Why does bread rise?
One-line takeaway
BLUF: Yeast eats sugars in the dough and gives off carbon dioxide. The gas cannot escape because gluten, a stretchy protein network formed when flour meets water and is worked, traps it in thousands of tiny bubbles. In the oven those bubbles expand and then set.
Bread is one of the oldest examples of humans farming a microorganism for a physical effect rather than a flavor.
Gas from yeast, trapped by gluten
Wheat flour contains two proteins, glutenin and gliadin, that link into gluten when hydrated and worked by kneading. Gluten is both elastic and extensible, so it can stretch around an expanding bubble without tearing. Meanwhile baker's yeast, Saccharomyces cerevisiae, ferments sugars released by enzymes breaking down starch, producing carbon dioxide and ethanol. The gas dissolves into the dough's water until it is saturated, then inflates gas cells already present from mixing. Yeast does not create new bubbles; it only enlarges the ones trapped during kneading, which is why a badly mixed dough rises unevenly. The alcohol mostly bakes off, leaving behind aromatic byproducts that give bread much of its smell.
A gas source and a gas trap
The deeper principle is that a rise needs a gas source and a gas trap, and the two are independent. Chemical leaveners make the same gas without yeast: baking soda releases carbon dioxide when it meets an acid, and baking powder carries its own acid. Beaten egg white or steam can do the job in other batters. Change the trap and you change the bread. Rye has little gluten, so rye loaves are dense and are usually held together by acidified starch from a sourdough culture instead. In the oven a final surge called oven spring comes from three things at once: gas expanding as it warms, dissolved carbon dioxide coming out of solution, and water turning to steam. Around 140F the yeast dies, and as starch gelatinizes and proteins set, the foam becomes a permanent structure.
Time is the baker's real ingredient
Bakeries today choose their leavening for time as much as taste. Commercial instant yeast gives a predictable rise in an hour or two. Sourdough starters carry wild yeasts alongside lactic acid bacteria, work more slowly, and produce acids that add flavor, strengthen the dough, and slow staling. Long cold fermentation in a refrigerator slows the yeast while enzymes keep working, which develops flavor without overinflating the dough, and it is why many pizza and baguette recipes call for an overnight rest. Home bakers use the same physics when they score a loaf: a deliberate cut gives the expanding gas a controlled place to break out instead of splitting the crust wherever it happens to be weakest.
What the yeast is not doing
Common misconceptions begin with what the yeast is doing. Yeast does not eat the whole loaf's sugar or need added sugar; flour enzymes supply plenty, and a spoonful of sugar mainly speeds the early stage. Longer rising is not always better, since an overproofed dough exhausts its gluten and collapses. Kneading is not strictly required, because time and hydration will align gluten on their own, which is the basis of no-knead recipes. Salt does not simply kill the yeast; it slows fermentation and tightens the gluten, and bread without it rises fast and tastes flat. And the holes in an open crumb are not a sign the bread rose more, only that the bubbles merged into fewer, larger ones.