To begin, preheat your oven to 180°C. Slice the rye bread into small cubes and spread them on a baking tray. Toast for 20 to 30 minutes until deep brown but not burnt. This intensifies the malty flavour. Proceed to soaking the bread.

Place the toasted bread into a large non-metallic bowl or pot. Pour over 2 litres of boiling filtered water. Cover and let steep for 4 to 6 hours at room temperature until cooled and richly infused. Move to straining the mixture.

Strain the bread infusion through a fine sieve or muslin cloth into a clean glass or enamel container. Discard the bread solids. This liquid forms the base of your Kvass. Transition to adding fermenting agents.

Dissolve the sugar in the warm bread liquid (not above 38°C). Add the active dry yeast and stir gently. If using lemon peel or mint, add them now for aroma. Cover loosely and let it stand for 12 to 16 hours in a warm place. Continue to bottling.

Once fermented, skim off any foam. Pour the Kvass into clean bottles, leaving a little space at the top. Add a couple of raisins to each bottle if desired. Seal tightly. Proceed to second fermentation.

Leave the bottled Kvass at room temperature for 1 to 2 days to develop natural carbonation. The raisins will float when the Kvass is fizzy enough. Transfer to the refrigerator after. Transition to cooling completely.

Chill the Kvass in the fridge for at least 12 hours before serving. This halts fermentation and balances the flavour. Prepare your glasses and optional garnish.

Pour the Kvass into glasses, optionally straining out the raisins. Serve cold with a slice of lemon or a sprig of mint. It pairs well with hearty Russian dishes or can be enjoyed as a light refreshment on its own.