What Causes Beef Rainbows?

Source - https://9gag.com/gag/anj1VRV/should-i-be-concerned-if-this-prepackaged-meat-is-shiny

I've seen those roast beef rainbows before, and I'll admit to having been a little freaked out by them.

I assumed that any greenish - even if it was iridescent greenish - wasn't good on meat. Turns out it's nothing to be worried about.

See, there are a few ways to make colors.

One is for there to be a light-absorbing dye. Like if you have a red shirt, it's been soaked in a chemical (a dye) that absorbs most of the colors of light except for the red wavelengths. That means that white light shone upon the shirt will reflect only the red light.

That's not what causes the roast beef rainbows.

Instead, they're formed by interference of light waves - as slightly explained in this Atlantic article or this Daily Mail article.

Source - https://www.dailymail.co.uk/news/article-2287287/Why-beef-rainbow-NOT-tainted-Popular-myth-debunked-Department-Agriculture.html

When light is reflected off of a wet surface, some of the light reflects from different depths of the liquid. As the same white light reflects from different depths, the reflected light comes back slightly out of phase from the other light. This causes a diffraction pattern which leads to shimmering, shifting colors - like the rainbow from an oil slick on a puddle or a butterfly's wing.

Apparently the rainbow on roast beef is common enough that the USDA even has a page answering the following...

Meat contains iron, fat, and other compounds. When light hits a slice of meat, it splits into colors like a rainbow. There are various pigments in meat compounds that can give it an iridescent or greenish cast when exposed to heat and processing. Wrapping the meat in airtight packages and storing it away from light will help prevent this situation. Iridescence does not represent decreased quality or safety of the meat.

So, as Pink Floyd told us, you should eat your [rainbow] meat...how can you have any pudding if you don't eat your meat?