Cindi - HappyMamatoThree > wrote:

> So, best substitution? most like creme fraiche?

Crème fraîche is cultured cream; sour cream is cultured cream. Ergo,
they are one and the same. Any differences are in gradation of acidity,
texture, fat content, and any additives - and that depends entirely on
the producer and perhaps also the particular labelling laws or customs
in a given locality. In America, the commercial sour cream I
encountered tended to be fairly thick in texture, almost firm, and not
particularly sour. It was not all that much different from crème
fraîche. In Russia, it would generally be the other way around, with
sour cream tending to be semi-liquid, pourable, and rather more tangy.
The same thing is also true in Germany. I can well imagine artisanal
American producers offering something similar. Considering that you are
making a potato gratin, I would not hesitate to substitute sour cream in
any case.

That said, here is a suggestion by Patricia Wells, in her _Bistro
Cooking_. The reasons are lost on me, but here it is.

Victor

Crème Fraîche

2 cups (50 cl) heavy cream
2 tablespoons buttermilk

1. Thoroughly mix the cream and buttermilk in a medium-size bowl. Cover
with plastic wrap and let stand at room temperature overnight or until
fairly thick.
2. Cover tightly and refrigerate at least 4 hours to thicken it even
more. The cream may be stored for several days, as the tangy flavor
continues to develop.
Yield: 2 cups (50 cl)