The dairy products you mention fall into perhaps three categories: creams and soured creams, yogurt, and fresh cheeses.

We assume you’re not asking what cream is, but what a recipe writer means when his recipe simply calls for cream. In general, it means a fairly thick, relatively high-butterfat cream. Double cream is the British designation for super-rich cream - with 48% butterfat. By contrast, whipping cream in the United States has between 30% and 40% butterfat. Light creams, or single cream in Britain, which average around 20% butterfat, are not as stable for cooking, are more prone to curdling in the presence of acids or high heat, and so are not called for as often in recipes.

Crème fraîche and sour cream are both manufactured cream products. Crème fraîche is a slightly tangy, slightly nutty, thickened cream. Before the age of pasteurization crème fraîche made itself as the bacteria present in the cream fermented and thickened it naturally. It is widely available in Europe, but much less so in the US, where almost all cream is pasteurized, and therefore has to be fermented artificially.

Sour cream was also 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 tablespoon of vinegar to 2 cups of cream and let the mixture stand out at room temperature for several hours until curdled.

You can also make a facsimile of crème fraîche by adding a tablespoon of buttermilk (don’t tell us they don’t have buttermilk where you live!) or a half cup of sour cream to a cup of whipping cream, heating it gently to 110°F (45°C), then putting it in a loosely covered bottle in a warm place and letting it sit for anywhere from 8 hours to a couple of days, until thick. Store it in the refrigerator, where it will thicken further, and keep for about three weeks.

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 like whipping cream, and it will not curdle if boiled.

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