As UW-Madison commemorates LGBTQ+ Pride Month in June and rainbow flags appear around Madison, a phenomenon known as structural color can also cause iridescent rainbow coloring on sliced deli meats.

The appearance of these rainbows are fairly common and a result of structural color on the meats’ surface. The microscopic structure of muscle fibers in certain meats, like beef, can cause light to hit the surface in a certain way and diffract light into a rainbow. The appearance of rainbows often occurs in deli meats like roast beef and pastrami due to their smooth surfaces.

Though this phenomenon may seem odd, the appearance of the rainbow does not correlate with spoilage or the meat’s freshness.

To learn more about iridescent meat, check out this article from America’s Test Kitchen.