Transfer the coarse bulgur and rice you washed in plenty of water into a deep saucepan. Add the water and boil over medium heat until it becomes soft.
After straining the water of the mashed bulgur rice mixture, pass it through a fine-hole strainer. You can discard the pulp part.
Leave the bulghur porridge that you filtered at the room temperature to cool completely.
For the yeast mixture that will make boza sour; Mix warm milk, fresh yeast and powdered sugar in a separate bowl.
Transfer the porridge mixture cooled at room temperature into a large pot. After mixing it with the yeast mixture you have prepared, keep it in the pot you have closed its lid in, in a sunless environment for 24 hours.
Open the lid of the boza mixture that you have kept for fermentation from time to time and let it vent.
After the boza wait enough, add the powdered sugar and then the water little by little to the boza and adjust its consistency.