What is the difference between crème fraîche and sour cream?
Crème fraîche is typically higher-fat and a little less sharply tangy than sour cream. The big win is that crème fraîche is more resistant to curdling when warmed, so it is a safer bet for sauces and soups. (Still, avoid boiling it hard or adding a ton of acid while it is on high heat.)
Why did mine not thicken?
Usually one of three things: your kitchen is too cool, your buttermilk did not have active cultures, or your cream is being a little fussy. Ultra-pasteurized cream can still work, but it may take longer or thicken less reliably. Fix it by moving the jar to a warmer spot (around 72 to 78°F), giving it more time, and making sure you are using cultured buttermilk (not just "buttermilk" flavoring). If it still will not budge after 24 hours, start over with fresh buttermilk.
Can I use milk instead of cream?
You can, but you will not get the classic thick texture. Crème fraîche depends on fat for that spoonable body. If you want a lighter cultured dairy, look into kefir-style methods instead.
Does it need to be covered while it sits out?
Yes. Loosely cover the jar (do not seal it airtight) with a lid set on top, or use a clean cloth secured with a rubber band. You want protection from dust and kitchen funk, while still letting the cultures do their thing.
Can I make it without buttermilk?
Buttermilk is the easiest reliable starter. Some people use plain yogurt or a spoonful of existing crème fraîche, but results vary. For first-timers, buttermilk is the no-drama choice.
How do I know it is done?
It should look thickened like loose yogurt, coat a spoon, and mound just a little when you dollop it. It will thicken more after chilling, and a tiny bit of separation is not unusual. If it smells pleasantly tangy and fresh, you are good. If it smells off or you see mold, toss it.
Is it safe to leave it out for 12 to 24 hours?
This is a standard method when you start with fresh dairy, active cultures, and a very clean jar. Keep it out of direct sun, aim for a normal room-temperature range (about 72 to 78°F), and always discard it if it smells off or shows any mold.