Strain the kvass through a sieve or cheesecloth, leaving 0.5 liters of sourdough. Fill with fresh water, put 3 tablespoons of sugar and 15 raisins, mix and put again for 3 days.
And so on until you get bored. To add color, you want to add a black crust to the black bread.
If you just drink kvass, you can sweeten it. For mayonnaise, you need to mix all the products in a blender.