The Crucial Questions

Possibly the three most important initial questions are:

• To what degree is the microorganism, or the desired form of the final product, affected deleteriously by agitation?

• How fast does the organism grow and how sensitive is it, and product formation by it, to increases in temperature?

• What are the aeration requirements of the system?

The answers to these questions will influence decisions about the type of aeration, mixing, and heat removal mechanisms that the large-scale bioreactor must have. Of course, these considerations are interconnected and affect the ability to control the macroscale variables of the process.

To what degree is the microorganism, or the desired form of the final product, affected deleteriously by agitation? Bioreactors can either be completely static, intermittently agitated, or continuously agitated. Frequent or continuous agitation would be desirable if it were tolerated, because it aids bulk transport of heat and O2, improving the ability to control the conditions within the bed. Further, evaporative cooling of the bed can dry it out to water activities that restrict growth, meaning that it is often desirable to add water during the fermentation. It is only feasible to add water while the bed is being mixed. However, agitation can also affect the process deleteriously. It may damage hyphae in fungal-based processes, which might adversely affect growth and product formation. Conversely, it may be desired that the final product be knitted together by fungal hyphae, such as in the production of a fermented food, and this would be prevented by agitation. Beyond this, agitation can crush substrate particles if they do not have sufficient mechanical strength or can cause sticky particles to agglomerate, in either case producing a paste in which O2 transfer is greatly hindered. Unfortunately, the balance between positive and negative effects of agitation has not been well characterized. It will be necessary to undertake your own studies at laboratory-scale in which the performance of agitated and non-agitated fermentations is compared, with both being forcefully aerated in order to minimize transport limitations, thereby isolating agitation as the factor responsible for any differences.

How fast does the organism grow and how sensitive is it, and product formation by it, to increases in temperature? Control of the temperature of the substrate bed is one of the key difficulties in large-scale SSF processes, especially in those processes that involve fast-growing microorganisms. At large scale, it may be difficult to prevent the temperature from reaching values that are quite deleterious to the microorganism. The various bioreactors differ in the efficiency of heat removal, with the temperatures reached depending on a complex interaction between the organism and the type of bioreactor and the way in which it is operated. These considerations may determine key decisions such as maximum bed depths.

What are the aeration requirements of the system? The majority of SSF processes involve aerobic growth. There are essentially two aeration options in SSF processes. One is to circulate air around the bed, but not to blow air forcefully through it. The other is to blow air forcefully through the bed. Agitation can influence the efficiency with which fresh air is delivered to the substrate particles. Note that in forcefully aerated beds the air phase plays an important role in heat removal. In fact aeration rates are typically governed by heat removal considerations since the air flow rates required for adequate heat removal are usually more than sufficient to avoid limitations in the supply of O2 to the particle surface.

These considerations will be crucial in determining the agitation and aeration regimes that are appropriate. The bioreactors can then be compared on the basis of their ability to provide the desired regimes. More advice on how these various factors should be weighted in selecting an appropriate bioreactor type are considered in Sect. 3.4.

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