Tuna (Thunnini spp.) is a saltwater fish species with several varieties with flesh that ranges from light to dark. The main types of tuna processed for human consumption are the skipjack (Katsuwonus pelamis) and yellowfin (Thunnus albacares), usually labeled as light tuna vs albacore (Thunnus alalunga) tuna. Unlike other fish species that naturally have primarily white flesh, the various tuna species feature muscle tissue that ranges from almost white to a very light pink to a dark red, with albacore being the only white-fleshed fish of the group. While white albacore tuna is a favorite in America, light tuna contains pinker and darker flesh which is thought to be more flavorful.
Coloration of the Flesh
The pink to red coloration of the tuna flesh comes from myoglobin and hemoglobin. 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.
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The intensity of coloration from white tuna vs red tuna 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.
Muscle Tissue and Colors
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.
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.
Canned Tuna Species
Light canned tuna contains more dark flesh and typically comes from yellowfin tuna or skipjack. In contrast, canned white tuna fish is albacore tuna, which features light-colored flesh with a more delicate flavor. Both albacore and light tuna are available canned in water or oil. The oil may be vegetable, sunflower or olive oil, which adds a significant number of calories to your tuna sandwich, salad or casserole.
In comparison to light tuna, albacore tuna has more calories and fat, and fewer omega-3 fatty acids. Overall, light tuna is considered to be slightly healthier than white tuna, and certainly more budget-friendly.
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. 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.