The ingredient in bread that causes it to rise is yeast. Yeast is a microorganism that belongs to the fungus family. When yeast is combined with water and provided with a source of food, such as sugar or starch, it undergoes fermentation. During fermentation, yeast consumes the sugars and produces carbon dioxide gas as a byproduct.
The carbon dioxide gas gets trapped in the dough, creating bubbles that cause the dough to rise and expand. This process, known as leavening, gives the bread its airy and light texture. Additionally, yeast also contributes to the development of flavors in the bread through the production of various compounds during fermentation.