During sourdough fermentation, arabinoxylans — the major structural fibers of wheat — break down. The standard assumption: the lactobacilli do it. They're the dominant microorganisms. They're metabolically active. They produce acids, gases, flavor compounds. If fibers are breaking, the bacteria must be breaking them.
New research from VUB shows the bacteria barely touch the fibers. The breakdown is driven by endoxylanases — enzymes already present in the wheat flour, dormant at neutral pH, activated by the acidic environment the bacteria create.
The through-claim: the microorganisms set the conditions; the substrate does the work. The bacteria's contribution isn't enzymatic degradation of fiber — it's acidification. As the dough drops below pH 5, wheat xylanases activate and begin cleaving arabinoxylans. The bacteria are catalysts of catalysis: they don't perform the reaction, they enable the reagent that performs it.
This reframes fermentation as a two-layer process. Layer one: microbial metabolism produces acid, flavor compounds, COâ‚‚. Layer two: the acidic environment activates dormant plant enzymes that restructure the dough's fiber architecture. The texture and digestibility changes attributed to “fermentation” are actually autodigestion — the wheat breaking itself down in conditions the bacteria created.
The practical consequence: bread made with arabinoxylan-enriched flour and sourdough culture was both higher in fiber content and more flavorful, because the endogenous enzyme activation produces a broader range of breakdown products than bacterial metabolism alone. The dormant enzyme, once activated, does more diverse work than the bacteria that woke it.