The rainbow effect is caused by light reflecting off the meat fibers and iron compounds (like myoglobin) in the muscle. When the meat is sliced, the smooth surface can act like a prism, creating an iridescent or rainbow shimmer.
This effect is more noticeable when:
- The meat is freshly sliced
- The cut surface is very smooth
- Light hits the meat at a certain angle
- The roast is medium or medium-rare
Is It Safe to Eat?
✅ Yes — usually completely safe if:
- The meat smells normal
- It was refrigerated properly
- There is no slimy texture
- There is no sour or rotten odor
When to Throw It Away
❌ Discard if you notice:
- Sour or rotten smell
- Sticky/slimy surface
- Gray or green patches that don’t change with light
- It has been left unrefrigerated for over 2 hours
Quick Tip
This rainbow sheen can also appear on ham, roast beef, corned beef, and deli meats, and it’s a known harmless optical effect.
✅ Since you cooked it yesterday and sliced it today, it’s very likely perfectly fine to eat.