There are no positive buffs, nausea is the only effect you can get from this, other than filling your hunger bar.
Kvass (or квас in cyrillic) does really exist. It is a Russian (or East European) beverage, normally made from fermented black or rye bread. It can be flavoured with fruit or herbs. It has been a cheap and common drink in Eastern Europe since the Middle Ages. Kvass in real life has quite a low alcoholicity (0.5% to 1%). I don't know if Plum kvass really exists, but why not?
Apple Juice •
Berry Juices •