are yellow-orange compounds that are quite widespread in nature but the first one isolated was beta carotene from carrots which gave the name to the whole family. But they are also responsible for the coloring of orange juice, red peppers, watermelon, tomatoes, egg yolks, apricots, corn, pink grapefruit, pink salmon and pink flamingoes.
Lobsters and shrimp dine on plankton which contains carotenoids and these compounds become concentrated in the shell. But there they are bound up with protein molecules and the carotenoid-protein complex has a dark green color. When the protein is heated, it is denatured. In other words it breaks down and disassociates from the reddish pigment, astaxanthin, which in turn now becomes visible. To a smaller extent this is also evident in cooked carrots which become more orange than they were before. The effect is not as great as with lobsters because carrots do not have much protein.
Carotenoids extracted from natural sources are widely used as food dyes. “Annatto” is an extract of a tropical shrub which is intensely colored due to the presence of the carotenoid, bixin. Cheddar cheese, margarine and butter are often colored with annatto. This dye is completely harmless; in fact, the natural yellow color of butter is due to carotenoids found in grass. When grass dries into hay, these compounds are destroyed. Winter butter therefore is often white and carotenoids are added to increase market appeal. So there is a lot of interesting chemistry that occurs when a lobster is plunged into hot water. However, I doubt that the lobster enjoys it very much.