Boil 1.5L of water
Toast 2 slices of black rye or pumpernickel bread until very dark
Wash a handful of raisins
Remove water from heat and add the toasted bread and raisins
Add lemon slices if desired
Cover the pot and let sit for at least 3 hours
Filter out the bread and larger pieces from the liquid
Add fresh raisins to the filtered liquid
Measure and add 75-100g sugar and 1 teaspoon dry yeast
Pour mixture into a hermetically sealed glass jar
Store in a warm place for 2 days while fermenting
After 2 days, open the jar carefully (may be under pressure)
Filter the fermented liquid into a second clean glass jar using cheesecloth or a fine filter
Add a squeeze of fresh lemon juice
Refrigerate to 4°C before serving
Serve cold
