The red pigment in rare meat, myoglobin, undergoes chemical changes as its temperature rises. When the meat's degree of doneness is rare (an internal temperature of approximately 130°F), the myoglobin in the interior still retains most of its red color. When the meat is medium-rare (approximately 135° to 140°F), the myoglobin is pink. At medium (approximately 145° to 150°F), just a trace of the pink remains. By the time the steak is well done (160°F), all myoglobin in the meat has turned drab brown. (Note: These descriptions apply after the meat has rested, not when it has just been removed from the oven at these temperatures.)
