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I just sliced this piece of tuna that had been in my freezer for about a week and had been thawed in the refrigerator for a day - to find this luminous orange colour to it. It was on the outside of the slice too but not as much.

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This is from the supermarket and I have no idea how long it had been sitting in their refrigerator, although they claim that their product is fresh.

Is this some kind of unusual preservative or maybe the fish has gone bad?

  • I don't know why tuna does that but I often see perfectly good canned tuna have similar shiny hues. – 0xFF Nov 9 '17 at 18:46
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From Cooks Illustrated

The rainbow effect is caused by the reflection of light off muscle fibers, technically known as double refraction or birefringence. It occurs when the muscle fibers are cut crosswise and can be observed for several days after slicing the meat. Light striking the ends of the fibers is reflected in two different directions, appearing to the eye as a rainbow of colors. Just like a real rainbow, the multicolor appearance is fleeting, visible only when the muscle fibers are cut at a specific angle relative to the grain of the meat and when the meat is viewed at a certain angle.

One more point to keep in mind: While the rainbow effect is harmless and doesn’t offer any indication of how fresh the meat is, there is another color change that is a sign of meat that has passed its prime: the development of a green pigment. Meat with a green cast is contaminated with bacteria and should be avoided.

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