I tried making cultured butter today from a supermarket variety crème fraîche. For some reason the butter hasn't split from the whey although I've "churned" it with the hand mixer for 20 minutes or more.
This recipe for cultured butter suggests that about 2 to 5 minutes is needed with a food processor, and my previous experience with the hand mixer is about 10 minutes for sweet butter.
I checked the ingredients list of the crème fraîche and I don't see anything unusual: cream* and an unspecified culture. That's it. No artificial thickeners for example.
What could be the problem?
I'll just note one avenue I've already explored: temperature. I've tried varying the temperature during churning from just above freezing to around room temperature.
* The cream is "högpastöriserad" which is Swedish for a pasteurization process where the cream is held at 80°C for 5 seconds. I'm not sure this term translates directly to a common English term.