Why Does Bread Rise?

When baker yeast (Saccharomyces cerevisiae) is mixed into dough, it metabolises sugars through fermentation, producing carbon dioxide (CO2) and ethanol as byproducts. The CO2 forms tiny bubbles throughout the dough, but these would simply escape if not for gluten — an elastic protein network formed when flour proteins (glutenin and gliadin) are hydrated and kneaded. Gluten strands trap the gas bubbles, allowing the dough to inflate like thousands of tiny balloons. During baking, the heat causes a final rapid expansion ("oven spring") as gas expands and dissolved CO2 comes out of solution, then the gluten and starch set permanently, and the ethanol evaporates. Sourdough relies on wild yeast and lactic acid bacteria, which produce the characteristic tangy flavour through a slower, more complex fermentation.