Tuna is a saltwater fish with several varieties ranging from lighter to darker flesh, according to The Earthlife Web. Unlike some fish that have white flesh, tuna has muscle tissue that ranges from a very light pink that is almost white in color to a dark red. When canned, white tuna, also known as albacore tuna, refers to the lighter flesh, and dark tuna, or light tuna, has darker flesh.
The pink to red coloration comes from myoglobin and hemoglobin, according to The Earthlife Web. Myoglobin is a protein that carries oxygen. It is used by the muscles and heart during periods of physical exertion. Hemoglobin is a protein found in the red blood cells that delivers oxygen from the lungs to muscles and brings carbon dioxide from tissues to the lungs. The intensity of coloration is related to the amount of myoglobin and hemoglobin that is present in the muscles, with the darker color having an ample amount of these proteins and the lighter having very little or none.
Dark, or red, muscle tissue has many capillaries and a high hemoglobin content. It is used for continuous swimming and is found in active fish, especially those, like tuna, that live in the ocean. Red muscle constitutes less than 20 percent of the total muscle tissue of a fish, according to The Earthlife Web. White muscle has thicker muscle fibers than red muscle, fewer capillaries, and less oxygen available for muscles to perform hard work. The activity of most white muscle is anaerobic, meaning it does not require oxygen to function. Instead, white muscle uses glycogen for fuel and exhausts the supply of glycogen quickly. White muscle is primarily used for swift and short bursts of movement. Pink muscle is between red and white. It is good for periods of swimming lasting a moderate amount of time at a relatively high speed.
Light canned tuna contains more dark flesh and typically comes from yellowfin tuna or skipjack, per the book, "The Diversity of Fishes: Biology, Evolution and Ecology." In contrast, white tuna contains lighter-colored flesh, and is often labeled as albacore tuna. Light tuna is canned in water, whereas white tuna is packaged in a vegetable oil. The flavor of white tuna is milder than light tuna. In comparison to light tuna, white tuna has about three times more mercury, calories and fat, but fewer omega-3 fatty acids. Overall, light tuna is considered to be healthier than white tuna.
Market Quality of Tuna
The quality of tuna is determined by several factors, including muscle color. There are four general grades to determine the quality of tuna, according to Revolutionary Chefs. The first grade is tuna with bright, dark and clear red flesh that is firm in texture and has little or no fat. Tuna with red muscle that is not so dark and clear and has less fat than first grade tuna is considered to be of the second grade. Third grade tuna has a mix of red and browner flesh that is opaque and has no fat. The fourth grade is tuna that is lighter in color with a brown or gray hue, soft and opaque.