What is Sour Cream?

Sour cream is made by adding lactic acid to cream and pasteurizing it

In the olden day in United States, the cream was harvested from the top of the raw whole milk. This cream was allowed to go sour from naturally present enzymes. This was called sour cream. Now a days after the cream is harvested, it is pasteurized and lactic acid is added to make sour cream. It has no live culture.
At home, you can buy heavy whipping cream. The protein structures are already modified, because it is ultra homogenized. Warm the heavy whipping cream to 115º F add  Dannon's Natural yogurt with live culture. Let it incubate for about six hours at an ambient temperature of 105º F and you will have old fashioned sour cream.