Sour cream with furry, black and green splotches on top is obviously spoiled, but when it hasn't quite reached the moldy stage, look for other, more subtle signs that the product has gone bad. If you have doubts about the freshness or safety of sour cream -- or of any other food -- because of its age or condition, simply toss it out.
Visual and Sniff Tests
A quick look at and a whiff of a container of sour cream can determine if it's fresh or not, but that method is subjective and ambiguous. Don't worry about a light layer of white, milky liquid floating on top of sour cream or about a tangy odor; both those conditions are normal. Stir the liquid whey into the rest of the sour cream to make it smooth and creamy. However, if the whey seems curdled or too abundant, or if the odor of the sour cream has moved from tangy to sour, discard the container.
If the sour cream has been in the refrigerator for a while, it may look and smell fresh to you but still harbor unsafe bacteria. According to Clemson University Cooperative Extension Service, sour cream stays fresh for up to two weeks, but the U.S. Department of Agriculture places the time period between seven and 14 days. Use your judgment after seven days and discard the product after it's been open for two weeks.
Check the Date
Because the U.S. Department of Agriculture requires a product date only for infant formula, not for sour cream, your container of sour cream may or may not have a "sell-by," "use-by" or "best-by" date stamped on it. The date stamp means that a store has until that date to sell the product. In any case, only buy a container that has not yet passed the date stamped and begin timing the two-week use period after you open the container.
Sometimes your sour cream is within its use-by date and may look OK, but it still can be unsafe. For example, if a sour cream dip sits out at a party for more than two hours, bacteria have had a chance to multiply, and you need to throw the dip away. Also, the FoodSafety.gov website recommends that you throw away a container of sour cream if your power goes out for longer than four hours, whether or not you've opened the container.