The bioreactor step is a key step in an SSF process. It is in this step that the bioconversion takes place. More details about bioreactor operation will be given later. At this point it is only necessary to understand the general features of a typical SSF bioreactor and how it might be operated (Fig. 2.2). The bioreactor has two important functions:
• to hold the substrate bed and provide a barrier against both the release of the organism to the surroundings and the contamination of the substrate bed by organisms in the surroundings.
• to control, to the degree that is possible, the key environmental conditions, such as the bed temperature and water activity, at values which are optimal for growth and product formation by the microorganism.
It is not possible simply to set the environmental conditions within the substrate bed at the desired value. The growth of the organism will tend to change the environmental conditions away from the optimal values and we must then intervene in order to try to bring them back to the optimum (Fig. 2.3). However, we can only manipulate a limited number of variables that are external to the bioreactor, called "operating variables". For example, we can change the agitation regime (if the bio-reactor is agitated), the temperature, flow rate, and humidity of the inlet air (if the bioreactor is aerated), the addition of solutions or substances or the temperature and flow rate of the cooling water (if the bioreactor has cooled heat-transfer surfaces). The success of these external interventions in bringing the bed conditions back to the optimum values depends on the efficiency of the heat and mass transport processes within the substrate bed.
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