Given the essentially anaerobic environment that exists in dough once the available oxygen is used, one would expect the primary physiological activity of yeast to be that of fermentation. However, the organism also undergoes some growth and cell multiplication during the fermentative process. For example, a test dough with a yeast content of 1.67%, based on flour, and fermented at 80° F (27° C), demonstrates no significant increase in yeast-cell count during the first two hours of fermentation with the actual rise in cell numbers being on the order of 0.003%. The most vigorous yeast growth was observed during the period between the second and fourth hours of fermentation, when the yeast cell count increased by 26%. Between the fourth and sixth hours, the rate of yeast multiplication declined again to about 9%, based on the original cell count.

Other findings indicate that the smaller the original quantity of yeast in the dough, the greater the percentage increase in cell numbers during the fermentation, with all other conditions being held constant. Thus a 0.5% yeast addition to a test dough produced an 88% increase in cell count after 6 hr of fermentation, while with a 2% original yeast level the corresponding increase in cell numbers was only 29%. This is not surprising given the fact that at the lower yeast level, the competition for nutrients is far less than at the higher yeast levels. Thus, each yeast cell has access or at least the opportunity for access to greater food supplies during fermentation.
….