Creme fraiche is one of those ingredients that can be tough to find at your local grocery store. While it is slightly different than American sour cream, you can substitute one for the other in a potato casserole and most other recipes. Commercial sour cream has a lower fat content than creme fraiche and often contains other additives to contribute to its texture. These differences will cause variations in your final dish, so keep this in mind when making the substitution.
A Culturing Experience
Sour cream, creme fraiche and plain yogurt are all very similar dairy products. All three are cultured with bacteria that feed off of the lactose in dairy. This culturing produces lactic acid, which in turn creates the creamy texture and sour tang found in these products. The main difference between the three is the butterfat content. Creme fraiche has the highest amount of fat, while yogurt has the lowest. In terms of flavor for any dish -- including potato casserole -- any of these products can be substituted for another.
Higher fat content adds to the creaminess and richness of dishes, but that's not where the important texture issues end. It turns out that heating fermented dairy products can cause them to separate, making a dish grainy instead of creamy. Fat content is directly related to the temperature at which this break occurs, so sour cream has a tendency to separate when cooked at high temperatures, while creme fraiche does not. For the standard casserole temperature of 350 degrees Fahrenheit, this won't be a problem.
Don't use nonfat sour cream as a substitution in a potato casserole or any cooked recipe -- it will lose its texture and can ruin your dish. Blending sour cream with a little bit of flour will help prevent separation at high temperatures; mix 1 teaspoon flour for each 1/2 cup of sour cream. Bringing sour cream to room temperature before cooking with it will also help prevent separation.
Simple Creme Fraiche
If you've got time, you might consider making homemade creme fraiche. It's simple and foolproof. Simply heat 1 pint of heavy whipping cream to about 90 F and mix in a couple of tablespoons of cultured buttermilk, making sure the buttermilk has live and active cultures. If cultured buttermilk is difficult to come by, yogurt with live cultures can be used instead. Blend well, cover and let sit at room temperature overnight. Refrigerate the homemade creme fraiche and use it within two weeks.