Leave a lump of flour, water and yeast on the counter and, a couple of hours later, it has doubled. Most people picture the yeast as a tiny bubble factory, blowing new bubbles all through the dough. That picture is wrong in a way that explains almost everything about bread: why you knead it, why a loaf has the crumb it has, and why a dough that rises a lot can still turn out dense.
Yeast eats the sugars in flour and releases carbon dioxide and ethanol, in roughly equal amounts by mole. The carbon dioxide first has to dissolve into the watery part of the dough. Once that liquid is saturated, the gas has nowhere to go but into pockets of air that are already there.
Those pockets come from you. Mixing and kneading drag air into the dough, folding it into thousands of tiny bubbles. Yeast then uses up the oxygen in them, so they shrink a little and are left mostly nitrogen. After that the dough's own bubbles are the only place the yeast's gas can go. As the researchers at Campden BRI put it, the mixing action entrains the air bubbles that are later inflated during proof and baking.
So the number of bubbles is fixed early, and the yeast only decides how big each one gets. Here is the catch: the same amount of gas shared between a few bubbles makes a few huge ones. Shared between thousands, it makes a fine, even foam.
Drag across the dough to knead: every stroke folds in more air. The numbers are a simplified model for feeling the idea, not lab data.
Flour contains proteins, mainly glutenin and gliadin. Add water and work them, and they link into a stretchy network called gluten. Think of it as a web of thin elastic walls. Kneading lines that web up, so the walls can stretch around a growing bubble without tearing.
The same hands also carry the air in. That is the double job: build the walls and make the bubbles the walls will surround. Try the interactive above. Fold in plenty of air, proof it for an hour at around 35 °C and you get a crowded, fine crumb. Click Barely mixed, give it the same hour, and the same gas swells just a few bubbles into cavities, which is how you end up with bread that is dense in places and full of tunnels in others.
Yeast is a living thing, so its speed depends on how warm it is. It works fastest somewhere in the mid-30s °C. In a fridge it slows to a crawl but does not stop, which is why bakers prove dough slowly overnight to get flavour without a huge rise. Push it too hot and the cells begin to die. In the slider above, drag the temperature to 4 °C and watch how long the dough takes to puff up compared with 35 °C.
Put the dough in the oven and it often jumps upwards in the first minutes. This is called oven spring, and it is usually finished within the first third of the bake. The warming dough releases dissolved carbon dioxide, ethanol (which boils at 78.4 °C) turns to vapour, and water starts to evaporate into the bubbles (water boils at 100 °C). All the gas expands as it heats. Bakery scientists still argue about the exact split between carbon dioxide, ethanol and steam, but the result is clear: the bubbles get bigger one last time.
Then the structure freezes in place. Starch swells and sets, the proteins firm up, and the foam turns from a stretchy dough into a solid crumb. Drag the oven slider on to see the dough spring, then lock. Whatever bubbles you had at that moment are the crumb you will slice.
The yeast supplies the gas, but you supply the bubbles. Next time your loaf comes out dense or full of holes, the culprit may not be the yeast at all. It may be the first ten minutes of mixing and kneading, when the bubbles were put in.