Substrate properties can be quite important in affecting how an SSF bioreactor performs. Some of these properties need to be determined to be included in biore-actor models. Others may not appear in models, but can influence bioreactor performance, and therefore it is necessary to think about these during the bioreactor development process. As yet, there has been relatively little effort to characterize these properties quantitatively for SSF systems, therefore much of this section will be qualitative.
Substrate properties related to the intra-particle structure are not included here. They might be important in influencing the resistance to diffusion of enzymes and nutrients and to hyphal penetration. However, as yet bioreactor models tend to ignore intra-particle phenomena.
The size and the shape of the prepared substrate particles influence the accessibility of nutrients to the organism. The greater the size of the smallest particle dimension is, the greater is the average depth of the nutrients within the particle (Fig. 19.1(a)). It may be difficult for the organism to utilize the nutrients located in the interior of the particle, especially polymers. This will affect both the rate and final amount of growth that occurs during the fermentation. In fact, the maximum biomass content is an important parameter of the logistic equation (see Eq. (14.6) in Table 14.1 and Eq. (16.3) in Table 16.1). It might be determined by the surface area available for growth, which in turn is determined by the size and shape of the particles. In other words, if the biomass covers the surfaces of all the particles at the maximum biomass packing density (biomass per cm2), further growth will not occur, even if there are still nutrients within the substrate particle. However, note
Fig. 19.1. Some important considerations about particle size and shape. (a) The effect of particle shape on nutrient accessibility. For two substrate particles of the same overall volume, the more spherical the shape then the greater the average depth from the surface and the lower the surface area to volume ratio. (b) Effect of particle size on porosity and pressure drop, illustrated with regularly-packed spherical particles. For the two different particle sizes, the percentage of the total volume occupied by the void spaces is identical. However, for forced aeration at the same superficial velocity, the pressure drop across the bed will be greater for the bed on the right, that is, with the smaller particles. (c) The overall particle size can increase early during the fermentation and then decrease during the latter stages. This is a result of two phenomena, the consumption of residual substrate and the expansion of the biofilm
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