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. Crème fraîche is widely available in Europe, but much less so in the US, where most all cream is pasteurized, and therefore has to be fermented artificially. There are commercially produced versions available in select gourmet shops, but it can be hard to find and fairly expensive. Considering how often it is called for in recipes these days, it is surprising that it is so rare.
Most people make a facsimile of crème fraîche by adding a tablespoon of buttermilk 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. You can also whip it like whipping cream.
But, Madeleine Kamman, whose formative years were spent in France, and who is one of the flavor mavens of our age, says the homemade crème fraîche is a poor substitute for the real thing. Since many people have not had the real thing, they won’t know the difference. But, Kamman says, if you run across real crème fraîche in a specialty shop - and are of adequate means - buy it.