Wash the berries, then press them through a sieve. Pour the juice into a pot and set it aside in a cool place.
Transfer the leftover berry pulp to another pot, cover the pulp with hot water, add the sugar, stir, and bring the mixture to a boil.
Remove the pot from the heat, stir in the potato starch dissolved in cold, boiled water, return the pot to the heat, and, stirring constantly, bring it back to a boil.
When the mixture cools to 104–122°F (40–50°C), add the chilled juice and stir in lemon juice or citric acid. This helps preserve the vitamins.
Kissel can be thin or quite thick; the thicker version is often served with ice cream.
For a medium-thick kissel, use 1 liter of water, 400–500 g of berries, 100 g of sugar, 2 g of citric acid, and 3 tablespoons of starch.
