1. Baker’s Yeast

Baker’s yeast is a kind of ferment called Saccharomyces Cervisiae. Ferment is a living mono-cell organism from fungi family. Therefore, it is a kind of fungi which cannot provide its food and continues its life by fermentation along with growth and reproduction.
Fermentation is a process through which chemical changes take place in an organic material via a biochemical catalyst referred to as enzyme. The mentioned enzyme is made by a special living microorganism.
Through growing and reproducing, ferments produce two kinds of enzymes:

1. Enortaz which changes starch (saccharine) to fructose and glucose.
2. Amylase is an enzyme which turns saccharine into water and carbonic gas in the presence of oxygen and into alcohol and carbonic gas when there is no oxygen.

So, the result of yeast reactions is the mentioned enzymes which change bread starch into saccharine and saccharine into alcohol, carbonic gas and energy. This process is called fermentation.
The produced carbonic gas makes the bread bulky (it rises), light and tasty. Through several years, bakers found that if the yeast is kept for a while before baking, it rises and becomes light. Ancient people believed that if yeast remains in a place for some time, special fungi will enter the yeast from the air and will start reproducing there.
However, after thousands of years, presently, bakers add yeast to their dough so that starchy materials and the saccharine of the dough are consumed by the existing fungi and produce some carbonic gas to release during fermentation. On the other hand, the heat of furnace increases the amount of released carbonic gas, so bread rises and swells and therefore, the final product is a dry, raised, light and good-smelling bread.