By Harold McGee
Red muscle fibers are used for prolonged efforts. They are fueled primarily by fat, whose metabolism absolutely requires oxygen, and obtain both fat (in the form of fatty acids) and oxygen from the blood. Red fibers are relatively thin, so that fatty acids and oxygen can diffuse into them from the blood more easily. They also contain their own droplets of fat, and the biochemical machinery necessary to convert it into energy. This machinery includes two proteins that give red cells their color. Myoglobin, a relative of the oxygen-carrying hemoglobin that makes blood red, receives oxygen from the blood, temporarily stores it, and then passes it to the fat-oxidizing proteins. And among the fat oxidizers are the cytochromes, which like hemoglobin and myoglobin contain iron and are dark in color. The greater the oxygen needs of the fiber, and the more it’s exercised, the more myoglobin and cytochromes it will contain. The muscles of young cattle and sheep are typically 0.3% myoglobin by weight and relatively pale, but the muscles of the constantly moving whale, which must store large amounts of oxygen during its prolonged dives, have 25 times more myoglobin in their cells, and are nearly black.