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P1: GRB Final Pages
Encyclopedia of Physical Science and Technology EN002G-67 May 25, 2001 20:8

Bioreactors 269

FIGURE 30 Dependence of the specific growth rate µ on the concentration S of the growth limiting substrate.

joining the biomass concentrations at t = t and t = 0 in metabolites (i.e., ones that are nonessential to the cell) are
Fig. 31; i.e., often produced after the growth has ceased.
An important operational variable for continuous flow
X
P = . (8) bioreactors is the dilution rate D, or the volume flow rate
t of the feed medium divided by the constant volume of the
This definition of productivity disregards any time in- broth in the reactor. The biomass productivity of a contin-
vested in preparing the fermenter prior to inoculation of uous culture is simply the dilution rate D multiplied by
the batch. Lower values of productivity result when the ad- the biomass concentration X in the harvest stream. In a
ditional preparatory time is taken into account. Often, the continuous flow well-mixed bioreactor, the biomass con-
product of a fermenter is not the biomass per se, but a com- centration varies with dilution rate, as shown in Fig. 32.
pound produced by the cells. Examples of such products Also, the productivity increases with increasing dilution
are antibiotics and monoclonal antibodies. The kinetics of rate until the dilution rate approaches close to the maxi-
product formation may mirror those of the cell growth, or mum specific growth rate (Fig. 32). Any further increase
they may be quite different. Products known as secondary in dilution rate causes a sharp decline in productivity and

FIGURE 31 Biomass productivity in batch culture.