"Why does over-proofed bread collapse?"

"Over-proofed dough has been inflated with CO2 past the structural capacity of its gluten network. The gas bubbles have expanded enough that the gluten walls between them have thinned to the point of tearing. When the dough is handled or placed in the oven, the gas escapes and the structure collapses. The gluten also weakens over time as yeast produces organic acids that degrade it."

"Why do some breads use cold fermentation?"

"Cold fermentation (4-8°C) slows yeast activity dramatically but doesn't stop it. The dough rises very slowly over 8-24 hours. During this extended time, the yeast and native bacteria in the dough produce small amounts of organic acids, esters, and other flavor compounds that can't develop in a 1-hour room-temperature proof. The tradeoff is time. The result is more complex flavor."

"What does salt do in bread dough?"

"Salt serves several functions in bread. It strengthens gluten structure by promoting tighter protein interactions. It controls yeast activity, slowing fermentation and preventing over-proofing. It affects flavor directly. And it retards enzymatic activity that would otherwise degrade gluten over long fermentation. Salt-free bread is possible but ferments faster and has weaker structure and less flavor."

"What is the difference between active dry yeast and instant yeast?"

"Active dry yeast has a protective coating of dead yeast cells around a live core. It needs to be proofed in warm water first to dissolve the coating and allow the live cells to become active. Instant yeast (also called rapid-rise) has smaller, more porous granules that don't need proofing and can be added directly to flour. Instant yeast is also slightly more potent. Both are Saccharomyces cerevisiae, just processed differently."

How Yeast Makes Bread Rise: Fermentation, CO2, and Gluten Science

Quick Answer

Baker's yeast (Saccharomyces cerevisiae) consumes sugars in dough through fermentation, producing CO2 gas and ethanol as byproducts. The gluten network in bread dough traps the CO2 bubbles, inflating the dough. Heat in the oven kills the yeast and sets the gluten network in its expanded state, producing bread's open crumb structure.

The Science

A lump of raw bread dough looks dense and inert. Leave it in a warm kitchen for an hour and it doubles in size, feels airy, and smells faintly of beer. Something biological is happening inside, and the thing that’s alive in there is a single-celled fungus that’s been domesticated by bakers for at least 5,000 years.

Saccharomyces cerevisiae and What It Wants

Baker’s yeast is Saccharomyces cerevisiae, the same species used in brewing beer and fermenting wine. In its natural habitat, it lives on the skins of fruit and grapes, where it has access to simple sugars. In dough, it gets those sugars from the flour.

Wheat flour contains some free sugars (glucose, fructose, maltose) and large amounts of starch. Yeast can use the free sugars directly. For the starch, enzymes naturally present in flour (amylases) break down some starch into maltose and glucose, which yeast then consumes.

The yeast’s goal is purely metabolic: consume sugar, reproduce, survive. The CO2 and ethanol it produces are byproducts it doesn’t need. For the baker, these byproducts are the whole point.

The Fermentation Chemistry

When yeast ferments sugar anaerobically (without oxygen, which is the condition inside a mass of dough), the reaction is:

C6H12O6 (glucose) produces 2 CH3CH2OH (ethanol) + 2 CO2 (carbon dioxide) + energy

This is the same reaction that produces alcohol in beer. In bread, the ethanol mostly evaporates during baking, though trace amounts contribute to flavor in the final loaf. The CO2 is the leavening agent.

One glucose molecule produces two molecules of CO2. At dough temperatures and the scale of fermentation in a loaf of bread, this produces enough gas to inflate the dough significantly, provided there’s a structure to contain it.

Gluten: The Balloon Walls

Without gluten, the CO2 would bubble up and escape immediately, the way it does in a glass of soda. Bread dough traps the gas because the gluten network forms a continuous, elastic, gas-impermeable matrix.

Gluten consists of two proteins, glutenin and gliadin, that when hydrated and worked, form long, cross-linked protein networks. These networks are elastic (they stretch) and cohesive (they hold together). When CO2 forms, it creates pressure within the gluten network. The network stretches but doesn’t break, trapping the gas in small bubbles.

Think of gluten as the rubber of a balloon and CO2 as the air being blown in. Weak gluten (under-kneaded, or made with low-protein flour) doesn’t stretch without tearing. The gas escapes and the loaf is dense. Strong gluten traps the gas and the loaf rises properly.

The gluten also sets during baking. At around 160-180°F, the gluten proteins denature permanently into the expanded position. The crumb structure you see in sliced bread is the frozen record of where the gas bubbles were.

Temperature and Yeast Activity

Yeast activity is extremely temperature-sensitive. At 39°F (refrigerator temperature), yeast metabolism is very slow but not stopped. At 70°F, it’s active and will proof dough in 1-2 hours. At 100-110°F, it’s working at near-maximum rate. At 140°F, it dies.

This range gives bakers enormous control. A long, slow cold fermentation at 38-40°F for 12-24 hours produces noticeably more complex flavor than a quick 1-hour room-temperature proof. The slower metabolism allows more time for enzymatic breakdown of flour proteins and starches, plus the gradual accumulation of flavor compounds from yeast metabolism.

A warm proof (85-95°F, the temperature inside a turned-off oven with just the light on) speeds things up dramatically, useful when you’re in a hurry. But speed comes at the cost of flavor development.

Salt, Fat, and the Other Variables

Salt at normal bread concentrations (1.8-2% of flour weight) slows yeast slightly by increasing the osmotic pressure around the yeast cells, drawing water out and inhibiting metabolism. This is useful: it gives bakers more control and prevents over-fermentation. Salt also strengthens gluten structure, which holds the gas bubbles more effectively.

Fat (from enriched doughs like brioche or challah) coats gluten strands and weakens the network. This makes the crumb more tender but also limits gas trapping. That’s why enriched breads are denser and have a finer crumb than lean bread. The fat shortens the gluten (the same mechanism as in fat in baking), which is why biscuits, croissants, and brioche all have distinctly different crumb structures despite all being baked from fermented or leavened doughs.

When Yeast Isn’t the Leavening Agent

Baking soda and baking powder are chemical leaveners that produce CO2 without biological activity. They’re faster (no proofing time needed) but produce no fermentation flavors. Quick breads, muffins, and cakes use chemical leaveners. Yeasted breads use biological leavening. The gas production mechanism is different, but the gluten physics are the same.

Sourdough adds another layer: wild yeast and lactic acid bacteria fermenting together. The bacteria produce lactic and acetic acids that flavor the bread and slightly weaken the gluten, producing the characteristic open crumb and sour taste. The wild yeast does the same CO2 production as commercial yeast, just more slowly and with more microbial complexity. See more in fermentation basics.

What This Means for You

Proof yeast in water between 100-110°F. Below 70°F, yeast is too slow for practical baking. Above 140°F, it dies. For flavor, slow-ferment dough in the refrigerator overnight. Cold fermentation lets organic acids and other flavor compounds develop without overproofing. The gluten needs adequate development before fermentation begins, so knead or fold adequately before the first rise.