Here, we will discuss what is important to control in SSF bioreactors and how it can be done. The simulations at the end of this chapter show that performance of SSF bioreactors will be better with control schemes; for example, when the temperature in the inlet air is decreased in response to a rise in the average temperature in the bed, or when water is added in response to a drop in the humidity of the off-gases.
As with classical submerged liquid fermentation, in SSF processes there is an optimal set of conditions that will lead to maximal cell growth and metabolite production. As discussed in Sect. 5.1, two of the key process variables are the temperature and water content of the solid bed. In order to maximize the performance of industrial-size SSF bioreactors, these variables must be maintained close to the optimum values for growth and product formation.
In small-size cultures the ratio of the heat transfer capacity of the bioreactor to the metabolic heat generation rate is high; consequently the metabolic heat can be dissipated effectively. Under these conditions, it is straightforward to keep the culture reasonably homogenous. However, this is much more difficult to achieve in industrial or pilot size SSF bioreactors (See Chap. 5). If operated without control, temperature differences between different regions of the solid bed can be as high as 20°C, resulting in disappointingly low productivity, the growth of contaminants, or complete failure of the fermentation run.
Manual control may be useful to regulate the conditions within the solid bed; however, large-scale SSF bioreactors are difficult to operate manually since many variables must be measured and manipulated simultaneously. Under manual control it is not possible to run the bioreactor effectively with just a single operator. In addition, the effects of the many manipulated variables interfere unpredictably with each other throughout the fermentation run. The operators then have a hard job trying to coordinate their respective control actions. Consequently, the biore-actor operation is not reproducible and optimum performance is unattainable.
These difficulties have contributed to the fact that only a minor fraction of the many SSF processes that have been successfully developed at laboratory scale have been scaled up for industrial production.
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