What makes bread rise?

What makes bread rise?

Dough goes into the bowl as a dense lump and comes out puffed to twice its size, full of holes. Nothing was added. So where did all that extra volume come from — and what put the holes there?

If the dough grew without gaining ingredients, then something inside it must be producing gas. And for gas to puff the loaf rather than simply escape, the dough must be able to trap it. So we need two things: a maker of gas, and a net to hold it. What living thing might be making gas, and what in flour might form the net?

Take the net first, because flour on its own has nothing net-like about it. Dry flour is a powder, and the two proteins that matter — glutenin and gliadin — sit in it separate, folded and inert. Add water and they hydrate, unfold and begin to link to one another; work the dough and you align and extend those links. The pairing is the clever part. Glutenin forms long chains that resist being pulled and want to spring back, so it supplies strength; gliadin is smaller and flows, so it lets the mass stretch instead of tearing. Elasticity alone gives you a lump that refuses to expand, extensibility alone a puddle. Gluten is what the two become together — which is why the word names something that did not exist before the water went in.

Now the gas. Yeast is a single-celled fungus, and given sugar and little oxygen it converts that sugar into carbon dioxide and ethanol. But flour contains very little free sugar, so where is the yeast getting it? From the starch, broken down into fermentable sugars by enzymes already present in the flour. The dough is quietly running a two-stage supply chain, which is one reason a long slow ferment behaves differently from a fast one: it gives the enzymes time.

Here is the step most accounts skip, and it answers "what put the holes there". The carbon dioxide does not appear as bubbles. It appears dissolved in the water of the dough, as gas is dissolved in an unopened bottle. Only once that liquid is saturated does it start coming out of solution — and it does not carve out new bubbles to do so. It migrates into pockets already present: the microscopic bubbles of ordinary air beaten in while you mixed. So the number of holes in the finished loaf was very nearly settled at the mixing stage, before a single yeast cell had done anything, and fermentation only inflates what mixing supplied. Hence an under-mixed dough gives a coarse, irregular crumb that no amount of extra proving will refine.

Then the loaf goes into the oven and rises again, quickly, in the first minutes. Why would heat make a dough expand rather than simply cook it? Several things push at once. The trapped gas expands as it warms, adding something like a tenth to its volume by the time the interior reaches 60 degrees. Carbon dioxide grows less soluble as the water heats, so the reserve still dissolved comes out into those same pockets. The ethanol from fermentation boils at 78 degrees, and a little liquid alcohol becomes a lot of vapour. And the yeast, warming toward its most active range before dying somewhere around 55 to 60 degrees, ferments hardest in the last minutes of its life. Then expansion stops — not for want of gas but because the walls have set: starch granules absorb water and gelatinise from around 60 degrees, the gluten proteins coagulate above that, and a stretchy foam becomes a rigid one.

This is why kneading and waiting both matter: kneading builds the stretchy net, and time lets the yeast eat and exhale. Heat later kills the yeast but sets the balloons in place, freezing the airy structure into crumb.

But "heat sets the balloons" hides a race. Everything above — expanding gas, gas leaving solution, alcohol vaporising, a last burst of yeast — happens on the way up to about 60 degrees, and everything that fixes the structure happens above it. The loaf grows for as long as it is still soft and stops the moment it is not. A dough proved too long arrives with its network already stretched to the limit, and instead of springing it slumps — not for want of gas but because it has spent the elasticity needed to hold the last push.

A sourdough starter differs from commercial yeast in a way worth naming, since it is not simply "wild yeast". It is a stable community of wild yeasts and lactic acid bacteria, the bacteria typically far outnumbering the yeast cells. The yeast still supplies most of the lift; the bacteria supply lactic and acetic acid — the tang — and drop the dough's pH substantially. That acidity is not only flavour: it changes how the gluten behaves and how the flour's enzymes work, which is why sourdough is slower, why its crumb and crust differ, and why it keeps longer. Commercial baker's yeast is one selected strain of the same species, supplied at a far higher cell count and producing no acid — buying speed and predictability at the cost of everything the bacteria contributed.

• Why baking soda makes cake rise by chemistry instead of by life.
• What steam in the first minutes of baking does for the crust, and why it lets the loaf grow further.

Every term the collection defines is gathered in the glossary.