This bread, with its mellow tang, is perfect for those who like their sourdough bread noticeably sour, but not mouth-puckeringly so. The signature sourdough flavor comes from a combination of lactic and acetic acids, created as the dough rises and ferments. Refrigerating the dough encourages the production of more acetic acid, which is the tangier of the two. Thus, this bread with its refrigerated starter has the ideal balance of sour flavor.