Pastrami sometimes appears iridescent.
Photo by Kevork Djansezian/Getty Images
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 comprised 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; navel, used for pastrami; and top round, 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.
Food Explainer thanks Edward Mills of Penn State and Andrew Milkowski of the University of Wisconsin-Madison.
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Correction, Sept. 10, 2013: Because of an editing error, this post misstated that top round is used for pastrami and that navel is used for roast beef. It's the other way around. (Return to the corrected sentence.)