The principle behind the leavening of bread dough by baker’s yeast is the same as in brewing, i.e. the anaerobic metabolism of glucose and other reducing sugars via pyruvic acid into ethanol and CO2. The difference is that the released CO2 is the important product in breadmaking because it is responsible for the texture of the bread. Ethanol may, however, contribute to the flavour of fresh bread. Originally, a portion of the risen dough medium was retained as a starter for the next baking session, or surplus yeast from brewing processes was used.
Specific yeasts for baking were first produced in Vienna in 1846, and baker’s yeast is now produced commercially under aerobic conditions because the yield of biomass can be maximized.
In the bread dough the yeast cells are subjected to anoxic or anaerobic conditions and must be able to release CO2 quickly. The carbon sources available to yeast cells in bread dough are hexoses, especially glucose, and the disaccharides maltose and sucrose, all of which are present at fairly low concentrations. Starch is not utilized by S. cerevisiae but can be hydrolysed by amylases present in the flour, and the glucose thereby released may be available to the yeast