What ingredient in bread is responsible for fermentation?

Yeast is the key ingredient in bread responsible for fermentation. When yeast is combined with sugar and moisture, it begins to metabolize these sugars, producing carbon dioxide and alcohol as byproducts. The carbon dioxide gas gets trapped in the dough, causing it to rise and create the airy texture characteristic of well-leavened bread. Fermentation not only contributes to the rise of the dough but also enhances the flavor and aroma of the finished product through the development of various acids and compounds during the fermentation process.

While sugar provides the necessary food for yeast, allowing it to flourish, it is the yeast itself that initiates fermentation. Flour contributes structure to the dough through gluten development, and salt plays an important role in flavor and controlling yeast activity, but neither of these ingredients is responsible for the fermentation process itself.