Leavening versus fermenting
I've been thinking about leavening and fermenting with bread making. The books I've been reading are Jeffrey Hamelman's Bread, Peter Reinhart's Whole grain breads and Artisan breads every day, and Chad Robertson's Tartine bread.
Both Peter Reinhart and Chad Robertson state that the sour flavor for a sourdough comes from the Lactobacillus sanfranciscensis bacteria, not from the yeast. The Lactobacillus produce lactic acid which makes the bread taste sour. This is what I call fermenting.