In sourdough bread baking what do the wild yeast and bacteria not eat during dough fermentation and why?
Flours contain a diverse array of carbohydrates which vary depending on the specific type of flour (wheat, rye, teff, etc). There will usually be a lot of starch (itself having variable amounts of different types of branching), some small mono and oligosaccharides (again of varying types) and &quot;fiber&quot;, which is a broad term for polysaccharides that are more resistant to digestion.
To add to the variables, the specific types of bacteria and yeast in a sourdough fermentation can vary in their ability to digest the different types of carbohydrates.
So one part of the answer to your question is that yes, there are carbohydrates that cannot be digested, but the specific molecules that are not digested will depend on the flour and the microbes. The cause of the undigestibility is either 1) the monosaccharides are linked in a way that they cannot be cut apart or 2) the microbes lack the metabolic capability to digest the monosaccharide.
But generally in a bread dough, the digestible carbohydrates have not been completely consumed, so some of the remaining carbohydrate is digestible. The reason it is not used up is that the baker stops the fermentation before it reaches that point so that the dough still has a good consistency.
It is also true that the fermentation activity slows down considerably before this point, even though it does not stop. There are two reasons for this. 1) the pH drops, which slows the metabolism of the bacteria and 2) the easily digestible carbs get used up first, so the later fermentation rate can be affected by the slow rate of breakdown of the more difficult to use carbohydrates. Ultimately, the dynamics of the fermentation are determined by the choice of flour, the microbial population and the conditions (temp, water content, whether and how often the baker works the dough, etc).