June 18, 2014 — From red to white to orange to blue, fish flesh can land almost anywhere on the color spectrum.
What's behind this huge variation? A lot of things — from genetics to bile pigments. And parsing the rainbow can tell us something about where a fish came from, its swimming routine and what it ate.
Red yellowfin tuna: A classic of the sashimi counter, the yellowfin tuna is also the Michael Phelps of the fish world. And its athletic prowess has a lot to do with its ruby red flesh.
Bruce Collette, a zoologist with the National Marine Fisheries Service, tells The Salt that the endurance-swimmer tuna need lots of oxygen to feed their muscles. The protein called myoglobin stores oxygen for muscles and acts as a pigment, turning the flesh of most tuna, and other open-ocean fishes like mako sharks and swordfish, pinkish red.
"It's true of land animals, too: If they're walking around a lot, they'll have more myoglobin and their meat will be darker," Collette says. But as Michaeleen Doucleff has reported, more muscle activity also makes meat tougher. That's why the lazy belly muscle of the tuna, known as otoro, is exceptionally soft.
The pigmented flesh in tuna occurs throughout the body but is concentrated in a head-to-tail strip of brownish flesh along each side — strong-tasting meat that's often discarded.
But take heed: Some sneaky seafood hawkers artificially enhance the color of tuna meat by exposing it to carbon monoxide gas. This method restores meat that has faded to an over-the-hill brown back to a fresh, luscious red. The practice is illegal in some nations, such as Singapore, but not in the U.S. So be wary of any tuna cut that seems just a tad too cherry-red to be real — it could be less fresh than advertised.