It is highly likely that the estimation of biomass will be based on the measurement of a component of the biomass. It may be desired to convert the measured value for the component into an estimation of the dry weight of biomass. The current section shows how this can be done. Note that this conversion is not necessarily essential. As mentioned in Chap. 14.2.1, it might be decided to relate all other growth-related processes to the component and not directly to the biomass.
15.3.1 Suitable Systems for Undertaking Calibration Studies
The indirect method of biomass estimation must be calibrated in a system that allows biomass measurement. Due to the difficulty in direct biomass measurements (Chap. 14.2.1), normally this cannot be done in the SSF system itself, especially for processes that involve fungi. If it could, then there would be no need to use the indirect method to monitor the fermentation process in the first place!
Note that, given the possibility that cell composition can change throughout the growth cycle, it is not appropriate simply to determine the relationship between the biomass and the component for a single sample. Rather, several samples must be removed throughout the growth cycle and the relationship between biomass and the component determined for each sample.
Given that the growth conditions can affect cell composition, it is desirable for the system in which the calibration is done to mimic as closely as possible the conditions in SSF. For this reason, it is not a good idea to calibrate the indirect method using results from SLF. It is highly likely that the biomass composition in SLF will be different from that in SSF. Two systems that are better for the calibration are "impregnated ion exchange resin" and "membrane filter culture". Both mimic, to some extent, the conditions in SSF.
In the membrane filter culture method, a medium is prepared that mimics the solid substrate as closely as possible, but in such a manner as to give a slab of substrate with a flat surface (Fig. 15.6). For example, a gel can be used to solidify a inoculum spread over whole surface
^membrane filter substrate slab samples removed at different times
'""for each sample
various membrane cultures incubated in a high humidity environment
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