Preheat the oven to 375°F (190°C). Tear or cube the Dark Rye Bread into bite-sized pieces, spread in a single layer on a baking sheet, and toast until deeply browned but not burnt, about 10–15 minutes — this caramelization gives the kvass its characteristic flavor.
Place the toasted bread in a large heatproof pot and pour in Water (about 2 liters). Bring to a boil, then reduce heat and simmer 10 minutes to extract flavor; remove from heat and let the mixture steep for about 1 hour.
Strain the liquid through a fine sieve or cheesecloth into a clean bowl, pressing the solids to extract as much liquid as possible. Discard the spent bread pieces.
While the liquid is still warm, stir in Granulated Sugar to taste (start with about 100 g / ½ cup and adjust up to 200 g / 1 cup if you like it sweeter) until fully dissolved. Allow the sweetened liquid to cool to lukewarm — roughly 30–35°C (85–95°F).
When the liquid is lukewarm, sprinkle in Active Dry Yeast (about 1 teaspoon) and stir gently to dissolve. Add a handful of Raisins to the mixture — they help feed the yeast and add depth of flavor.
Cover the pot loosely with a cloth or lid and leave at room temperature to ferment for 12–24 hours, tasting occasionally; you want a pleasantly tangy aroma and a little effervescence but not a strong sourness.
Once the kvass has reached your preferred flavor, strain it again into clean bottles or jars. Add a few fresh Mint leaves to each bottle if you like a minty note. Seal the bottles and refrigerate for several hours to chill and slow fermentation.
Serve the kvass cold. Open bottles carefully, as natural carbonation can build pressure. Keep refrigerated and consume within a few days for best flavor and freshness.
