What it is
An old Slavic tonic fermented from rye bread or beets — probiotic and traditional, but modern bottled versions are often just sugary soda that never truly fermented.
Why your biology objects
Authentic kvass is a lactic-acid ferment carrying live bacteria and organic acids (beet kvass adds betalains) — but it also holds trace alcohol (~0.5–1.2%), and the commercial default is frequently sweetened soda. Real when fermented; check whether sugar or culture is doing the work.