This drink is so hard to explain. I think it is in a category by itself. It's Russian bread soda! What else do you want me to say?
I really don't think that Katz's description of "ever so mildly alcoholic, full of healthful Lactobacilli, thick and milky, almost viscous" does this drink any justice at all. It's definitely acid and sour, but not unpleasantly so. It would definitely benefit from a flavoring agent of some sort - something like cumin, mint, orange, lemon, or ginger would all work. I did almost make bottle bombs out of this concoction - next time I will bottle in plastic bottles - which is recommended for sodas. This is because you can tell, by feel, the carbonation and because if it does burst there will be no flying glass.
I will be making this again! I really think that you could give this to someone, and they would have no idea what they were drinking and would be surprised to learn that its made from bread!
If anyone wants to point me in the direction of good commercial version I'd love that!
Kvass Part I
Kvass Part II