Every loaf of bread is, technically, a sponge full of gas made by billions of tiny fungi. The holes you see when you slice it are where the gas was. And the walls around those holes are a protein net, built by whoever kneaded the dough. Making bread is mostly persuading two kinds of chemistry to work together: one living, one not.
Start with the living part. Baker’s yeast, Saccharomyces cerevisiae, is a single-celled fungus, and a single gram of fresh yeast holds around ten billion of them. Like us, they need energy, and they get it from sugar. But flour contains very little free sugar. It is mostly starch, which is sugar molecules linked into enormous chains, too large for yeast to use. This is where the flour’s own enzymes come in. The moment flour meets water, enzymes called amylases, naturally present in the wheat, begin snipping the starch chains into smaller sugars. The yeast feeds on those.
Deep inside a ball of dough there is little oxygen, so the yeast doesn’t burn its sugar completely, the way your muscles do with every breath. It ferments it instead, breaking each sugar molecule into carbon dioxide and ethanol, the same alcohol as in wine. The carbon dioxide is the point. Each yeast cell releases a tiny trickle of gas, and with billions of them working, the dough fills with it. The alcohol is a by-product, which mostly evaporates later in the oven, though it contributes to the smell of fresh bread.
On its own, though, the gas would simply escape. Try making bread from rice flour and you’ll see: the yeast bubbles away happily, and the dough stays flat, because nothing holds the bubbles in. Wheat has a trick no other common grain can match.
Wheat flour contains two proteins, glutenin and gliadin. Dry, they do nothing. Add water, and they unfold and start to link up. Glutenin molecules are long and join end to end into chains, tied together by strong chemical links called disulfide bonds, which make them springy. Gliadin molecules slip between those chains and act like a lubricant, letting them slide past each other, which makes the dough stretchy. Together they form gluten: a network that is both elastic, so it springs back, and extensible, so it stretches without tearing. It’s the combination that matters. Too much spring and the dough can’t expand; too much stretch and the bubbles burst.
Kneading is how you build this network. Every fold and push lines up the protein chains and links them into longer, stronger sheets, which is why a dough that starts sticky and shapeless turns smooth and elastic after ten minutes of work. A well-kneaded dough can be stretched so thin you can see light through it without it tearing, a test bakers call the windowpane. Each sheet of gluten becomes the wall of a gas bubble, and as the yeast fills the bubbles, the walls stretch, and the whole dough rises like a crowd of tiny balloons.
Then comes the oven, where the bubble structure is fixed in place. In the first few minutes, as the dough heats, the yeast goes into a final frenzy, and the gas already inside the bubbles expands as it warms, just as any gas does. The loaf puffs up suddenly: bakers call this oven spring. Around sixty degrees, the yeast dies. A little hotter, and the starch granules soak up water, swell and set into a soft gel, while the gluten proteins firm up permanently, much as an egg white sets. The gas escapes, but the walls now hold their shape. What was a stretchy dough full of bubbles becomes a solid sponge full of holes.
The outside, meanwhile, is drying out and getting much hotter than the inside, which can never rise much above the boiling point of water while it is still moist. Above about a hundred and forty degrees, sugars and proteins on the dry surface react with each other in a cascade of reactions called the Maillard reaction, producing hundreds of new molecules, brown colours and the toasty, nutty smell that fills the kitchen. It’s the same chemistry that browns a dosa, roasts coffee and makes toast taste of toast.
So every slice of bread is a record of all this. The size of the holes tells you how much gas the yeast made and how strong the gluten was. The colour of the crust tells you how hot the oven was. And the fact that it rose at all tells you that a few billion fungi and two stubborn proteins spent a few hours doing exactly what you asked them to.