What causes the shiny, rainbowlike appearance on raw tuna and beef?
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.