Kvass is a Russian bread soda and the word Kvass means 'yeast' in Slavic. There is more info on the wiki page.
This 'bread soda' takes a few days to prepare so I haven't had the pleasure of tasting it yet. Does anyone know what I'll be up against? Katz says that "it is ever so mildly alcoholic, full of healthful Lactobacilli, thick and milky, almost viscous. I think it's delicious but some people wince at its sour flavor." A much nicer and more generous description of Kvass comes from Randy Mosher's "Radical Brewing", in which the drink is compared to champagne, albeit a grain champagne!
Kvass (adopted from Wild Fermentation by Sandor Ellix Katz)
4 cups of cubed rye bread (from this bread recipe)
2 tablespoons lemon juice
Toast bread. When cool, place in a mason jar with 6 cups hot water and lemon juice. Stir, cover, and leave for 8 hours.
Come back tommorrow for the rest of the recipe!