Vital kvas.

As you need to take are natural water, not boiled, and fresh, natural yeast bread of rye of millet flour, raisin, or not canned grape, not manufacture, better is fermented good raisin, brown, sugar, sugar, vital baked yeasts.
As of products you can to see, that kvas is a microbiologic product, and natural, not thermic, chemical manufacturing, and microflora here is vital. As in jar on 5 l. add 0.5 l. of warmed water and add pinch as one gr the yeast, two tbl.sp. of sugar, 4 raisins, and all to mix. As yeasts need to dilute in dregs, diluting and sugar. As rusks or pieces of bread chop on pieces as one cm, are on 2 handfuls. Add in jar, and shake, as wetting theirs. As a top of jar wrap with gauze and put the jar in a warm place, and three times in a day shake jar, that rusks are the same wetting. And in three days take one tea.sp.for taste and if a taste is sour, thus fermenting is ready, and in a day she will be sourer, and if yes, thus kvas is ready!