That dark, nearly black area in the middle of your tuna or swordfish steak is nothing bad or unhealthy, although you may not like its strong flavor. It is a muscle that is rich in myoglobin, a blood pigment.
The black part you see in the picture is Bloodline/Red Muscle. The bloodline has a lot of blood to provide oxygen. Though many folks dislike the taste and smell of bloodline, it's edible - full of nutrients. Vitamin A, D, E, Iron, EPD, DHA and minerals.
Myoglobin changes color, however, depending among other things on how much oxygen is available to it. The dark, purplish red color of freshly cut tuna is due to deoxymyoglobin, which in air changes first to bright red oxymyoglobin and then to brown metmyoglobin.
If you look at the whole fish, tail meat would be unsuitable for Sashimi just like Yellowfin. However, because the belly meat contains a lot of fat, generally known as Toro, you can pretty much eat any part of Blue Fin Tuna as Sashimi, as marked below in the green line.
Customers erroneously think all brownish-looking fish is old or bad, and will pay more for "fresher looking" fish. So in the 1990s, the FDA allowed companies to gas the tuna and keep it artificially bright red for long stretches at a time. "There's nothing wrong with a hot chocolate looking tuna loin," says Gomes.
According to Lacademie, raw tuna that has spoiled will appear off-colored, usually with a milky look or with shades of brown. Meanwhile, healthy raw tuna is a deep red or a light pink color. If your fish has started to change color or you see some sort of growth (like mold) toss that fish in the trash.
Fresh tuna, depending on its fat content, ranges in color from pale pink to deep red. (The more fat, the lighter the color.) Tuna tends to oxidize quickly, which cause it to turn an unappetizing grey or brown.
Otoro is the name of the fattiest part of a tuna. Otoro has a characteristic sophisticated taste and a distinctive fat marbeling which makes the pieces to melt in your mouth. The area on the abdomen near the gills is called harakami, and the center of the abdomen is called haranaka.
Toro is the term for the fatty part of the tuna, found in the belly portion of the fish.
The most expensive and most desirable cut is otoro. This high-quality cut is from the fat under-belly of a tuna fish. The fat adds a sweet note to the taste and the texture is always described as “melt in your mouth” soft. Interestingly, this part of the tuna used to be thrown out before refrigeration was invented.
The choice comes down to personal preference: If you like a less “fishy” tuna experience, go for white tuna; if you want stronger flavor, light tuna is a better choice. Nutritionally, all three of these fish are pretty similar.
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.
Fish dark meat is the same thing as chicken dark meat: tougher muscles used continuously for support and locomotion (as opposed to white meat muscle, which is only used for short bursts of activity). Sure, the taste is a bit stronger, so we understand separating the dark from light meat (ie the loin).
Tuna that has gone bad may also have a dull appearance, though it does oxidize quickly, which can also contribute to a less bright appearance. Instead of signaling spoilage, though, those dark centers in a tuna filet are caused by the presence of myoglobin (via MyRecipes).
What does Blackened Tuna taste like? It tastes spicy, peppery, savory, and bold. It contrasts nicely with the “meatiness” of the toothsome tuna steaks. It has enough flavor on its own and does not need a sauce.
In tuna, the myxosporidian parasite Hexacapsula neothunni colonies form white, spherical, 2-4 mm diameter masses in the freshly-caught tuna. As the fish deteriorates post mortem, the colony begins to release proteolytic enzymes that liquefy the muscle adjacent to the colony.
Is canned tuna fish good for you? Yes, canned tuna is a healthful food rich in protein and contains many vitamins and minerals such as B-Complex vitamins, Vitamins A and D as well as iron, selenium and phosphorus. Tuna also contains healthy omega 3 essential fatty acids DHA and EPA.
#1 grade tuna should have a fine and smooth texture, it should not be coarse or grainy. Lastly, fat content. Higher fat content means higher quality and better taste. Usually high fat content will be grade #1 tuna and sold to the sushi market.
Mercury levels in canned white tuna, which is exclusively albacore, are almost three times higher than those found in smaller skipjack tuna commonly used in canned light tuna products.
As you can see, chicken is higher in calories, protein and fat (which includes saturated fat) than a can of tuna calories, but they're still pretty comparable.
Light Tuna – There are a variety of fish that fall in the “light tuna” category. This tuna typically comes from either Skipjack or Yellowfin tuna. The meat is darker in color – somewhere between a light tan and tan-ish pink. It has a soft texture and is more flavorful than white meat tuna.
Yellowfin tuna can be a brighter, lighter color. Bluefin tuna are considered the most valuable because they can store a lot of fat in their musculature and this means greater flavor and texture.