Why does bread rise?

Bread rises because yeast eats sugars in the dough and gives off carbon dioxide. That gas doesn't blow new holes. It seeps into tiny air bubbles that were folded in during mixing and inflates them, while a stretchy web of gluten proteins keeps it from leaking out. In the oven the gas expands, more comes out of solution and water turns to steam, which gives a final burst called oven spring before the loaf sets.

What is the yeast actually doing?

Baker's yeast is a single-celled fungus. The grains in a packet are billions of dried, dormant cells, and as Washington State University's Dr. Universe column puts it, they need warm water and food to wake up. Once they do, they break down sugar for energy and release carbon dioxide and ethanol. That process is fermentation.

The details are a bit more choreographed than "yeast makes gas". Campden BRI, a UK food research organisation, describes yeast first using the oxygen trapped in the dough's air bubbles. Air is mostly nitrogen with some oxygen, so as the oxygen is used up, the bubbles first shrink by about 20 percent. After that, fermentation without oxygen takes over and carbon dioxide starts to build.

Even then the gas doesn't go straight into bubbles. It dissolves in the water around the yeast first. At typical proving temperatures of 32 to 38 °C, water holds about 1.0 gram of carbon dioxide per kilogram before it is saturated. Once the water is saturated, carbon dioxide starts moving into the bubbles and the dough begins to swell.

Where do the holes in bread come from?

From the mixer, mostly. Campden BRI's account is that mixing traps air bubbles in the dough, and those bubbles are what get inflated during proving and baking. The yeast supplies the gas. The structure of the crumb owes a lot to the bubbles you started with.

A decent analogy is a sheet of bubble wrap with half-filled bubbles and a slow pump attached. The pump doesn't add new bubbles to the sheet. It fills the ones that are already there, and how big the finished pockets get depends on how much gas arrives and whether the plastic holds.

Why does gluten matter so much?

Gluten is the plastic in that analogy. Kristine Nolin, a chemist at the University of Richmond, describes it as a mixture of proteins, mainly glutenin and gliadin, which together make up 85 to 90 percent of the protein in wheat flour. Glutenin forms long coiled chains that give dough strength and elasticity. Gliadin is smaller and slips between them, making the dough more viscous.

Water and kneading line those proteins up. Kneading breaks some of the bonds inside the glutenin coils, and sulphur-containing cysteine units on neighbouring chains link up into disulfide bonds. As more links form, the network grows. Bread flour, at 12 to 14 percent protein, gives you more to work with.

Nolin compares the network to a balloon that traps gas during rising and baking. The balance matters. Too weak and the gas escapes and the dough collapses. Too strong and the dough can't stretch enough to rise. Salt tightens the network. Butter and other fats coat the flour, keep water away from the proteins and weaken the network, which gives a more tender result.

Press Play to inflate the existing pockets, then compare the three networks. Scrub the progress slider to inspect a still frame.

What happens in the oven?

The extra rise in the oven is called oven spring. WSU's Breadlab head of baking, Mel Darbyshire, describes it simply as a rapid rise caused by heat. What drives it is where the sources disagree a little.

Campden BRI says carbon dioxide coming out of solution causes most of the early pressure increase, with help from nitrogen and ethanol, and that oven spring is usually over in the first third of the bake. It also reports an older calculation by the cereal scientist Bloksma, which concluded that steam provided more than half of the oven spring volume, with ethanol and carbon dioxide making up much of the rest. Those two accounts don't sit neatly together, and the safest reading is that several gases contribute and their shares depend on the assumptions.

The order of events is clearer. Ethanol boils at 78.4 °C and water at 100 °C, so steam arrives late. By then, starch has begun gelatinising, which happens above about 60 °C and makes the crumb much stiffer. Near the end of the bake, steam pressure breaks the thin walls between bubbles, turning a foam of closed cells into a sponge of connected ones. The crust hardens, the loaf stops growing, and browning reactions colour the outside.

What does sourdough change?

Sourdough swaps packet yeast for a living community of wild yeasts and lactic acid bacteria. A 2021 review in Microorganisms puts the usual ratio at about 100 bacteria for every yeast cell. The bacteria make most of the lactic and acetic acid, which is where the sour flavour comes from. A sourdough's pH sits between about 4.0 and 4.8, against 5.1 to 5.4 for bread made with baker's yeast.

The rise still comes mainly from gas. A review by De Vuyst and colleagues credits leavening to the yeasts and to the heterofermentative bacteria, the kind that make carbon dioxide alongside their acids. Every starter differs with its flour, water, temperature and feeding schedule.

That review also pokes at a favourite story. The idea that a sourdough's microbes are tied to its place of origin, the authors write, often seems to be an artefact of how samples were collected and identified rather than a firm finding.

Sourdough still contains gluten

It is not. Fermentation changes some of the proteins, and labs have used selected bacteria plus added enzymes to break gluten down far enough to test with coeliac patients. That is research with special processes, not what happens in a bakery loaf.

Testing of commercial wheat sourdough shows the gap. Gluten Free Watchdog reported one country sourdough at 104,000 parts per million of gluten in a 2015 test, and two more loaves, one of them marketed as having its gluten "neutralized", came in above the top of the test's range in later tests. The US limit for labelling food gluten-free is under 20 parts per million. If you need to avoid gluten, sourdough only works when it is made from gluten-free flours and a gluten-free starter.

Kneading, baking powder and the flat loaf

Next up could be what kneading actually does to glutenin chains, or how baking powder raises a cake with no yeast at all. If your last loaf came out flat, that is a perfectly good starting question too, and Learnfold will turn it into a short course with a new linked page for each follow-up, an evening's rabbit hole that can wander as far as why spilled coffee dries in a ring. It writes with AI, which makes mistakes, so learn how to check an AI explanation before trusting a number like the gluten figures above.