No, sourdough doesn’t have to be sour. Perfectly fermented sourdough isn’t very sour at all. It may have a slight tang to it, but some people might not even taste a sour note at all, especially in those items made with a “stiff starter” like mine. “Sourdough” is just a term used to describe a bread that has been naturally leavened with wild yeast captured and grown in a sourdough starter. Sourdough is not supposed to be sour. The baker can manipulate the sourness, to an extent, through the health and maintenance of the starter, the time and temperature of fermentation, and also by under proofing or over proofing the dough. True sourdough will never be as sour as what you find in the store/commercial bakery. Commercial bakeries usually add acid flavor by means of vinegar, enzymes, or citric acid, and use yeast to compensate for the lack of rise when pushing fermentation. The commercial yeast also gives the bread a more closed crumb (less holes in the dough), and speeds up the rising process. This prevents the final bread from being as nutritious and gut friendly as true sourdough.