Fig. 10.6. The 50-kg capacity bioreactor of Pérez-Correa and Agosin (1999). Mixing is achieved by rotating the basket that holds the bed while maintaining the mixing blades stationary. The bed dimensions are given in the text

mesh, and is mixed by an agitator that undergoes a "planetary motion", that is, the agitator rotates around its central axis while this central axis simultaneously rotates around the central axis of the bioreactor (as in Fig. 10.1(a)). The bioreactor is fitted with a water jacket. It was used successfully in the production of gibberellic acid by Gibberella fujikoroi. The bioreactor contained 12 kg of moist wheat bran at a moisture content of 50% (wet basis). The bed was mixed for 10 s every 2 h, but this can be adapted as necessary according to process requirements. The air flow rate was 15 L min-1 kg-dry-matter-1, which corresponds to a flow rate of 90 L min-1. Since Gibberella fujikoroi is a relatively slow-growing organism, with the process taking 11 days, the major challenge was aseptic operation of the bioreac-tor. Given the low heat generation rate, temperature control was not difficult. The bed temperature was maintained within 1.3°C of the desired temperature of 28.5°C by maintaining the inlet air temperature at 28°C until 50 h and then reducing it progressively to 22°C at the end of the process (250 h).

The 50-L solids mixer of Schutyser et al. (2003b) presented in Sect. 9.3.1 could be used with intermittent mixing although use of this bioreactor in such fermentations has not yet been reported. Note that if the bioreactor were to be operated in the intermittent mixing mode, then aeration should be from top to bottom in order to ensure that the sides of the bed would be well aerated during the periods of static operation. Introducing air at the bottom of the bioreactor would tend to aerate only the central axis.

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