Prepare the utensils for cooking yogurt: give boiling water or proterilize over steam (small containers can be placed in a steamer), then wipe dry.
Bring 1 liter of milk to boiling (90oS), but do not boil. Remove from heat and cool to 36-42°C. Check the temperature by dropping some milk on your wrist – it should be slightly hot.
Break 1 bag of dry sourdough culture with warm milk according to the instructions on the package, stir until smooth and add to the main volume of milk. Instead of dry sourdough, ready-made yogurt of industrial production can be used: stir 125 ml (2 tbsp) with the milk so that there are no clots left, and enter the remaining milk. For convenience, use a mixer or blender.
If you are a lucky yogurt owner, pour the sourdough milk into sterilized serving glasses and place in the yogurt for 8-10 hours.
However, yogurt can also be made using a thermos, preferably soup. But in this way, the initial temperature of milk with sourdough is very important, because the thermos only keeps it, gradually reducing, unlike yogurt, which maintains the desired mode for the whole time cooking.
Quickly mix the milk with the sourdough and immediately pour it into the thermos, just before pouring scalded boiling water and wiped dry. Tuck it in a warm blanket, plaid, old fur coat and leave it in a warm place for 8-12 hours. Do not move the thermos so as not to interfere with the process of squirting milk
Properly cooked yogurt has a thick uniform consistency. It can be used as a sourdough for the next time, depositing approximately 2 tbsp in a sterile jar.