Pale Translucence

Most of the muscle in most raw fish is white or off-white and delicately translucent compared to raw beef or pork, whose cells are surrounded by more light-scattering connective tissue and fat cells. Especially fatty portions of fish, such as salmon and tuna bellies, look distinctly milky compared to flesh from just a few inches away. The translucence of fish muscle is turned into opacity by cooking treatments that cause the muscle proteins to unfold and bond to each other into large, light-scattering masses. Both heat and marination in acid unfold proteins and turn fish flesh opaque.