Kvass is an ancient beverage consumed in Russia and throughout the Slovic regions. It is low alcohol, similar to beer, and is much loved. It is also used when making a traditional Borscht. This recipe takes several days to complete.
Preheat oven to 200 degrees F and spread bread cubes on a baking sheet; bake until the bread is fairly dried out, about 1 hour.
In a large pot, bring 3 quarts of water to a boil, remove from heat, and stir in bread; cover with a clean towel and let rest in a cool, dark place for 8 to 10 hours.
Pour the liquid through a fine meshed sieve, or strainer, and gently press bread to get the liquid (don't press too hard or your finished product will be cloudy).
Mix the yeast with the warm water and a pinch of sugar; set aside and let proof 10 minutes. Stir into the strained liquid along with 1 cup sugar, stirring until sugar is dissolved; cover and let stand another 8 to 10 hours.
Strain again and pour into a 1 gallon container; add raisins, cover tightly with plastic wrap and a rubber band. Let stand in a cool, dark place 4 to 5 days until the yeast sediment has settled to the bottom, leaving a clear liquid
Carefully pour the clear liquid into a clean container or individual bottles, trying not to disturb the yeast sediment. Chill well.
VARIATIONS: add 2 tablespoons fresh mint leaves with the yeast and sugar. Or substitute 2 to 3 cups of berry juice for some of the water to give it a fruity flavor. It can also be served with the yeast sediment. This gives it a richer flavor and more vitamins.