You ever notice how when that Italian sub you’re nomming on hits the light just right, it get’s all sparkly? Not in the Edward Cullen way (hopefully), but more in the double rainbow all the way across the sky kind of way.
Fear not: it does not mean your sandwich has been dunked in bubble solution. Apparently, it’s just boring old science, as per usual.
Simply put, that iridescent sheen is a result of light diffraction. In a piece of (cooked) meat, the muscle fibers all align just so; when cut, the meat forms a freaky magic meat pattern, which bounces light back at us, so we can magically see a rainbow. Basically, your sandwich is as equally amazing as the sky after a thunderstorm. You ought to savor that mess.
There are a whole bunch of other factors that dictate whether you’ll have a beautiful sandwich or not. The color of the meat, for example: roast beef and cured ham are prime suspects, because they’re dark enough to show off a sheen. Chicken and turkey? Not so much. It also depends on the cut (against the grain, folks!), the fat content (the lower, the prettier), and the curing process used.
In other words, there is nothing magical or icky going on with your glowing meat. In fact, it probably means that stuff is pretty choice.
That said, if you’ve got a leg of beef (or whatever) and it’s uncooked and shining? Wipe it down with a paper towel. If the sheen goes away, so does that meat – ideally into a trash can. You’ve got a bad batch, buddy.