What is yeast bread primarily leavened with?

Yeast bread is primarily leavened with yeast, which is a living organism that ferments and produces carbon dioxide gas as it consumes sugars. This gas is what causes the dough to rise and creates the light, airy texture characteristic of yeast breads. The fermentation process not only contributes to the leavening but also to the flavor and aroma of the bread.

In contrast, air is not the main leavening agent in yeast bread, although it can play a role in incorporating some volume during mixing. Steam is a leavening method used in certain types of breads, like baguettes, to create a crisp crust but is not a primary leavening agent for yeast-based bread overall. Baking powder is a chemical leavening agent that works differently from yeast and is typically used in quick breads rather than yeast breads. Thus, the unique properties of yeast make it the essential ingredient for leavening in yeast breads, leading to their distinctive texture and flavor.