Q. What causes some sliced deli meats to possess an iridescent sheen? On occasion, a slice of ham or beef will exhibit the sort of rainbow spot one might see on an oil slick or the inside of a seashell. Why?
A. It's because of the particular way light bounces off the surface of the deli meat, a phenomenon known as "diffraction." A piece of meat is composed of strands of fibers that are tightly packed together in parallel bundles. After meat is sliced, the cut ends of the fibers form a series of grooves, like the top of a picket fence. White light is composed of a spectrum of different colors, and each one of those colors has a specific wavelength. When white light hits the grooves on the surface of a deli meat slice, some of the light is absorbed and some of it is reflected. Each component color wave of the reflected light bends at a different angle depending on its particular frequency. The result of this spread of color waves is a kaleidoscope or iridescent effect, similar to the colors we see in soap bubbles, CDs and fish scales.
Diffraction depends on the grooves being structurally intact and perfectly aligned. This is why you're much more likely to see this rainbow effect on processed deli meat that's cooked and/or cured than on raw meat. The former has a firmer, tougher texture and the picket fence structure keeps its shape well when deli meats are sliced. Raw meat, on the other hand, is softer and more delicate, and the ends of the meat fibers are easily damaged when the meat is cut, which means that light is reflected in a haphazard way that doesn't result in rainbows.
So why do some deli meats shimmer while others remain dull? The color of the meat matters. Dark cooked meat like roast beef and bright cured meat like ham are more likely to show iridescence because the background colors provide a starker contrast to the pearly greens and orangey reds that you're most likely to see coming off of shiny meats. Turkey and chicken are too pale to showcase such sparkle. It also matters whether the deli product contains ground or "restructured" meat (in which chopped meat is molded together) or is composed of a single piece of muscle. In the former case, the jumbled up meat fibers are no longer aligned correctly to diffract light.
The way a joint of meat is sliced at the deli counter or before being pre-packaged is also paramount. Only cuts that are sliced against the grain, or perpendicular to the direction of the meat fibers, show iridescence, since the protruding severed ends of the fibers produce the fine grooves. (For beef, these cuts are brisket, which is used for corned beef, and top round, used for pastrami, and navel, used for roast beef.) The sharpness of the blade with which the meat is sliced makes a difference, too. The sharper the slicing instrument, the cleaner the cut, the smoother the surface, and the more intense the display of rainbow hues. Blunt knives produce rough surfaces; the picket fence grooves will be too disrupted to produce iridescence.
The commercial curing process can cause sliced deli meats to have an especially smooth surface, which is why you sometimes see dazzling rainbows on cured hams. Cured meats are first injected with a brine or marinade and then tumbled in revolving metal drums to allow the brine to penetrate evenly throughout the meat. This process causes proteins to seep out from cells and fill gaps between muscle fibers, creating an even, consistent texture that's more likely to diffract light when sliced.
Fat content also has an impact on the light-reflecting properties of meat: A particularly fatty cut of meat is unlikely to diffract light. A slice of roast beef that's richly and evenly marbled with fat won't shine. Fat is either liquid (at room temperature) or semi-crystalline (when chilled), and neither of these states possess the right grooved structure to create a rainbow sheen.
Since diffraction is a purely physical phenomenon and has nothing to do with microbial growth, iridescent deli meat poses absolutely no safety risk, nor does it have any effect on taste. But this might not always be the case with raw meat, which can occasionally exhibit iridescence. Raw meats are more prone to bacterial contamination, and a colorful glow could be caused by light reflecting off a surface film of liquid produced by microbes. To determine whether your iridescent raw meat is unsafe, lightly wipe the surface of the meat with a paper towel. If the sheen disappears, then the meat is likely harboring slime-producing microbes, and you should discard it.