A critical part of the fermentor is the aeration system. With
large-scale equipment, transfer of oxygen throughout the growth
medium is critical, and elaborate precautions must be taken to
ensure proper aeration. Oxygen is poorly soluble in water, and in
a fermentor with a high density of microbial cells, there is a
tremendous oxygen demand by the culture. Because of this, two
different devices are used to ensure adequate aeration: an aerator,
called a sparger, and a stirring device, called an impeller (Figure
15.2b). The sparger is typically just a series of holes through
which filter-sterilized air can be passed into the fermentor. The
air enters the fermentor as a series of tiny bubbles from which the
oxygen passes by diffusion into the liquid. Stirring of the fermentor
with an impeller (Figure 15.2c) accomplishes two things: It
mixes the gas bubbles generated by the sparger and mixes the
organisms through the liquid, ensuring that the microbial cells
have uniform access to the nutrients.