Why do shellfish change color when cooked?

Shellfish change color when cooked due to a chemical reaction that occurs between the proteins and pigments in their shells. The main pigment responsible for the color of shellfish is called astaxanthin, which is a carotenoid pigment that is also found in other animals such as salmon and shrimp. When shellfish are cooked, the heat causes the astaxanthin to break down and release its color, resulting in the characteristic red or pink color that we see in cooked shellfish.

In addition to astaxanthin, other pigments such as beta-carotene and lutein can also contribute to the color of shellfish. These pigments are also sensitive to heat and can undergo similar color changes when cooked. The specific color that a shellfish turns when cooked depends on the combination and concentration of pigments present in its shell.

It's worth noting that the color change in shellfish during cooking is not only a visual transformation but also an indicator of doneness. When shellfish are fully cooked, the proteins in their shells have coagulated and the flesh becomes opaque. This change in texture and color signals that the shellfish is safe to consume.