Leave sourdough dough fermenting long enough and it stops feeling like dough at all — slack, wet, tearing instead of stretching. That's rarely just overproofing. It's an enzyme the flour was carrying the whole time, dormant until the starter's bacteria acidify the dough enough to switch it on, and from there it spends the rest of the ferment quietly cutting the gluten network apart.
Why acid switches on an enzyme yeast dough never meets
Wheat flour carries its own protein-cutting enzymes, left over from the grain's own biology, and in a plain yeasted dough they barely do anything. The dough's pH stays close to neutral — typically around 5.3 to 5.8 — and that isn't the range these enzymes work well in. A sourdough starter changes that math. Its lactic acid bacteria produce acid continuously as they ferment, and by the time a sourdough dough is ready to shape, its pH has commonly dropped to somewhere around 3.8 to 4.5. That's close enough to where the flour's own proteases turn on, and once they do, they start snipping the long protein chains — glutenin and gliadin — that give gluten its stretch and its snap back.
None of this is damage in the sense of something going wrong. It's the same chemistry running a little further the longer the dough sits. A short sourdough ferment barely touches it. A twelve-hour cold bulk ferment gives it enough time to visibly loosen the dough, which is why a loaf pulled a few hours later than usual can go from workable to sticky and slack without anything else in the recipe changing.
The same breakdown that weakens gluten also deepens the crust
That protein-cutting doesn't only loosen structure. Every gluten chain it opens releases smaller peptides and free amino acids into the dough, and those amino acids are the raw material the Maillard reaction needs once the loaf hits the oven — the browning reaction that darkens a seared steak's crust, not the plain sugar-browning of caramelization. A straight yeast dough, usually baked within a few hours of mixing, hasn't had nearly as much time to build up that pool of amino acids. So sourdough gets a second, separate contribution to its flavor, sitting alongside the sourness the acid adds directly. One comes from the acid tasting sour. The other comes from what the acid enabled somewhere else entirely.
Why the schedule runs on temperature, not a clock
Two things are speeding up together in a sourdough ferment: the bacteria making acid, and the enzyme that acid switches on. Both are ordinary biological reactions, and like most of them, their rate roughly doubles for every 18°F (10°C) the dough warms. A bulk ferment written as eight hours assumes one specific dough temperature. Run the same dough ten degrees warmer and both processes — acidification and the gluten breakdown that follows it — finish in closer to four hours, so a baker following the clock instead of the dough ends up well past where they meant to stop. A plain yeast dough, leavened by gas production alone, drifts too when a kitchen runs warm, but it has no second, acid-driven reaction stacking on top of the first. That's the actual reason sourdough recipes lean so hard on dough temperature instead of a fixed number of hours.
The starter's own ratio, and what happens when it's off
A starter is maintained on its own ratio, separate from the dough's hydration: roughly equal parts by weight of mature starter, fresh flour, and water, often shorthanded as a 1:1:1 feed. That balance keeps the yeast and bacteria supplied with new sugar before the batch gets too acidic relative to how many organisms are actually there to use it. Feed with too little fresh flour against the old starter and the mix stays acidic and sluggish, already leaning toward the same breakdown discussed above before it even meets the final dough. Feed with too much fresh flour and the population gets diluted past the point where it can reliably double in a normal window, so the starter takes longer to wake up than the schedule expects. Keeping to the ratio, not a fixed jar size, is what lets a baker maintain a small starter indefinitely or build a big one up before a weekend bake.