The Batch Fermenter

The above generalised forms of equations can be simplified to fit particular cases of bioreactor operation. 

Starting from an inoculum (X at t=0) and an initial quantity of limiting substrate, S at t=0, the biomass will grow, perhaps after a short lag phase, and will consume substrate. As the substrate becomes exhausted, the growth rate will slow and become zero when substrate is completely depleted. The above general balances can be applied to describe the particular case of a batch fermentation (constant volume and zero feed). Thus, 

Suitable rate expressions for rs and rx and the specification of the initial conditions would complete the batch fermenter model, which describes the exponential and limiting growth phases but not the lag phase. 

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The size of beads was uniform and consistent, the mean size of beads with 3% alginate and based on measurement of 20 samples the mean value for the beads diameter was 4.85 mm, with a standard deviation of 0.3 mm and calculated variance of 0.1 mm. The standard deviation was less than 5%. The data for the batch fermentation experiment with 50gl 1... 

If a CSTF is considered (Fig. 5.56), which has a volume V, volumetric feed flow rate F, with influent substrate and biomass concentrations S0 and X0 respectively, then suppose that the substrate and biomass concentrations in the fermenter are 5 and X. A material balance can be established over the fermenter in the same manner as for the batch fermenter. This is ... 

The ratio of the CSTF productivity to that of the batch fermenter is obtained by dividing equation 5.143 by 5.145 to give ... 

As with the batch fermenter, the growth has to be initiated by the addition of an inoculum in this case it is represented by the stream at a volumetric flowrate of F, with biomass concentration X, and substrate concentration S,. This is mixed with the fresh-feed stream which has a volumetric flowrate of F0, biomass concentration X0 and substrate concentration 50 to produce the entry stream of flowrate FA, biomass concentration XA and substrate concentration SA. 

In contrast to the batch fermentation based methods of determining kinetic constants, the use of a continuous fermenter (Fig. 3.71) requires more experiments to be performed, but the analysis tends to be more straightforward. In essence, the experimental method involves setting up a continuous stirred-tank fermenter to grow the micro-organisms on a sterile feed of the required substrate. The feed flowrate is adjusted to the desired value which, of course, must produce a dilution rate below the critical value for washout, and the system is allowed to reach steady state. Careful measurements of the microbial density X, the substrate concentration S, and the flowrate F are made when a steady state has been achieved, and the operation is then repeated at a series of suitable dilution rates. 

If the final cell concentration to be reached is in the stationary phase, the batch fermenter is a better choice than the CSTF because the residence time required for the batch as shown in Figure 6.12(b) is smaller than that for the CSTF. 

When analyzing a fed-batch fermentation, we cannot assume constant volume as in the batch fermenter, so the balances for cell density, substrate, product, and volume read according to Eqs. (8.10)—(8.13). 

Figure 4.38 gives results for three cases with different initial conditions for the glucose and cell concentrations at the beginning of the batch fermentation. The base case is 100 g/L of glucose and 1 g/L of cells. For these initial conditions, the batch time is about 40 h and the final ethanol concentration is 44 g/L. If the initial glucose... 

Compared with the fed-batch fermentations the performance of the batch fermentations was poor. Only a small fraction of the hexoses was fermented (Table 3), compared with almost 100% during the fed-batch fermentations with TMB 3000. As a consequence, the specific ethanol productivity was very low in the batch experiments. The levels of HMF did not decrease significantly during the batch fermentations. On the other hand, furfural was completely converted. The cell viability was close to 0% at the end of all three batch fermentations. 

The differential equations shown in the Microbial Kinetics section are used to describe the batch fermentation mode. These... 

Equation (7-93) may have to be modified by subtraction of a death-rate term idCx. may well increase during the batch fermentation in which case the net growth rate of (viable) cells eventually becomes negative, and the concentration of (viable) cells will start to decrease. 

In the stationary phase of the batch fermentation, enzyme activity begins to decrease due to an increasingly unfavorable environment. In the membrane fermentor, protease (the enzyme) continued to be excreted by the growing biomass for 50 hours. The ratio of enzyme excreted per unit mass of cell produced in the membrane system (70 units/mg) was almost twice that from the batch system (45 units/mg). This would indicate that under the more favorable environment in the membrane fermentor, the organism is more efficient. 

Figure 4. Dependence of biocatalytic activities for the batch fermentation of ethanol from glucose with immobilized yeast cells as a function of incubation time (time for cellgrowth in the carrier) for various initial cell loadings in epoxy carriers.

Generally, the maximum concentration of PDO was obtained in batch and fed-batch cultures. Continuous culture has the advantage of relatively high productivities, but the PDO concentration was lower than that of batch fermentation. Giinzel et al. (1991) studied the batch fermentation of PDO by C. butyricum DSM in a 2-L whisk and a 1.2-L airlift fermenter the PDO concentration was 50-58 g/L. Cameron et al. (1998) studied the fed-batch fermentation in a 5-L fermenter with K. pneumoniae ATCC 25995 a PDO concentration and productivity of 73.3 g/L and 23-2.9 gl (Lh) were obtained. Menzel et al. (1997) studied the continuous fermentation of... 

Industrial amino acid fermentations are usually done in batch, fed-batch, or continuous modes of fermentation. The batch fermentation is the simplest of all where the fermentation starts with inoculation from the seed fermenter and ends after fixed interval of time, when the nutrients deplete. The major disadvantages of batch fermentation are the low substrate concentration of the medium and prolonged lag phases and decreased productivity and longer unproductive downtime. The batch fermentation has simpler operations for sterilization, inoculation and external connections, ability to run successive phases in the same vessel, and low contamination risk. 

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