Why does bread rise when yeast is added?

Yeast cells consume sugars in the dough and produce carbon dioxide and ethanol through fermentation. The carbon dioxide bubbles are trapped in the gluten network, which stretches and inflates as the dough proofs. That expansion is what gives bread its airy texture. Heat in the oven then kills the yeast and sets the structure, while steam and gas expansion provide oven spring. Baker’s yeast, Saccharomyces cerevisiae, has been used for bread-making for thousands of years.