Use hamburger within one day and cook it to 160 degrees Fahrenheit.
Red ground beef is associated with freshness and quality, but your nose is the best judge. The red color is caused by a number of factors, and may or may not indicate how fresh the meat is. If ground beef smells bad -- even if it's still pink or red -- toss it out. When you cook ground beef, don't rely on changes in color to determine doneness. Beef can turn brown prematurely or even remain pink after it's completely cooked through.
The Role of Pigment
All beef and hamburger contain myoglobin, a natural pigment that is responsible for the red color consumers prize. When beef is first cut, the flesh appears purple to deep red. As the pigment is exposed to oxygen, the meat turns bright red. The interior portions of a package of hamburger, however, turn brown or gray because no oxygen is available. The meat is still fresh, but the pigments haven't been oxidized.
Cuts and Color
Myoglobin and its interaction with oxygen causes the exterior portions of hamburger to turn bright red, while the inside may be pink, brown or gray. Two packages of hamburger may be slightly different shades of pink or red because of the cuts of meat included. Beef from bulls often has more pigment and a higher pH level than meat from steers or cows, according to the U.S. Department of Agriculture. Hence, this meat is redder in color. Most commercial hamburger contains a combination of meat from different cuts. Depending on the percentage of each type of meat in a package, the color of the hamburger can vary.
The Fresh Factor
Consumers generally associate a brighter red color with fresher meat, and this assumption has some validity. Hamburger ground from older carcasses often has a light red or pink color, rather than bright red.
It's in the Packaging
How hamburger is packaged can also affect the color of the meat. The most common type of packaging is a meat tray covered with plastic. The plastic allows enough oxygen in to turn the meat red while still keeping it fresh. Some packages contain no oxygen, however, but instead have been treated with small amounts of carbon dioxide or carbon monoxide. These gases won't prevent spoiling, but they do preserve the meat's red color longer.
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