Rye or wheat flour for a first sourdough starter?
Either works. The wild yeast and bacteria live on the surface of the grain, so they come into the jar with the flour whichever you choose. Rye is a fine pick to get going, just don't expect a rye dough to rise as high as wheat: its pentosans stop a gluten network forming, so it holds less gas even when the starter is perfectly healthy.
Answered by Hendrik, The Bread Code
From the book
Please also note that you can only make bread with great oven spring when making wheat based doughs. When starting with this hobby I always wondered why my rye breads would turn out so flat. Yes, rye has gluten, but small particles called pentosans (arabinoxylan and beta-glucan) prevent the dough from developing a gluten network the way it can with wheat. Your efforts will be in vain, and your dough will stay flat. Only spelt- and wheat-based doughs have the capability of retaining the CO₂ created by the fermentation. The more bran your flour contains, the more water your flour can absorb.
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