Hamburger that is entirely gray in color, feels slimy or smells bad has gone bad. Fresh meat is naturally purplish in color. Meat that is exposed to oxygen turns brownish in color; thus, a brown color does not necessarily mean the hamburger is bad.
Hamburger should be cooked or frozen within two days of being purchased to ensure fresh quality. To destroy bacteria, ground beef should be stored at less than 40 degrees Fahrenheit and cooked to 160 degrees Fahrenheit. Ground beef should never be left at room temperature for more than two hours.
To safely thaw frozen ground beef, leave it in the refrigerator. If this is not an option, microwave the meat or put it in an air-tight bag in cold water, changing the water every 30 minutes to keep it cold. Transporting freshly purchased hamburger is another consideration; bring a cooler for meat if the trip is going to take longer than 30 minutes.