Commercial sour cream contains from 18 to 20 percent fat and has been treated with a lactic acid culture to add its characteristic tang. Sour cream often contains additional ingredients such as gelatin, rennin or vegetable enzymes. Light sour cream contains about 40 percent less fat than regular sour cream because it's made from half-and-half. There's also a nonfat sour cream, which is thickened with stabilizers. Refrigerate sour cream in its carton for up to a week after the date stamped on the bottom of the container. If any mold forms on the cream's surface, discard all of the sour cream.