I wonder if sourdough starters like the endosperm more than they like bran and germ. If so, maybe this explains why when I feed a starter with AP flour, i.e., 100% endosperm, it thrives both faster and more vigorously than if I feed it whole wheat flour, where it’s getting only 83% endosperm, and then 14% bran and 3% germ (rough figures). With AP flour, is it getting more of what it likes?
I thought starter liked whole grain flour more, but I haven’t tested it. The only test I’ve done was to divide an AP starter and feed one jar whole grain red something (can’t remember) and the other jar whole grain rye. The rye grew faster. I called it the Starter Olympics.
I do think starter loves sugar/starch. I believe the vegan version of my sourdough spelt sandwich bread recipe rose faster in my various test batches than the non-vegan version because the oat milk I bought was super carb-y.
When making a starter using wholegrain has its advantages. The yeast and bacteria grow on the grain so using the wholegrain is a better option.
Once a starter is established the yeast and bacteria do love the sugars which are concentrated in the endosperm. However that isn’t to say adding in some wholegrain doesn’t have its advantages. It adds to flavour for one thing and it adds to the health of the starter. Even when mature the environment + food can alter a starter. So adding into the feed some whole rye if your starter is sluggish can be beneficial.
What you might also be experiencing is the different qualities of bread flour and wholegrain. For example the characteristics as supposed what’s going on under the microscope. Just like a bread flour loaf encourages larger bubbles and a higher rise because of the gluten formation not being interfered with by the bran then so to in a starter so some degree.