Have you ever noticed that the ground meat you just brought home from the supermarket is red on the outside but dark purple or brown on the inside? Is this an indication of meat past its prime? Fortunately, no.
The color in meat comes from a muscle protein called myoglobin. When the meat is freshly cut, this protein is deep purple. As the meat sits in its packaging (or in the butcher’s display case), the myoglobin will convert to bright red oxymyoglobin on the meat’s exterior, where it is exposed to more oxygen. Inside the meat, where less oxygen can penetrate, it will slowly convert to brown metmyoglobin. Color changes of this nature are purely cosmetic—they have no bearing on the meat’s flavor or wholesomeness.