For taking challah, (how) do you account for the flour in sourdough starter?
I understand that one should take challah when baking bread made with at least 10 cups of flour. (That article says 14 for saying the b'racha but down to 10 for taking at all.) How does this requirement interact with sourdough starter?
A sourdough starter is a fermented mixture of flour and water. One "feeds" the starter from time to time with more flour and water to keep the fermentation process going. To make bread, you take some of the starter, feed it, let it ferment for a few hours, and then prepare your dough with it and more flour and water.
When deciding whether you've met the threshold for taking challah (and then the threshold for doing so with the b'racha), do you count the flour you fed the starter? Do you proportionally count some volume of the pre-fed starter? Or is it only the flour that goes into the bowl right before kneading that counts?