Which ingredient is primarily used as a leavening agent in baking?

Yeast is primarily used as a leavening agent in baking because it is a live microorganism that ferments sugars present in the dough, producing carbon dioxide gas and alcohol in a process known as fermentation. The carbon dioxide gas gets trapped in the dough's structure, causing it to rise and develop a light, airy texture.

This property of yeast is crucial for many baked goods, especially in bread-making, where the fermentation process contributes not only to leavening but also to the flavor and aroma of the final product. In contrast, while salt, sugar, and flour are essential ingredients in baking, they do not serve as leavening agents. Salt enhances flavor and controls yeast activity, sugar provides sweetness and may help with browning, and flour is the primary structure-building ingredient in baked goods.