Why Can Yeast Make Dough Rise?

When dough rises, yeast does not push the whole mass upwards like a tiny air pump. Yeast obtains energy from fermentable sugars in the flour and produces carbon dioxide and ethanol. The carbon dioxide enters small bubbles already present in the dough, increasing their internal pressure and volume.

Producing gas is only half the process. Once wheat flour is hydrated and mixed, a viscoelastic gluten network develops. It acts like a connected set of stretchable walls that partitions and retains the gas. Bread therefore depends on both gas production and gas retention: active yeast cannot raise a weak structure if the gas escapes, while a strong network will not expand much without adequate fermentation. Gluten-free bread can also rise, but other structural ingredients must help stabilise its bubbles.

Early in baking, trapped gas expands as it warms, while steam from water and ethanol also enlarges the cells. Starch gelatinisation and changes in protein structure then set the flexible foam into the visible crumb. The central point is not simply that yeast “makes bread fluffy”. Microbial metabolism supplies the gas, and the material structure of the dough preserves that gas as form.

https://openstax.org/books/microbiology/pages/8-4-fermentation
https://pubs.acs.org/doi/10.1021/jf1003946