14.2.1. The Problem of Measuring Biomass in SSF
The first experimental step in developing a kinetic model is to undertake a fermentation and plot the biomass content of the fermenting solid substrate against time (Fig. 14.2(a)). However, this immediately raises an experimental difficulty that is not faced in typical SLF processes (Fig. 14.3). In order to measure the dry weight of biomass directly, it is necessary to separate the biomass from solids. Many SSF processes involve filamentous fungi and, due to the penetration of the mycelium into the solid substrate, it is often impossible to remove the biomass quantitatively from the substrate, meaning that indirect methods of biomass measurement have to be used. Even in fermentations that involve unicellular organisms, although it may be possible to suspend many of the cells that are adhered to the particle surface and let the solid material sediment, the measurements are likely to be inaccurate (see Sect. 14.2.2).
The difficulty in measuring biomass dry weight in SSF raises the question of whether it is really necessary to use the dry weight of biomass as an indicator of growth. In fact, in SLF it is usually so simple to measure the dry weight of biomass (Fig. 14.3) that thought is often not given to whether this is the best parameter. So why do we need to measure the dry weight of biomass? Our aim in writing the kinetic sub-model of the bioreactor model is to write an equation that describes changes in a key variable to which we can relate other key processes that have important effects on bioreactor performance, such as metabolic heat production and O2 consumption. However, does this variable have to be the dry weight of biomass? Are heat production and O2 consumption actually related to the amount of dry biomass in the system? Or are they related to the amount of actively metabolizing biomass in the system? Given that we are typically limited to indirect measurements of growth in SSF, is it really necessary to convert the indirect measurement into dry weight? The answer is that no, it is not essential to write the kinetic sub-model in terms of the dry weight of biomass; we can use any growth-related parameter to which the important growth-related processes can be linked. For example, it may be possible to couple all the important growth-related activities to experimentally determined respiration kinetics.
Having said this, it is important to note that many of the current bioreactor models do in fact base their kinetic sub-models on changes in the dry weight of biomass. Therefore this book recognizes that indirect measures of growth will typically be converted into estimates of the dry biomass. The point is that the approach presented in this book is not the only possibility; other approaches to modeling the kinetics are possible. These other approaches will follow the general principles that we develop here in terms of dry biomass measurements.
dry and plot directly
Calibration curve y it tn o o optical
density p o y it tn
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