I bought some ground beef around 2 weeks ago and used half of it, putting the rest in a freezer bag in my fridge (not the freezer). It has since turned brown (not red, as when I bought it).
Is it safe to use to make hamburgers?
I'm not going to comment on whether or not it is safe, because that could be any number of issues other than the brown coloring of the meat. However, the brown in and of itself is not an issue. When meat is exposed to air it turns a brown color. This does not effect the falvor of the meat, but the color turns a lot of people off. Grocery stores will actually throw this meat away, not because it is bad but just because people won't buy it. Terribly wasteful.
From the USDA: Optimum surface color of fresh meat (i.e., cherry-red for beef; dark cherry-red for lamb; grayish-pink for pork; and pale pink for veal) is highly unstable and short-lived. When meat is fresh and protected from contact with air (such as in vacuum packages), it has the purple-red color that comes from myoglobin, one of the two key pigments responsible for the color of meat. When exposed to air, myoglobin forms the pigment, oxymyoglobin, which gives meat a pleasingly cherry-red color. The use of a plastic wrap that allows oxygen to pass through it helps ensure that the cut meats will retain this bright red color. However, exposure to store lighting as well as the continued contact of myoglobin and oxymyoglobin with oxygen leads to the formation of metmyoglobin, a pigment that turns meat brownish-red. This color change alone does not mean the product is spoiled
First, do not eat that. Regardless what color the beef is, two weeks is entirely too long to refrigerate ground beef. It is unsafe and should be thrown out. Raw ground beef only keeps in the refrigerator for 1-2 days.
Ignoring storage time, regarding color, brown meat is as safe to eat as red meat. As others have indicated it is simply oxidation occurring. Many grocery stores, at least in the USA, [used to?] blast and wrap their meats with carbon monoxide which imparts a bright pink color that lasts well beyond the shelf life of the meat. If you freeze hamburger or steak for a few months there's a good chance it will come out much browner than when you put it in. This is because some oxidation still occurs in the freezer, unless you hermetically sealed it somehow. Similarly, as long as you're within the 3-4 month freezer period for ground beef, you're OK.
I think it's worth pointing out that, unlike steak, you're probably going to cook ground beef (or minced beef as we English call it) to death (literally) so any talk of food poisoning is slightly hysterical. The worst you're likely to do is ruin a potentially good meal with some horrible tasting meat.
Before anyone shouts at me for this irresponsible suggestion, I should point out that it is true that there are bacteria which produce heat stable toxins which can cause food poising, but they're quite rare, so... You'll be fine. Probably...
I just called the meat department about the same problem, except mine was only in the refrigerator for two days but had already turned brown. I was going to cook a large pot of soup but wanted to be safe. He said, do not use it. The expiration date is today, and it is the 3rd day since I bought it, but had turned brown before today. I would never risk food poisoning! Not worth it! I am taking it back for a refund. You can't even trust expiration dates either but have to look at the meat and don't use if it is brown.
I ran into a similar situation, a package of raw, presumably never frozen, ground beef sat in the refrigerator for at least three weeks. The instructions on the package read "freeze or use by October 9 (of this year). Not wanting to waste meat, or poison my roommate, I decided to cook it, knowing that heating the meat would produce an odor which would indicate whether it was edible. The nature of scent is that when chilled, odors indicating spoilage are harder to detect, and heat will activate, liven, or bloom any odor indicating hazard. Upon cooking the meat for a few minutes the meat had an inedible odor which made my stomach feel slightly ill. Had I let my roommate eat it, I would surely be accused of poisoning her.