The simple stirred tank fermenter

A culture will grow in a reproducible form if it is supplied with the similar medium under the same conditions of temperature, pH and oxygen concentration. A shake flask will give constant aeration if the medium does not exceed 500 ml but above this volume insufficient oxygen can be entrained at the surface of the culture. The stirred tank fermenter overcomes this primary obstacle to microbial growth in larger volumes by providing agitation via an impeller rotating at the bottom of a circular vessel.  Additional mixing of the culture may be provided by one or more baffles on the sides of the vessel in Figure 1. Oxygen normally as sterile air enters the fermenter underneath the impeller by a sparger. The sparger breaks up the flow of air into bubbles which are broken into yet smaller bubbles when hitting the impeller. The combination of an impeller running at > 250 rpm, baffles and sparger enable a highly efficient oxygen transfer to the culture. If the speed of the impeller and the rate of flow of oxygen to the fermenter are linked via a processor to an oxygen probe in the culture the oxygen saturation of the culture can be monitored and regulated continuously.

                                  Figure 1.  A simple  fermenter with temperature, pH, and  oxygen control.

At high biomass loads a vessel is subject to some cooling to the atmosphere, but may even be warmed by the metabolic energy of the culture itself. Shake-flask temperature is regulated by the air temperature of an incubator, but a fermenter is often too large both spatially and in terms of heat capacity to be regulated in this way. Instead, a jacket surrounds the fermenter vessel linked to a thermostatically controlled water supply in Figure 1. This can be supplemented with warming or cooling coils in the culture itself. Finally the pH of the fermenter is kept constant with a probe linked to alkali and acid pumps.