A popular sour cream recipe on AllRecipes.com consists of mixing milk and white vinegar before stirring in heavy cream and letting the mixture stand for 24 hours at room temperature. Buttermilk can be substituted for milk and white vinegar.
Cultures for Health suggests using heavy cream and a packet of sour cream starter culture. The cream is heated to 145 degrees F and held at that temperature for 45 minutes before being returned to a temperature of 77 degrees F. The starter culture is then mixed into the cream until it dissolves. The jar is sealed and placed in a warm area for 16 to 18 hours, at which point it is transferred to the refrigerator where it can be stored for up to two weeks.
The type of cream used can have drastic effects on the sour cream, according to Cultures for Health. Pasteurized heavy cream and whipping cream result in thick sour cream, while half-and-half and raw cream yield thinner results. Ultrapasteurized and ultra-high temperature cream should be avoided for homemade sour cream. Dry milk powder can be added to runny sour cream to help thicken it.
If sour cream starter culture is not available, Cultures for Health suggests using cultured buttermilk or mesophilic yogurt. The type of starter culture can affect taste.