Get your Kvass on MyFooDen
Kvass is a fermented non alcoholic beverage that is cereal based / bread based with a sweet and sour taste. If it is flavored, then you might identify berries, herbs or honey.
Simply put, it's best served cold, from the refrigerator, which by extension means that you should store it in the refrigerator in a tightly closed container, ensuring it does not lose carbonation.