Prepare the first fermentation by toasting the cubed sourdough rye bread, mixing it with sugar and water in a jar, and leaving it to ferment for 3-7 days.
Decant the fermented kvass and add optional sweeteners/flavourings for the second stage of fermentation in a swing-top bottle.
Leave the bottle to ferment in a warm, dark place and enjoy the finished kvass within 3 days.
