The science of fermentation
Bakers recognize that stiff doughs (low water content) will take longer to ferment. Likewise, a dough that is made slack (higher ingredient water content) will ferment much faster. This is due to the concentration of soluble solids in the free water of the dough.
For example, as the ingredient water increases, causing the dough to become softer, the soluble solids are diluted, causing a decrease in osmotic pressure on the yeast and thereby increasing its activity.
Dough temperature is the basic means by which the baker controls fermentation in the bakery. It has been estimated that a 1°C rise in dough temperature will accelerate yeast activity by about 10 percent.
Correspondingly a decrease will cause a similar slowdown of yeast activity. As dough ferments, heat is generated, raising the dough temperature and accelerating the yeast activity. This temperature increase can be monitored and indicates the progress of fermentation. At the end of a standard fermentation time, higher temperatures than expected may indicate an excess of fermentation has been accomplished.
The optimum temperature range for yeast activity is between 32 to 40°C. Temperatures higher or lower than this range will slow yeast metabolism. The general practice in bakeries, depending on the dough system used, is to ferment doughs between 23 to 30°C, so as to maintain proper dough handling through production. One must also bear in mind that the quantity and the type of aromatic substances will vary with the temperature.
The initial pH of a dough should range between 4.0 to 6.0. However, if the pH is higher or lower than this range a slower yeast activity is seen. As a dough is fermenting the pH will begin to drop because of acid production. A freshly made dough will have a pH slightly above 6, but the pH will drop during fermentation (due to the formation of lactic and acetic acid).
Salt is one ingredient that should be mentioned because of the strong retarding effect it has on yeast fermentation. As salt in a formulation increases by as little as 0.2 percent, the yeast activity will slow. Normal salt ranges for most breads are 1.75 to 2.25 percent, based on flour weight.
The primary function of yeast is to produce carbon dioxide gas, which expands the dough during proof and the early stages of baking (oven spring). Yeast prefers slightly acid conditions to work best. A pH ranging from 4.5 to 6.0 gives the best results. Bread doughs are generally in the region of pH 5.5.
Some ingredients used in the bakery, such as mold inhibitors in some bread improvers, lower the pH of the dough and do have a retarding effect on yeast fermentation. This effect is usually taken into account when deciding on the yeast level to use in a given formula. There are other ingredients which can retard yeast activity such as spices or raisins.
Carbon dioxide cannot form a gas bubble on its own. In fizzy drinks, microscopic projections on the side of the bottle provide those sites, which is why when you release the pressure as you open the bottle you see "streams" of gas running from the sides. In bread dough the nucleating sites are provided by the nitrogen gas bubbles, trapped in the dough during mixing. The yeast has used up the oxygen from the air.
During proof stages the carbon dioxide goes into solution until the solution is saturated and then any more, which is generated, makes its way into the nitrogen gas bubbles, which grow in size, and the dough expands. The more yeast and the warmer the temperature the faster the expansion; bakers get oven spring because the maximum gassing rate occurs at 40 - 45°C.
In bulk fermentation stages bakers also get dough expansion from carbon dioxide generation but most of that is lost when the dough is knocked back and divided, so the yeast has to start over again.
Yeast also contributes to dough maturity/development. Though its role is minor compared to improvers in no-time doughs, it is more significant in bulk fermentation where the enzymes, especially the proteolytic ones (they modify the gluten proteins), play a significant role.