Figure 11.8 Digitalised image of
conglomerates of P. chrysosporium.
Dark bioparticles correspond to
nucleus of original pellets.
The application of the liquid-phase pulsation might be considered a useful tool to
increase mass transfer rate, and consequently the oxygen availability; however the high
aerobic requirements associated with these bioprocesses constrain the use of this
alternative. Considering these facts, we proposed the so-called gas-phase pulsing
bioreactors. In this bioreactor scheme, the pulsing flow is generated by means of the
hydraulic transmission of a perturbation in the form of pulsed gas (air or oxygen) to the
culture medium in the bioreactor. A design for the pulsed bioreactor corresponding to a
fixed-bed bioreactor is shown in Figure 11.9. A few modifications should be considered
in the case of other bioreactor configurations such as expanded or fluidised-bed ones. In
this case, the scheme of the bioreactor is almost identical, except for the existence of a
settler in the upper part, which allows gas/liquid/pellet separation. Besides, a continuous
supply of air is introduced to maintain proper fluidisation. The two important objectives
of the introduction of gas in a pulsing form are: i) the maintenance of high oxygen
tension and ii) the control and regulation of hyphal extension and pellet size by a shearing
stress associated with the pulsation.
Control of pellet morphologyAlthough pulsation was found to regulate different filamentous fungi growth by
controlling pellet size efficiently (Moreira et al., 1996), here we present results
corresponding to the production of citric acid from A. niger in a pulsed fluidised
bioreactor.
Pulsing bioreactors 343