Why Are There Rainbows on My Roast Beef?

I pulled a slice of roast beef from the deli bag only to discover that it had a greenish tint. When I held the beef up to the light, it looked like a rainbow had spread across its surface. 

This shiny display of colors made me wonder if the meat had gone bad. But since it didn’t smell funny or feel sticky to the touch—two telltale signs of meat that’s spoiled—we ate it anyway. 

Later, after raising my concern with with our Senior Science Editor Paul Adams, I learned that this iridescence has an interesting explanation, and a lot more people than just me have wondered about it. 

Usually, color—including the red and brown colors of meat—is caused by the presence of pigment. But iridescence on meat has nothing to do with pigment. The shimmery rainbow is actually due to a phenomenon called structural color. This means the physical structure of an object causes it to diffract light into a spectrum of colors.

In beef, the microscopic structure of the muscle fibers can cause light hitting them to diffract and form a rainbow on the cut surface of meat.

Light can also diffract off other relatively smooth, flat surfaces such as butterfly wings and peacock feathers, giving much the same effect.

These rainbows on meat are so common—and so many people are mystified and/or concerned about them—that the USDA has even posted an explanation on its website. 

According to Paul, the surface of the meat needs to be smooth enough for the light to be diffracted in a unified formation for it to be noticeable; with a rough surface, the light will bounce off in all directions and be too diffuse to detect. 

That degree of smoothness generally only comes when meat is sliced or cut by the commercial-grade equipment used in a deli or by a butcher. So you’ll see rainbows on cold cuts and the surface of a raw steak more often than on the meat you cook and slice at home. 

Paul also explained that while rainbows can appear on almost any type of meat, including poultry, it’s more visible on darker red meats. Dark color absorbs more light in general, so the light that is diffracted is more visible against the dark background.

Iridescence on its own has nothing to do with the freshness of meat. A greenish tint, on the other hand, could be of concern. According to the USDA, a benign reason for a green cast to meat is that pigment in the muscle has changed color from heat or processing. 

But a green color can also come from spoilage organisms like green or gray mold, so you should always use caution before eating meat that looks green: Smell it to see if it has sour or other off odors; touch it to see if it feels sticky; and be sure to consume it within the sell-by date.