Filmjölk is a mildly tart yogurt that cultures at room temperature, so there’s no need for fancy yogurt makers or other equipment. It’s a good idea to heat your milk until steaming and then let it cool to room temperature, as this helps to reorganize the milk protein so that the filmjölk thickens more easily. It also helps eliminate any competing microbes in the milk that may change the final product's flavor or texture.To make future batches of filmjölk, reserve ¼ cup from one batch and store it in the fridge for up to 1 week. Then, use it as your starter for the next batch.
Bring the milk to a simmer over medium-high heat. When it starts to steam, reaching a temperature of 180F, turn off the heat and allow the milk to cool to room temperature.
Whisk the filmjölk starter into the room-temperature milk and then pour it into a 1-quart mason jar. Cover it tightly, and let the milk culture at room temperature for 24 hours or until the thickened slightly. Store in the fridge for up to 2 weeks.