In sourdough bread baking what do the wild yeast and bacteria not eat during dough fermentation and why? I bake bread using a sourdough starter. The starter is a culture of lactobacillus and some kind of yeast. They apparently eat the sugars or carbohydrates in the dough during fermentation but when I look at the nutritional info for fully baked sourdough bread I see 56gm of carbohydrates per 100gm and my question is, why do these organisms stop eating and what is it exactly that they leave behind? They seem to consume something in the dough then they stop as if they have run out of food at which point you need to bake the bread else it becomes &quot;over-proved&quot;. Why do they stop eating and what is actually left over when the fully fermented dough goes in the oven?
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).