Sour cream is pasteurized cream that has been soured by a bacterial culture. When buying it, always check the expiration date. Believe it or not, sour cream can get even more sour when it's old. You can make your own sour cream by adding 2 tablespoons of buttermilk to 2 cups of fresh cream. Allow this mixture to set at room temperature for 24 hours and don't disturb it. This sour cream will keep in the refrigerator for 3 days. In contrast, real sour cream will store in the refrigerator for 2-3 weeks.