In this view we unfreeze the snapshot of the fermentation described in Sect. 2.4.2 and concern ourselves with phenomena that occur on the timescale of seconds to minutes. It is again convenient to consider the macroscale and the microscale separately.
The dominant transport processes at the macroscale will depend on the bioreac-tor and the way it is operated. At this scale, the bed will typically be treated as a single pseudo-homogeneous phase, that is, as a single phase that has the average properties of the solid and air phases that comprise it. Figure 2.6(a) shows the various heat and mass transfer phenomena that occur within and between the various phases that were identified in Fig. 2.4(a), for a well-mixed, forcefully-aerated bioreactor. It is typically important to describe these transport phenomena mathematically in mass and energy balance equations; this topic receives detailed attention in Chap. 18 and is not discussed in detail here.
Figure 2.6(b) shows the microscale transport processes that occur in a typical SSF process in which a fungus grows aerobically using a polymer as its main carbon and energy source. At this scale, the particle and the inter-particle air are treated as different subsystems. Many of the transport processes shown are largely unaffected by the bioreactor and the way it is operated, that is, they are intrinsic to SSF systems due to the presence of the solid phase. At the substrate preparation stage, it might be possible to improve the efficiency of the inter-particle processes. For example, cooking may weaken or disrupt cell walls, reducing the barrier to penetration and diffusion, and may also hydrate polymers, making them more accessible to enzymes. In addition, the use of small particle sizes will reduce the distance over which diffusion must occur. However, these manipulations cannot entirely eliminate the importance of the intra-particle diffusion in SSF processes. These processes include mass transfer processes such as:
• the diffusion of O2, CO2, and water vapor within static regions of the gas phase and their convective movement in regions of air flow, with the extent of static and flowing regions depending on whether the bed is forcefully aerated or not. Note that, even if air is blown forcefully through the bed, static layers of air are formed around any solid surfaces such as particle surfaces or hyphae;
• the diffusion of O2, CO2, water, nutrients, protons, products, and enzymes within the biofilm phase and the substrate particle;
• exchanges of O2, CO2, and water vapor between the various phases. Note that evaporation is typically treated as a phase change within the bed at the macro-scale, whereas with a microscale view it is treated as a transfer between subsystems.
• Also, within the particle there will be the reaction of enzymes with their substrates. This is especially important in the context of SSF where the major carbon and energy source is quite often a macromolecule.
There will be various biological phenomena:
• translocation of nutrients within hyphae;
• growth, including processes such as the extension of hyphae or the expansion of a biofilm. In either case the biomass occupies volume that was previously occupied by either gas or substrate;
• physiological responses to the environment. Stress responses may be especially important in SSF, due to the combination of low water, low O2 inside the particle, and high bed temperatures;
• genetic response mechanisms, such as induction and repression;
Compared to our understanding of these processes in SLF processes, relatively little is known about SSF. This is due to the fact that cell physiology is more difficult to study in SSF than in SLF. In particular, the well-mixed continuous-culture technique, which is a powerful tool in the study of microbial physiology in SLF, cannot be applied to SSF.
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