Red Tunas

The meaty color of certain tunas is caused by the oxygen-storing pigment myoglobin, which these fish need for their nonstop, high-velocity life. Fish myoglobin is especially prone to being oxidized to brownish metmyoglobin, especially at freezer temperatures down to –22°F/–30°C; tuna must be frozen well below this to keep its color. During cooking, fish myoglobins denature and turn gray-brown at around the same temperature as beef myoglobin, between 140 and 160°F/60 and 70°C. Because they are often present in small quantities, their color change can be masked by the general milkiness caused when all the other cell proteins unfold and bond to each other. This is why fish with distinctly pink raw flesh (albacore tuna, mahimahi) will turn as white as any white fish when cooked.