Meat contains a variety of compounds including iron and fat. There are various pigments in meat compounds that can give it an iridescent or greenish cast when exposed to oxygen, light, heat and processing. When exposed to oxygen or light, some of the nitrate-modified iron content of the meat undergoes a chemical change that alters the meat's pigmentation and you will see the colors of the rainbow. This rainbow-effect is a sign of oxidation and not necessarily spoilage. Wrapping the meat in airtight packages and storing it away from light will help prevent this situation. Iridescence does not mean that the meat is unsafe or that the quality is compromised.