Rye Bread Kvass Starter

Cut the rye bread into 1.5–2 cm cubes, spread on a baking sheet, and toast in an oven preheated to 180°C for 15–20 minutes until deep golden and crisp, watching closely so the croutons don't burn or turn bitter.

Put the croutons into a large jar or pot, pour over 3 L of boiling water, cover with a towel, and let steep for 4–5 hours, until the liquid cools to room temperature.

Strain the liquid through cheesecloth into a clean container, pressing the croutons well to extract all the liquid. In a glass of the warm liquid, dissolve the yeast with 1 tsp of sugar (from the total amount) and let sit for 10 minutes until a foamy cap forms.

Add the remaining sugar and salt to the strained liquid, pour in the activated yeast, and stir. Cover with cheesecloth and leave at room temperature of 22–25°C for 12–18 hours, until light foam appears and a sweet-and-sour aroma develops.

Strain the kvass again through several layers of cheesecloth, pour into clean bottles, and drop 3–4 raisins into each.

Seal the bottles tightly and refrigerate for at least 8 hours — during this time the raisins will gradually rise to the top once the kvass is sufficiently carbonated.

Serve chilled as a drink, or use as a sour base for borsch or cold soup.