Leavened bread relies on yeast, a single-celled live organism for rising. They belong to the genus Saccharomyces – this literally means “sugar fungus.” Yeast also affects flavour and texture in the bread through a process called fermentation. Different types of bread are made depending on the type of yeast used, its primary fermentation stage and the way it is introduced into the recipe. Most leavened breads are made with packet yeast, either fresh or dried. The exception is sourdough which uses natural yeast – some sourdough starters are very old, and are guarded jealously by bakers.
Unleavened bread has no yeast in it. It gets its rise from raising agents present in the flour – normally bicarbonate of soda in self-raising flour. These are generally called “soda breads” and are generally much flatter than leavened breads. Different cultures have different types of unleavened breads, such as roti/chapati and tortilla.