The USDA says "iridescent beef" is perfectly okay.
People are like moths to the flames that are rainbows. The next time there's a rainbow outside, notice how many people drop everything, even important things, to Instagram it. It would be a perfect time for aliens to take over Earth. Rainbows where rainbows shouldn't be, however, cause alarm. Take beef, for example.
How many times have you (if you eat beef) foregone a package of sliced roast beef for a different package because said beef was slightly iridescent? If you don't eat beef, perhaps you've seen a package of said rainbow meat and it reminded you why you no longer eat it. The Internets are clogged with threads like, "Why does deli roast beef look like a rainbow?," and the ever gravid concern, "Subway shiny roast beef?" And while everyone should be spared urbandictionary.com's definition of what a "Beef Rainbow" is, the truth of the matter is, there's nothing inherently wrong with rainbow meat.
Beef rainbows aren't a sign of spoiled, tainted, or (sorry) magical beef. There's enough speculation over the integrity of rainbow beef that the USDA's website has a section on "Iridescent Color of Roast Beef" near similar topics like "What does 'natural?' mean" and "what is beef?" According to the USDA, "When light hits a slice of meat, it splits into colors like a rainbow." This is something called a "diffraction grating," essentially what happens when light waves bend or spread around a surface and create a pattern. It's the same thing that happens to make rainbows on the surface of a DVD. It's understandable that folks mistake diffracted light as a sign of spoilage, especially since the main color created by meat diffraction gratings is green. There is a reason why in Dr. Seuss's Green Eggs and Ham, the central conflict of the protagonist is his strong apprehension against eating green meat.
Speaking of ham, beef is not the only meat known to have rainbows. However, when cooked beef is sharply sliced against the grain of the muscle fiber, this, coupled with the moisture in the beef, creates an excellent surface for producing rainbows. "In my opinion," Dr. Thomas Powell, Executive Director of the American Meat Science Association, told me, "The reason it shows up in roast beef is because the cuts of meat that are used in most roast beef are more prone to iridescence, particularly in the round," hence the reason why the USDA singles out roast beef as being especially colorful.
In a way, it's sad that meat rainbows are given a bad rap, especially since diffraction gratings in nature are relatively rare -- and I say this as someone who doesn't eat red meat. Sure, one can see the the vibrant iridescence of peacock feathers or the milky rainbows of an abalone shell and marvel at the rich tapestry that is nature. But it is under the flourescent light of our grocer's deli section where we can look at a rainbow on a slice of beef and know the natural diffraction grating responsible for it is shared with very few things, including the antennae of seed shrimp, and the shells of animals that haven't lived for hundreds of millions of years. A rainbow worth Instagramming as much as any other, for sure.
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