I am having a bit of trouble with my preferment, sponge. In my bread baking books they talk about a sponge, that if properly prepared, and ripe will impart a slightly sour flavor to the finished loaf. Mine does not and I'm wondering why. My sponge is a 64% hydration and made as follows: 1/4 cup water with 1/8 tsp of dry yeast mixed in and set aside. In a small bowl I put 80 gm of water. I then add 30 gm of the yeasted water (2 TBLSP). Add 30 gm of whole wheat flour and 140 gm of unbleached A.P. flour. Mix well by hand. Cover and set aside on counter for 12-14 hrs. (No salt added). Also, I have tried letting it rest on the counter for 3 hrs and then refrigerating it over-night. Neither method yields the slightest sourness. I then proceed to make my loaf as normal (125 gm of unbleached A.P flour, 30 gm of water, 38 gm of milk, 3/8 tsp yeast, 1 TBLSP of butter, 1 tsp sugar. After an autolyse of 20 min I add 1 1/8 tsp of salt. 3hr bulk ferment which includes 3 folds. A very nice tasting loaf but alas NO sourness. P.S. I am NOT attemping to create a sour dough, just a slight tangyness. Obviously, I'm doing something wrong but I don't know what it might be. Any thoughts?