Sour cream made from kefir sounds excellent, I'll give it a try.

Quark, no, I've never even heard of it before now. I've been reading recently that store-bought yogurt, while still good as it's a fermented food, contains much less benefit than yogurt that has been home-made, which I can well believe, given that everything from the shops is pasteurised. I've been meaning to try making my own yogurt and I've got 2 reasons now, as I'm curious to try quark.