presented great conglomerates of mycelia with sizes ranging from 3 to 7 cm, producing
bed compacting after a few days and disabling a longer operation.
As far as external morphology is concerned, the appearance of the pellets of A. niger
corresponding to air pulsation is compact, with negligible peripheral growth of mycelium
around the pellet. In contrast, the pellets from the system without pulsation have a greater
radius because of hyphal extension and branching (Figure 11.11). This phenomenon
displaces the growth mechanism to dispersed mycelia by shifting the pellet distribution to
the formation of conglomerates of pellets, which are constantly being increased by the
linking of free hyphal fragments.
Gas pulsation implies not only the control of hyphal extension but also an improved
efficiency of metabolite production, in this case citric acid (Table 11.3). The optimum
Figure 11.10 Distribution of A. niger
pellet size after 22 days of operation
for the different pulsation frequencies.
Pulsation frequency F; inoculum I;
nonpulsed bioreactor NP.
Pulsing bioreactors 345