Sour cream is a type of food product created by the controlled fermentation of a milk product with bacteria. This foodstuff is often used as a topping in Earth cuisine, and can be created for consumption by Humans with a food synthesizer.
During the wait for Jadzia Dax and Worf to marry in 2374, Julian Bashir broke from kal'Hyah to order a steak with mushrooms and a baked potato with sour cream and chives. (DS9 episode: "You Are Cordially Invited")
When Erika Hernandez first encountered a replicator, aboard the USS Titan in 2381, she tested the machine's comprehension by ordering a quesadilla of Jack cheese and black beans, with sides of hot salsa, sour cream and guacamole, and a mojito. She found all the elements to be subtly off, including the paste-like sour cream. (ST - Destiny novel: Lost Souls)
Sour cream could be used as an ingredient in numerous meals and desserts, including kugel, bregit lung, chocolate cake with raspberry frosting, azna and fried mochana shoots. (ST reference: Star Trek Cookbook)