What Units Should Be Used for the Biomass

Once a direct or indirect measurement method has been selected, it will be used to give an estimate of the amount of biomass in samples removed over the course of the fermentation, allowing the construction of a kinetic profile. However, there remains a question: "What units will be used to express the biomass concentration in the kinetic profile?" The importance of this question becomes apparent when it is realized that various different units have been used to construct kinetic profiles in the past. These various methods are compared in Fig. 14.4, which also indicates the meaning of the various symbols used below:

• grams of biomass or component per gram fresh sample. In this case the sample is removed and weighed directly, the amount of biomass or component then being measured and divided by the fresh weight of the sample (i.e., X/M);

• grams of biomass or component per gram dry sample. In this case the sample is removed, dried in an oven at around 50-70°C and then weighed, after which the biomass or component is measured and divided by the dry weight of the sample (i.e., X/D). Note that if the analytical method used would be adversely affected by a drying step, the sample can be divided, with the water content being determined by drying of one fresh sub-sample and the other fresh sub-sample being used for biomass determination.

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