Exponential growth in batch culture may be prolonged by the addition of fresh me- dium to the vessel. Provided that the medium has been designed such that growth is substrate limited (ie, by some component of the medium), and not toxin limited, exponential growth will proceed until the additional substrate is exhausted. This ex- ercise may be repeated until the vessel is full. However, if an overflow device was fitted to the fermenter such that the added medium displaced an equal volume of cul- ture from the vessel then continuous production of cells could be achieved (Fig. 2.5). If medium is fed continuously to such a culture at a suitable rate, a steady state is

achieved eventually, that is, formation of new biomass by the culture is balanced by

the loss of cells from the vessel. The flow of medium into the vessel is related to the

volume of the vessel by the term dilution rate, D, defined as:

=D

F

V

(2.9)

where F is the flow rate (dm

3

h

–1

) and V is the volume (dm

3

).

Thus, D is expressed in the unit h

–1