In cereal grains the principle carbohydrate is starch (70% the total kernel weight at 14% moisture basis). These starch granules are composed of long linear (amylose) and branched (amylopectin) polysaccharide chains. If you can visualize many single sugar units (glucose) linked together in either a long line or in a many branched tree-like structure you can get a good visual picture of what these structures look like. In the previous articles on fermentation we have seen that yeast is limited to glucose and fructose as a potential energy source.

In a bread dough or sourdough culture we have a potential food source (the starch) that, unmodified, is largely unavailable for fermenting by the yeasts and LAB present because they cannot metabolize these complex aggregations of sugar. What is needed is the input of enzymes that will degrade the long starch chains into simple sugar units that can then be metabolized by the yeast and LAB present in the culture or dough. The enzyme best able to reduce these longs chains of sugars, and thus most critical to the success of fermentation, is amaylase.



Wheat kernels and all cereal grains for that matter, are the seeds of the plant. They represent the successful reproduction of the wheat plant, and if left intact, will germinate into the next generation. The seed is protected by a tough fibrous coat called the bran layer. Inside are the germ, which is the embryonic seedling, and a large quantity of starch, the endosperm. This starch is the stored food source for the developing plant. As with the yeasts, this starch is unusable by the developing seedling. Upon germination the the germ secretes enzymes which breakdown the starch into its simple sugar units thus making them available to the developing plant.



In modern wheat production practices the wheat is harvested before these enzymes have been secreted by the germ making the flour enzymatically stable and usable in a wide range of bread production settings. This practice avoids the onset of a condition known as “sprout damage” in which the enzymatic content of the wheat (and thus finished flour) is uncontrolled and subject to wide ranges in variability. The result of this harvesting practice means that the flour is deficient in the necessary enzymes to support yeast and LAB alcoholic fermentation. As a result modern flour production practices involve adding these enzymes to the flour in controlled amounts so that the correct level of enzyme content is present in the flour which will become active upon mixing with water. The enzymes are added to the flour in the form of malted barley flour, fungal amalyase, and various other synthetic forms amylase.

Amylase (in its two forms alpha (α) and beta (β)) is the principle enzyme needed to transform starch into usable simple sugars for the yeast and LAB. α-amylase works by breaking the long chains of starch at random points throughout its structure. β-amylase, on the other hand is only able to work on the ends exposed by the α-amylase. Thus the two forms of amylase work in tandem with α-amylase exposing more and more sites for β-amylase to work on over time. β-amylase in its action on the exposed ends of the starch chain yields maltose a two unit sugar composed of two glucose units. As we have seen, maltose (along with glucose, fructose, and sucrose[glu+fruc]) can be used directly by hetero-fermentative forms of LAB but must be further reduced to single glucose units to be usable by the yeast. The further reduction is accomplished by the enzyme invertase secreted by the yeasts themselves.



The enzymatic activity of a flour is indicated by its ‘falling number’. For more information please read our falling number article. There too you will find further information on the amylases.



Once there is ample usable sugar yeast respiration can begin and is described with the formula:

What can be seen from the above equation is that for every molecule of glucose (C6H12O6) the yeast cell is able to produce 2 molecules of ethanol (useful in the brewing process but evaporated during the bake) + 2 molecules of carbon dioxide (CO2) - the gas that raises the loaf of bread + energy (what the yeast will use to support its life’s activities) and acid (a contributor to the sharp taste of naturally leavened breads. The alcohol, CO2 and acid are waste by-products of yeast respiration but are what bakers use to make their loaves of bread. The energy is the principle reason the yeast undertakes this process.