One thing that can be said definitively about crème fraîche is that while it may indeed be crème, it is by no means fraîche. In fact it’s sour, very very similar in taste and texture to sour cream, though with some important functional differences.
Crème fraîche’s main difference is that like just about everything French, it’s higher in milk fat. About 30% fat compared to about 20% for American sour cream. Sounds good you say, and yes, indeed it is. The extra proportion fat makes it richer, yet it also leaves less room in the mixture for protein (which as you may recall is present in milk in a proportion of about 3.5%). The difference in protein between sour cream and crème fraîche may only be about a percentage point or so, yet it’s enough to make them behave differently in the kitchen.
As you recall, proteins are long, stringy chains of amino acids that in their natural state are found wadded up like clumps of yarn. Heat them and they loosen into long, straight filaments useful for thickening runny sauces. Heat them too much though, and they clump back up again, separate out from whatever liquid they’re in, and sink. And thus we have one of the main functional problems with sour cream: it curdles (which us why you only stir sour cream into stroganoff after the meat has finished cooking). Crème fraîche doesn’t have this problem, which means you can heat it.
Another neat thing about crème fraîche is that unlike sour cream, you can whip it into a foam. For you see, to perform the milk fat miracle, any dairy product must contain a minimum of 30% fat. Crème fraîche squeaks in right under the wire with a bare 30%. But it’s a rare recipe that calls for a 100% crème fraîche
whipped topping. Most (like this/last week’s olive oil cake recipe) are some combination of heavy cream and crème fraîche, so as to a) cut the sour tang of the crème fraîche somewhat and, b) to bolster it fat-wise so it whips up better.