Beef: Red on the outside, brown on the inside
Take bright-red ground beef and put it in a freezer for a few days. After taking it out and letting it thaw, it looks fairly red on the outside, but brown on the inside. Why is that?
From the answers to this question, I understand that fresh beef initially becomes bright-red upon exposure to oxygen, and then browns after long-exposure to oxygen.
I also understand that oxygen can pass through the plastic packaging.
What I don't understand is why the the ground beef on the inside turns brown before the outside? If oxygen is the cause of browning, shouldn't the opposite happen?
The meat is brown on the inside not due to too much oxygen but due to a lack of oxygen.
Oxygen can pass through the packaging but not, generally, through the meat itself. Thus, the interior of the meat runs out of oxygen faster than the exterior (which is still exposed to oxygen from the air) and browns for this reason.
I'll quote the full-explanation from the USDA, just for reference:
Why is pre-packaged ground beef red on the outside and sometimes dull, grayish-brown inside?
Oxygen from the air reacts with meat pigments to form a bright red color which is usually seen on the surface of meat purchased in the supermarket. The pigment responsible for the red color in meat is oxymyoglobin, a substance found in all warm-blooded animals. Fresh cut meat is purplish in color. The interior of the meat may be grayish brown due to lack of oxygen; however, if all the meat in the package has turned gray or brown, it may be beginning to spoil.