cream of tartar in egg yolk buttercream
I was looking at an egg yolk/sugar syrup buttercream recipe in Paula Peck's Art of Fine Baking, which calls for a small amount of cream of tartar in the sugar syrup. I'm curious about the role of cream of tartar here. I know I've seen it in recipes involving stiffly beaten egg whites--does it also have a stabilizing effect in egg yolk buttercream?
I don't have the book in front of me, but this recipe seems to be the same one (the Classic Egg Yolk Buttercream):
The cream of tartar is just used in the simple syrup portion of the recipe. The acid (cream of tartar is tartaric acid) turns the simple syrup into an "invert" simple syrup. The sugar crystals become smaller and finer, improving the texture of some baked goods, in this case making a smoother icing. In an icing, an invert simple syrup will also "hold onto" the other ingredients more, keep the icing trgether so it doesn't weep. You can also use lemon juice -- it's the acid that's key.
Cream of tartar in egg whites helps create structure and stability -- essentially the acid provides bonding sites for the molecules that help form the equivalent of girders in a building.