Oxygen from the air reacts with meat pigments to produce the brilliant red color visible on the surface of stored beef. Oxymyoglobin, a molecule found in all warm-blooded animals, is the pigment responsible for the red color in meat. The hue of fresh-cut meat is purplish. Due to a lack of oxygen, the interior of the meat may be grayish brown; however, if all of the meat in the package has turned gray or brown, it may be spoiling.