I made creme fraiche using Suzanne Goin's simple recipe of 1 cup cream plus 2T buttermilk. I heated the cream to "body temperature" which to me meant that it felt barely warm to my finger. That was 48 hours ago and it's still not very thick (just a bit thicker than the cream itself). I read up on it later and learned that ultrapasteurized cream is not recommended for this (though one site said you could use it, but that it would take MUCH longer (but didn't say how long)). Of course ultrapasteurized is what I used, since that's what was in the refrigerator. Is it ever going to thicken? Note that in NY state this time of year, my house is not particularly warm; should I just be more patient, or should I give up?