Raw milk or fresh milk with a short shelf life must be boiled first. Raw milk is untreated and may contain small amounts of various germs and bacteria.
If you drink untreated fresh milk, it is usually completely harmless. However, if this is processed into yoghurt, the initially very low number of germs can increase significantly. In this case, the yogurt becomes slimy and smells unpleasant.
Very important: Only stir in the yoghurt ferments well after it has cooled down. If the milk is too hot, the bacteria die.