Kvass
Kvass is made by the natural fermentation of bread, such as wheat, rye, or barley, and sometimes flavoured using fruit, berries, raisins, or birch sap collected in the early spring. Modern homemade kvass most often uses black or regular rye bread, usually dried, baked into croutons (called suhari), or fried, with the addition of sugar or fruit (e.g. apples or raisins), and with a yeast culture and zakvaska ("kvass fermentation starter").