I'm used to the crust on sourdough bread being thick and chewy, but had always attributed that to the longer rise, and baking with steam, that one normally does with sourdough. However, this week I made a hybrid sourdough* in my bread machine, and its crust was thicker, browner, and chewier than non-sourdough breads made in the same machine. Since the baking cycle and atmosphere are the exact same as, say, last week's whole wheat loaf, it's not how it was baked.
So my question is: what is the physical or chemical property of sourdough starters that results in a heavier crust?
Note that while there are a number of questions on the board about sourdough crusts, all of them focus on manipulating the baking environment, and not on properties of the dough itself.
(* hybrid sourdough: some sourdough starter plus a little commercial yeast)