As a previous post noted hydration and temperature are key in selecting for
acetic vs. lactic acids in your sourdough. I personally maintain my stored
starter at 100% hydration because it is easier to do the math when using my
starter with different recipes. Nancy Silverton's white starter is at
145%, Peter Reinhart offers several different starters between Crust and
Crumb and The Breadbaker's Apprentice. My personal experience has taught
me to maintain my stored starter at a hydration that that I find
convenient, and then to vary both hydration and temperature in my builds to
control the flavor of my breads.
If your goal is sour you will want to use a firm build as one of your
steps. Reinhart's San Francisco Sourdough in Crust and Crumb is a good
example. Allowing this build to rise for 4-6 hours and then refrigerating
it overnight will also favor the bacteria that promote sour flavors. You
can use this method regardless of the hydration of the starter with which
you begin.
I recognize that hydration levels can be confusing for many bread bakers
but once you have mastered this concept you can truly become an artist with
your bread, controlling flavor and texture without reference to recipes. I
now bake almost exclusively using an Excel spreadsheet that I designed.
Jim Cook