I found both dark and light rye bread at the store but it was the factory-made kind. I definitely made mistakes with both batches including adding way too much bread to the light kvass.the bottles need a day to carbonate then we shall see how well they turned out. Not that I have any reference for the flavor.
I am from a place where kvass is made and drink it quite often. The difference between store bought and homemade stuff can be massive. Stuff that comes in cans tastes like bread flavoured soda or beer that was sweetened too much. The stuff restaurants make tends to be very sour, on the other hand.
~7 Pieces of Rye Bread(preferably dark or Russian rye)
~1/2 gal [1.9L] Water
~2/3 cup [150g| sugar(can be substituted with wort, honey, molasses, ect)
Handful of raisins or any other dried berry (cut in half if you like) ~10
Optional: Pinch of bread yeast or tea sppon of sour dough starter to accelerate the fermentation
Toast the bread so dark as to be almost burned
Boil enough water to cover the toast and pour over toasted bread pieces
Wait until room temp and add raisins and sugar
Ferment 3-5 days depending on your fermentation activity. Bottle with more raisins and a tea spoon of sugar and set on counter 2-3 days Refrigerate and drink
It is slightly sour with a unique flavor that is very much determined by the type of bread used. I am sure playing with the amount of sugar can change how sour it is. These were commonly home made in the pre-industrial era and from what I understand, a lot of slovik people still make it at home. Kvass seems to be pretty versatile as far as ingredients go.