Beef: Red on the outside, brown on the inside

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.

It's hard to initially judge the freshness of ground beef by looking for color. Very fresh ground beef is red-purple. The plastic wrap they use in grocery stores is oxygen permeable- that is oxygen can get through. This means that a few hours later, the part of the ground beef that is exposed will turn that bright red (oxymyoglobin) that we associate with fresh meat.

The inside will still be that deeper purple red color. The color difference can make people think that fresh ground beef has spoiled, when in fact it's fine. In fact, if you open s package and expose the purple- red meat to air, it will change to a bright red color.

Note that this is different from slightly older meat. As the meat starts to age the myoglobin changes to metmyoglobin, which is grey- brown in color. This doesn't indicate that the meat has spoiled, but does have an unappealing color when raw. It doesn't effect the cooked product- it cooks the same.

Meat that is really old often is grey or grey green. That is an indicator of age & long exposure to light- oxygen. Steer clear.

In general, color isn't a good indicator.
It is better to use your nose and smell for sourness, or feel for a tacky or slimy texture. If you detect shy of these, pass.

Besides, bacteria and other pathogens can harm far before the ground beef gives you signs of spoilage. You should be careful and always buy the freshest possible product.

These changes also happen in frozen meat. Proper wrapping can minimize exposure to oxygen,

When freshly slaughtered meat is cut into steaks, the muscle tissue comes into contact with oxygen in the air. The myoglobin in the meat binds this oxygen, forming oxymyoglobin and giving the meat a red color. However, if fresh meat sits for a period of time, generally over the course of several days, the structure of the myoglobin changes. The iron molecule in the middle is oxidized from its ferrous to ferric form and a different complex is formed called metmyoglobin. This compound turns the raw meat a brown color. The meat is usually still safe to eat when cooked, but the brown, unappealing color turns off most consumers. To avoid having your fresh meat turn brown, use it as soon as possible after purchasing it.

some of it comes from the dyes some places use to redden the meat to make it look more attractive. my butcher explained that some places use this dye but it can also come from oxygen which reacts with both the dye and iron. the meat should be good through out if it was properly cared for (proper refrigeration or freezing well wrapped)

I haven't seen this in a long time, but purchased ground beef from a larger store and opened to make hamburgers. It smelled fine, inside and out, but that red-dye actually showed up on my hands after forming patties.

This is an old trick by the meat department at large grocery stores. They often take the meat that was prepacked and browning and grind it. It is now browning ground beef. they then take the not so pretty meat and place it outside of the view of the purchaser. This is a great way to keep food costs down as nobody would likely by brown hamburger meat. Meat does not brown from the inside out but the outside in.