Sour cream is a tasty cultured dairy product, but if it has been sitting out all day, it's best to toss it, even if it seems okay. Without refrigeration, harmful spoilage microorganisms can proliferate in the sour cream after one to two hours at room temperatures above 40 degrees Fahrenheit.
Can Sour Cream Go . . . Sour?
Sour cream, as its name implies, has a distinct, pleasantly sour flavor, which is created by the administration of beneficial lactic acid bacteria to dairy cream. These bacteria, including Streptococcus lactis and Leuconostoc citrovorum, are safe for human consumption. Unfortunately, dangerous spoilage microorganisms, including yeasts and molds, can also flourish in sour cream. These microorganisms can make you very sick and thrive in temperatures between 40 and 140 degrees. This is why consistent refrigeration below 40 F is necessary to keep your sour cream safe.
But it's Just Been One Day
Harmful pathogens multiply very quickly and will spoil your sour cream if it is left out of your refrigerator for one hour in temperatures warmer than 90 F and two hours in temperatures between 40 and 90 F.gov. Although your sour cream may appear edible after a day of sitting around your home unrefrigerated, don't take the change of eating it; you could get food poisoning. You may also see mold or smell an unpleasant odor, all signs of spoilage. Properly refrigerated sour cream can last up to two weeks past its printed date.