A traditional Russian bread kvass is a fermented and carbonated beverage in which the main components are Rye bread, yeast, sugar, water and raisins. In Russia kvass is used in some cold recipes such as okroshka.
Position a rack in the upper third of an oven and preheat the broiler. Arrange bread slices on a baking sheet. Broil the slices until dark brown, turning them over only once, about 3-4 minutes per side.
In a large bowl, mix together active dry yeast, sugar and water. Place bread and raisins in 3 Mason jars and fill the jars with water.
Cover the jars with lid or plastic wrap and let it stand on the countertop for 6-8 hours at room temperature, stir the mixture occasionally.
Carefully remove the toasted bread and raisins and discard them. Using a strainer pour the kvass into empty jars or plastic containers and place in the refrigerator overnight.