The flour you use in sourdough bread makes a big difference to how sour the bread becomes. Switching to whole grain flour will increase the sourness by a long stretch, even if you changed nothing else.
As the dough ferments, it eats up sugars and starches in the flour. This takes away the sweeter notes of the flour, giving an increase in sour flavor.
Many bakers throw out the hooch that develops on top of sourdough starter. Mix it right back into the starter during feedings to give your bread an extra sour flavor.
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