Theoretically, a fermentation to produce a metabolite should also be more productive in continuous culture than in batch because a continuous culture may be operated at the dilution rate which maintains product output at its maximum, whereas in batch culture product formation may be a transient phenomenon during the fermentation. The kinetics of product formation in continuous culture have been reviewed by Pirt (1975) and Trilli (1990). Product formation in a chemostat may be described as:
Change in product concentration = production -
where p is the concentration of product and qp is the specific rate of product formation (mg product g~' biomass h^1). At steady state, dp/dt = 0, and thus:
where p is the steady-state product concentration. If qp is strictlyrelated to ¡x, then as D increases so will qp; thus, the steady-state product concentration (p) and product output (Dp) will behave in the same way as x and Dx, as shown in Fig 2.10a. If qp is independent of fj, then it will be unaffected by 1) and thus concentration will decline with increasing D but output will remain constant, as shown in Fig. 2.10b. If product formation occurs only within a certain range of growth rates (dilution rates) then a more complex relationship is produced.
Thus, from this consideration a chemostat process for the production of a product can be designed to optimize either output (g dm 3 h or product concentration. However, as Heijnen et al. (1992) explained,
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