Sour cream has long been a traditional ingredient in Russian, Eastern European, and German cooking, and has gained popularity in the rest of Europe, North America, and other parts of the world in the past 50 years or so. It was traditionally made by letting fresh cream sour naturally – the acids and bacteria present produced a generally consistent flavor and thick texture that went well with both sweet and savory dishes. These days, commercially produced sour cream is made by inoculating pasteurized light cream with bacteria cultures, letting the bacteria grow until the cream is both soured and thick, and then repasteruizing it to stop the process.
Sour cream cannot be made at home with pasteurized cream; the lack of bacteria in the cream will cause the cream to spoil instead of sour. If you have access to unpasteruized heavy cream, you can add 1 Tbsp of vinegar to 2 cups of cream and let the mixture stand out at room temperature for several hours until curdled.
If you can’t get unpasteurized cream, you can still make a version of crème fraîche, which is also a soured cream. The taste is generally milder than that of sour cream, but it may be an acceptable substitute for you in recipes that call for sour cream. You can make crème fraîche by adding 1 cup of buttermilk to 2 cups of heavy cream and leaving it out in a warm place (80° to 90°F, or 26° to 32°C, is ideal) for as few as eight hours and as many as 24 hours. One of the benefits of crème fraîche is that it can be whipped.