As far as whites instead of whole egg, it's probably not so much the protein content as it is the phospholipids, especially lecithin. A lot of the thickening for something like that is from the emulsification, as much as the protein networks. Egg whites have nearly the same protein content as yolks, but *none* of the fats and emulsifiers.

And dammit, I *still* don't have a food icon on LJ.