If you think "crème fraîche" is a fancy way of saying "sour cream," you're not entirely wrong. They are both cultured cream, which is heavy cream allowed to sour. At a warm room temperature, bacteria turns the lactose — a sugar — in the cream, into lactic acid. That's where the tang comes from in both crème fraîche and sour cream.
But they're not exactly the same, and there's more to it than the fact that one has a French name and one has an English name.
Crème fraîche has a higher fat content, about double what sour cream has. Crème fraîche is about 40 percent fat, which gives it lower acidity and makes it more stable. The lower acidity means it can be used on things like fresh fruit desserts.
It still has that tang that contrasts nicely with the sweet fruit, but it's not, you know, sour. The fat also helps keep crème fraîche from separating at high heat, so it can be added to things like soup or scrambled eggs while they're still cooking on the stove.
Sour cream, on the other hand is tangier and more liquid. According to the U.S. Food and Drug Administration rules, it has to have at least 18 percent fat in the United States. It has higher acidity, which means it can break down gluten in baked goods, like cakes or muffins, for a moister texture. But its lower fat content means it curdles when added to hot food. That's why we often use it as a condiment or add it to soup at the last minute, maybe a dollop just before serving.
Sour cream was first developed in Eastern Europe. Sour cream is more common (and less expensive) in the United States, where it's usually made industrially. Crème fraîche was developed in France, as you probably guessed from its name.
They're made about the same way, though there are some slight differences. Start with pasteurized milk, so first bacteria have to be added to the cream. In France, crème fraîche is traditionally made with naturally occurring bacteria. American sour cream is also usually thickened with gelatin or rennin, an enzyme that curdles milk.
You can substitute crème fraîche for sour cream and vice versa, typically as a one-to-one ratio. Just keep in mind the subtle flavor differences, and that sour cream tends to curdle when added to food that's cooking.