Crème fraîche (French for "fresh cream") is a heavy cream slightly soured with bacterial culture, but not as sour or as thick as sour cream. Originally a French product, today it is available throughout Europe. Crème fraîche is made by inoculating pasteurized light cream with lactobacillus cultures, letting the bacteria grow until the cream is both soured and thick and then pasteurizing it to stop the process.
Thus, it cannot be made at home with pasteurized cream—the lack of bacteria in the cream will cause it to spoil instead of sour. If unpasteurized heavy cream is available, a substitute for crème fraîche can be produced by adding one tablespoon of vinegar to two cups of cream and letting the mixture curdle. In general, crème fraîche and sour cream can be used interchangeably in most recipes, but crème fraîche has two advantages over sour cream: it can be whipped, and it will not curdle if boiled.