If your burgers or mince have developed a brown colour in the centre - do not be alarmed and definitely do not throw the item away - this is perfectly normal and safe to eat.
Why does this happen?
This is all a matter of science. A pigment exists within beef called myoglobin and this pigment contains iron. This pigment starts out purple when beef is first cut and as it is exposed to oxygen in the air it turns red.
Enzymes within the meat use this oxygen and while doing so, remove electrons from the iron within the myoglobin. This turns the myoglobin and the meat brown. There are other enzymes present that give electrons back to the iron within the myoglobin - turning the myoglobin and the meat back to purple and red.
Minced beef and burgers have a high surface area, which means the myoglobin is saturated with Oxygen from the air. Once the mince is packed or the burgers have been pressed, the myoglobin on the outside areas can continue this cycle. However, the mince that is in the centre or has a burger disc attached to it only has enough oxygen for the enzymes to turn the myoglobin brown. It is not enough for the other enzymes to turn the myoglobin purple and red again as the air and oxygen cannot penetrate through.
This means that the centre of the mince and any surface of burgers that are covered may be brown upon delivery. Once the mince is broken up or the burgers are unpackaged and exposed to the air, they will begin to turn red again as the Oxygen allows the enzymes within the myoglobin to continue the cycle.
An explanation and biology lesson rolled into one!