When you make bread kvass this is your starter – left over rye sourdough. I made my first lot of this fermented drink around 2 months ago and am still using the same bread I started with. . When it’s ready (i.e. sour enough for me), I drain the fermented liquid and decant it into a swing-top bottle to fizz up in a second ferment, then I remake a sugar/water solution to add the bread back into. . I think this re-using of bread could go on indefinitely, like water kefir or milk kefir grains do. (And now I’m wondering if somewhere, someone has decade old bits of rye bread that they are still using in kvass?!!) . I often flavour my swing-top bottle second ferment with fresh mint from the garden. Sometimes, I’ll add a few raisins to get it really bubbly.