Why do bread and beer contain yeast?

The presence of yeast in bread and beer primarily serves the function of fermentation, which is a crucial process in both these products. When yeast is added to the ingredients, it consumes the sugars present and, through fermentation, produces carbon dioxide and alcohol. In the case of bread, the carbon dioxide produced helps to leaven the dough, creating the light and airy texture that is characteristic of baked bread. For beer, the alcohol and carbon dioxide develop the beverage's flavor and carbonation.

This consumption of sugars is a vital step in both processes; it highlights the biological role that yeast plays beyond merely enhancing texture or flavor. While yeast does contribute to these aspects, its primary function revolves around the fermentation process, making it essential for transforming basic ingredients into the final products of bread and beer.