If it’s cold, be bold
It can often be confusing and annoying when you're looking at recipes that call for sour cream, creme fraiche, or plain yogurt. Frequently you might be in possession of only one of the three and it never seems to be the one called for in the recipe. And keeping all of them stocked all the time seems problematic, since, while they are all a fermented product which lasts a decent length of time, they are still dairy and the chances of using them all up before they expire would require a level of preparation and planning that I personally do not possess. I am guilty of throwing away more expired and never-opened or barely-used tubs of these items than I care to admit.
And on their surface, shouldn’t they be sort of interchangeable? I mean, they are all creamy, acidic, thick and tangy, with a bit of chalkiness on the finish. Can’t it just be “use any of the above”? And if so, is there one schmear to rule them all?
And the answer is a frustrating yes and no. Argh.
Sour cream is made by adding lactic acid and bacteria to a combination of cream and milk and letting it thicken and sour. Creme fraiche was originally a product of unpasteurized cream, which would naturally just thicken as it sat in warmer temperatures. It is still made that way in Europe where they are smart enough to continue to embrace the delicious alchemy of unpasteurized products. Here in the States where we are all "Bacteria, eww!” and all of our cream is pasteurized, creme fraiche is made by adding extra bacterial goodies to assist in fermentation and thickening.
Yogurt, on the other hand, is made in a similar way to sour cream, only it is made by fermenting milk instead of cream. Greek yogurt, that thicker product which is more similar in texture to sour cream and creme fraiche is made the same way as regular yogurt, only the whey is removed to thicken it even further.
So essentially the major difference as it relates to your cooking is in fat content. Sour cream is about 20 percent fat. Creme fraiche is around 30 percent. Yogurt is only around 10-12 percent fat. And while they are all fermented products with a good acid content, the types of fermentation and level of acid vary.
Which is why they are both interchangeable, as a garnish or in any cold application where you can easily swap one for the other, and also not at all interchangeable, in baking or cooking applications where making a switch can mean a disaster. In some hot sauces or soups, for example, yogurt would not have enough fat content to blend smoothly and would curdle a bit making for unpleasant look and texture. In baking, where you need to be scientifically precise, especially about fat and any acid that is assisting in leavening, swapping out can make for a total flop.
So the rule I follow is pretty simple. If it’s cold, be bold. If it’s hot, not.
The one I keep stocked? A good quality sour cream. It has the middle level of fat content, so it works well for all cold applications and most cooking ones. I don’t like to eat plain yogurt as a breakfast product, so I only buy that when I specifically need it for a recipe. And creme fraiche I either buy as needed or make fresh, usually because I have leftover buttermilk on hand.