Yeasts are eukaryotic (organisms having a membrane bound nucleus), single celled fungi. They are found in sugar-rich mediums such as flower nectar and fruits.
Yeast is used as a leavening agent while baking bread. The most commonly used yeast in baking is Saccharomyces cerevisiae. When added to the bread dough, it initiates the process of fermentation. The fungi feeds on the sugars present in the dough producing ethanol and releasing carbon dioxide. The carbon dioxide creates air pockets and causes the dough to expand. This gives the baked bread a soft and spongy texture.
Ethanol, which is a by product of the fermentation process is widely used in beer and wine making. In case of bread making, the alcohol produced is driven off when the dough is baked.
The fermentation of sourdough breads is often initiated by naturally occurring yeasts present in air. Yeast for baking is available in the form of compressed cakes or granulated powder.