15. To make sourdough bread, a sourdough “starter” is used. This is made from a mixture of just flour and water that undergoes a fermentation process for about two weeks. Throughout this time, more flour and water are added at regular intervals to feed the starter. After the starter is ready, whenever one uses it, he adds new flour and water to the starter, then takes some of it to cause fermentation. This portion is used to prepare dough by adding more flour and water to it.
16. Starter is potur from hafrasha. The starter itself is not fit for consumption, so it itself is potur from hafrashas challah (עי' דרך אמונה בה''ל פ''ו הי''ט ). Thus, even if one makes a large starter with enough flour to require hafrashas challah, he does not do hafrasha, as it is potur.
17. Flour in the starter counts toward the shiur. However, when sourdough starter is used to ferment other flour, the quantity of flour in the starter counts toward the shiur of flour in the dough for the chiyuv hafrashas challah (שו''ע סי' שכ''ד ס''ג ). E.g., if a starter has equal parts flour and water, as is common in starter recipes, and 100 grams of starter is added to 1600 grams of flour, 50 grams of the starter is flour and combines with the 1600 grams of flour to require hafrashas challah.
