Have you ever sliced into beef or pork and noticed a rainbow-like sheen? While it might seem alarming, this iridescent effect is a natural result of light refraction. When light hits the meat’s surface — especially proteins, fat, and iron — it bends and scatters, creating prism-like colors. Dr. Thomas Powell of the American Meat Science Association explains this is most noticeable when meat is sliced thinly against the grain.

This phenomenon isn’t limited to beef; it also occurs in pork, ham, bacon, and fish. According to the U.S. Department of Agriculture (USDA), this shimmering effect is completely natural and has nothing to do with spoilage or bacteria.

Still, to ensure meat is fresh, check for other spoilage signs like bad odor, sticky texture, or odd discoloration. The rainbow sheen is common in processed meats, such as ham and roast beef, because they are often thinly sliced, allowing more light interaction.

If you want to reduce the rainbow effect, store meat properly and cut it with the grain. In the end, these colors are a harmless, fascinating result of light and meat structure — not a reason to worry.