The Scientific Reason Tuna Is Bright Red

If presented with chocolatey brown tuna at your local sushi haunt, you might recoil and ask to have the dish replaced with something fresher — and ask for some real wasabi while you're at it. However, many folks are unaware that the naturally red meat turns brown only three to five days after being removed from its natural habitat. So why don't we ever see mahogany tuna in the seafood section at the grocery store? In short, because few would purchase it if it looked that way.

Getting tuna from the ocean to the dinner table in a matter of days is difficult — and expensive — so suppliers often treat the meat with carbon monoxide (CO). Myoglobin, a protein that turns flesh red, is responsible for giving the ocean fare its vivid crimson hue. After the fish is harvested, it is exposed to oxygen, subjecting it to oxidative browning and turning the flesh brown. It doesn't mean the tuna isn't safe for consumption, but it does indicate that it's not as fresh as possible. Treating the fish with carbon monoxide through wood smoking prevents this browning from occurring and guarantees it will retain a red or pinkish color.

Contrary to what some may think, tuna's bright red pigment isn't an indicator of its quality. It could mean it is very fresh, but it is more likely that it was treated with CO. The FDA has classified this treatment as generally safe, but other organizations have banned the practice.

While treating tuna with carbon monoxide is an accepted method of retaining its color in the United States, other regions around the world have outlawed the practice. Singapore, Canada, Japan, and the European Union have all banned the preservation technique. The latter reasoned that the process could be used as a means of deceptively concealing spoiled fish. However, others argue that enhancing tuna's color with CO is a favorable way of enticing potential consumers to include more of its nutritious meat in their diet.

Many consider superfreezing tuna a better option for maintaining its radiant red pigment. This is a process where the fish is quickly frozen to at least -76 degrees Fahrenheit. At this temperature, all biological activity ceases, and oxidized browning can't take effect. When the seafood is thawed, it retains the same ruby-red hue it had when it was caught. Japan commonly utilizes this technique, with 80% of the tuna it processes being superfrozen.

If you plan on making sushi at home and are concerned about it being decayed, you should worry less about how your chicken of the sea was processed and look for signs of spoilage. A strong fishy smell or sliminess on the fish is a sign that it is past its prime and should be thrown away. Just like you shouldn't judge a salmon's quality by its color, tuna's bright red complexion isn't necessarily an indicator of its freshness.