Creme fraiche offers a distinct tart flavor similar to that of sour cream, but it has a thinner consistency. Creme fraiche doesn't curdle easily under heat, which makes it suitable for cooking and adding richness to sauces and soups. If you don't have creme fraiche, several alternatives with a similar tanginess are equally versatile.
Creme fraiche isn't available in all grocery stores, but you can make it at home. Heat buttermilk and heavy cream and allow the mixture to sit at room temperature for 24 hours or until the liquid becomes thick and creamy -- similar to a slightly thinned sour cream. Creme fraiche thickens faster at room temperature, but once it's thickened you'll need to stir it, cover and refrigerate for at least 6 hours before using. Store it in the refrigerator for up to 2 weeks in an airtight container. Homemade creme fraiche is thinner than store-bought, but still has the same flavor.
Plain yogurt offers a tangy flavor similar to creme fraiche and a consistency that is just slightly thinner. You can use low-fat yogurt or full-fat yogurt varieties instead of creme fraiche. However, yogurt cannot be used at high temperatures like creme fraiche can since it is prone to separating. If your recipe requires you to simmer creme fraiche, you can still use the yogurt, but don't allow it to reach a boil. Use yogurt in equal parts to creme fraiche.
Sour cream is a common substitute for creme fraiche since the two foods are very similar to one another. Sour cream separates at boiling temperatures, so if you're heating it you must keep it at a simmer or below. Sour cream is thicker than creme fraiche, so if your recipe needs a thinner addition use a small amount of heavy cream and a whisk to thin it out. Use sour cream in equal parts to creme fraiche.
Creme fraiche is tart, but still creamy. In sauces and soups, you can use heavy cream instead of creme fraiche. Add the heavy cream toward the end of cooking to thicken and and richness to the sauce, but don't allow it to boil -- this will break up the fat proteins. For additional tart flavor, you can add a little bit of lemon juice or zest to the cream. Since cream is much thinner than creme fraiche, use 1 part cream for every 2 parts of creme fraiche.
Soy-based sour creams are suitable alternatives if you're lactose intolerant or vegan. These have the same consistency and tart flavoring as regular sour cream and also separate in high heat. Use in equal parts to creme fraiche.