Whey is produced in (fresh) cheese production and I don't throw it away. Add the sourdough mixture and the flour to the whey (alternatively: water). Mix well. The items should be left to ferment for at least 24 hours. Differences in dough maturity arise from the room temperature. I assume that most of them don't have a proofer with a constant 25 ° C. A swim test can be used to determine whether the dough is ripe (see pictures). Put a small piece of dough in a glass of water. When it swims, the sourdough is ripe. When it comes to diving, he still needs time.