Of all the topics covered in this book, continuous culture puts the greatest strain on mathematical ability. There are here several equations that are not easy to derive, but must be known because they may not be presented to you in an examination question. These equations really have to be learned by heart. Fortunately, it is not very difficult to put numerical values into the equations to produce the answers.

There are two modes of continuous culture, the turbidostat and the chemostat.

The turbidostat

The turbidostat is illustrated in Fig. 13.1. The culture vessel contains medium that can be stirred, aerated and kept at constant temperature and pH value (by adding acid or alkali) as needed. The medium is inoculated and allowed to grow enough to become turbid. Pump 2 then circulates a small part of the culture through a photometer which measures this turbidity. The output from the photometer goes to a control unit which activates pump 1 whenever the turbidity rises above some set value, so that fresh sterile medium enters the culture vessel. The volume of the growing culture remains constant owing to the overflow tube, and the turbidity settles to the set value.

The turbidostat allows organisms to be grown at their maximum rate in any particular medium.