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Fig. 2.5. At large scale in SSF there are variations with time during the fermentation for any given position and variations with position across the bioreactor at any given time. Temperature gradients are shown here as an example. Therefore the aim is to minimize temporal and spatial deviations from the optimal conditions (in this case, deviations from the optimum temperature for growth, Topt)

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Fig. 2.5. At large scale in SSF there are variations with time during the fermentation for any given position and variations with position across the bioreactor at any given time. Temperature gradients are shown here as an example. Therefore the aim is to minimize temporal and spatial deviations from the optimal conditions (in this case, deviations from the optimum temperature for growth, Topt)

The size and shape of the substrate particles, along with the manner in which the bed is packed, will determine the sizes of the inter-particle spaces and the degree of continuity between them. The substrate particles are moist and will have a thin liquid film at their surface.

The microbial biomass is distributed as biofilms on the surfaces of the substrate particles, in the case that the process organism is unicellular, or as a network of hyphae, in the case that the process organism is a fungus (Fig. 2.4(b)).

The inter-particle spaces are filled with gas. For growth of a unicellular organism, the inter-particle spaces are well defined, and the biofilm is treated as part of the particle. For processes involving fungi, a network of aerial hyphae grows into the inter-particle spaces and the boundary between the inter-particle space and the particle is located at the surface of the thin liquid film that surrounds the particle. Note that even when the inter-particle spaces appear to be completely filled with hyphae, there is still a gas phase within the network, since the mycelial structure prevents the hyphae from occupying more than 34% of the available volume (Au-ria et al. 1995). In true SSF systems there will be no or very little liquid water within the inter-particle spaces, although small droplets may be held within the network of aerial hyphae of fungi.

Examining the particle with an even greater degree of detail, such as a cross-section through a substrate particle, we would typically see:

• that the substrate particle contains one or more types of macromolecule that confer the "solid" structure. The polymer or polymers that confer this structure may or may not be degraded by the microorganism during the process. If they are degraded, then the structure and properties of the particle at the end of the fermentation will be different from those of the original substrate particle. Substrate properties depend on the source of the substrate particle and how it was prepared, and this has consequences for hyphal penetration and accessibility of the nutrients, factors that can affect process performance. As a simple example, grains or pieces of stems have a cellular structure at the microscopic level, meaning that cell walls are present. On the other hand, particles made by granulating flours or meals have a more amorphous structure.

• that there is a spatial distribution of biomass (Fig. 2.4(c)). In the case of a unicellular microorganism, the biofilm is restricted to the exposed surfaces of the particle. The intercellular spaces are filled with water, giving the biofilm the consistency of a thick paste. In the case of a filamentous fungus, it is possible to distinguish aerial, "biofilm", and penetrative hyphae. Penetrative hyphae are those that have penetrated into the moist solid matrix. Aerial hyphae are those that are in direct contact with the air in the inter-particle spaces. Biofilm hyphae are those that are above the solid surface but are submerged in the liquid film at the particle surface. Depending on the extent of this liquid film, which might be stabilized by the presence of the hyphal network, the biofilm hyphae may represent a significant proportion of the overall biomass.

• that the particle surface, the location of which is used to define biofilm and penetrative hyphae, may be indistinct, especially if the organism attacks the polymer that gives structure to the substrate particle. For fungal growth, the highest biomass concentration would typically be just above and just below the particle surface, where both nutrients from the substrate and O2 from the gas phase are most readily available simultaneously.

• that, if we could visualize specific chemical components, we would see gradients in protons (i.e., in the pH), enzymes, polymers, hydrolysis products, other nutrients and gases within the substrate particle. During the rapid growth period the O2 gradient is quite steep, with the O2 concentration falling from a high value at the outer surface of the biofilm to essentially zero at 100 |im under the surface of the biofilm (Oostra et al. 2001). The substrate concentration gradient is typically in the other direction, such that the concentrations of soluble nutrients near the surface of the biofilm are quite low.

• that the substrate particle is moist. Water within the particle might be free water or involved in capillary sorption or hydration of macromolecules. There is a water film at the particle surface, the thickness of which will depend on the biomass properties and the water content of the particles. Note that the continuity of the surface water film with the liquid phase within the substrate particle and the continuity of the water phase within the particle itself depend on the substructure of the particle, given that intact cell walls disrupt continuity. This has consequences for molecular diffusion within the particle.

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