March 23, 2019
Red Tuna: Not Always Fresh Tuna

Red Tuna: Not Always Fresh Tuna

by Berkeley Wellness  

Redder tuna means fresher tuna, right? Not necessarily. Fish suppliers have a trick up their sleeves: They may treat the tuna with carbon monoxide to make it look redder. Here’s how it works: There’s a pigment in the fish flesh that turns bright red when it picks up oxygen from the bloodstream. After the fish dies, exposure to air causes oxida­tion, which turns the flesh brown, indicating a decline in quality. Treating fish with carbon monoxide permanently changes the pigment color to bright red, so the color of the fish never changes.

The process itself is safe, but the practice can be deceptive, since unscrupulous fish dealers may use it to make old fish appear fresh. It can turn even three-week old tuna from brown back to red and reduce its odor. In one experiment, researchers at the University of Florida found that tuna gassed with carbon mon­oxide maintained its bright red color for 11 days in the refrigerator and for seven days when stored at slightly higher temperature. Treated tuna also had less odor after four days in the refrigerator than untreated tuna.

Of more concern, old fish can make you sick: As fish decay (with or without carbon monoxide treatment), histamine can form in the tissue. Histamine can be toxic at high levels, resulting in a foodborne illness called scombroid poisoning, whose symp­toms are often confused with an allergic reaction. While carbon monoxide can delay some bacterial growth, histamine may still form and is not destroyed by cooking. The FDA requires carbon monoxide-treated fish to be labeled as such, but that’s typically not done, and no one enforces the law. The European Union, Canada, Japan, and Singapore ban the practice.

Bottom line: Tuna that looks unnaturally red has likely been gassed, but it usually takes a trained eye to distinguish the color differences between fresh and treated fish. Shop and dine at reputable establishments that have a fast turnover of fish (even if they sell treated tuna, there’s a better chance it’s at least fresh). Keep in mind, also, that not all tuna is bright red when fresh. Some top-quality species are pale pink or brownish red.

Also see Tuna Fish: Lean Protein Packed with Omega-3s.