The Fed Batch Fermenter

This bioreactor mode refers to a tank fermenter operated semi-continuously (Fig. 3.24). The rate of the feed flow, Fo,may be variable and there is no outlet flow rate from the fermentor. As a consequence of feeding the reactor volume will change with respect to time. 

The balance equations then become for constant density 

Here the quantities VSi and VXi represent the masses of substrate and biomass, respectively, in the reactor. In a simulation, dividing these masses by the volume V gives the concentrations Si and Xias a function of time and which are needed in the appropriate kinetic relationships to calculate rs and rx. 

It can be shown by simulation that a quasi-steady state can be reached for a fed-batch fermenter, where dXi/dt = 0 and //=F/V (Dunn and Mor, 1975). Since V increases, pi must therefore decrease, and thus the reactor moves through a series of changing steady states for which pi=D, during which Si and pi decrease, and Xi remains constant. A detailed analysis of fed batch operation has been made by Keller and Dunn (1978). 

All three bioreactor modes described above can be simulated using simulation example BIOREACT. 

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One of the advantages of the fed-batch fermentation is the fact that the residual substrate concentration may be maintained at a very low level. This may result in a removal of catabolite repressive effects and avoidance of toxic effects of medium components. 

The material balance for the biomass in the fed-batch fermentation gives ... 

In total, about 15 L of dilute-acid hydrolysate was pumped into the reactor using a peristaltic pump (Ul-M Alitea AB, Stockholm, Sweden). Before addition the pH of the hydrolysate was increased from 1.8 to 3.0 by the addition of about 22 mL of 2 M NaOH/L of hydrolysate. The reason for the pH adjustment was to avoid disturbance of the C02 signal owing to large base additions during the fed-batch fermentation. 

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. 

Fig. 4. Specific ethanol productivity in two fed-batch fermentations with TMB 3000. Dilute-acid hydrolysate A was used. qe for the fed-batch fermentation with...

CNBr was negligible and resulted in around 78% yield recovery. The fed batch fermentation allowed the control of oxygen, nitrogen, and glucose sources, improving yields from tens of milligrams per liter previously obtained in standard batch fermentation. 

In the fed-batch fermentation, nutrients can be added making it an open system for substrates, but still a closed system for biomass and biomass-derived products. As shown in Figure 30.4B, in this type of process, the volume is not constant. The flow rate of nutrients can vary during the course of the fermentation. Fed-batch operation is very commonly used in industrial processes for the production of baker s yeast, enzymes, amino acids, and many other metabolites. 

Lowering the temperature of P. pastoris oMnPl-1 shake flask or bioreactor fermentations from 30 to 25 °C did not result in appreciable changes in cell growth rate or production of recombinant enzyme as indicated by cell density and rMnP activity measurements (Figs. 1 and 2). A higher cell density was achieved at 25 °C (52.6 g/1) than at 30 °C (42.1 g/1) in the fed-batch fermentations, but this did not result in a corresponding increase in rMnP activity (Fig. 2). 

Khanna and Srivastava [19] reported optimising the nutrient feed concentration and the addition time for the production of PHB. A mathematical model was developed to describe the batch kinetics. This equation was then extrapolated for the fed-batch culture by including the dilution rate. At the end of the fed-batch fermentation, 32 g/1 biomass and 14 g/1 PHB was obtained with a productivity of 0.28 g/l/h. 

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... 

During the fed-batch fermentation, we can get total glucose feeding GlUj, by integrating the pump flow rate, F ( ml / h ) ... 

Figure 5 Glucose concentration profile in the fed-batch fermentation process...

High cell density fed-batch fermentation of A. eutrophus was carried out for the production of PHB in a 60 L fermentor by Ryu et al. The PHB production was carried out by maintaining constant pH using NH4OH solution and PHB accumulation was induced by phosphate limitation. The fed-batch fermentation resulted in final cell concentration of 281 g/L and PHB concentration of 232 g/L and a productivity of 3.14 g/L/h. 

The process flow diagram is shown in Figure 1.17. The plant comprises four 2 m (seed 1) and four 20 m (seed 2) seed reactors and six identical 120 m main fermenters (nominal volumes). All reactors share similar geometry. Table 1.9 notes some key mass balance information. The fed-batch fermentation results in a final liquid harvest volume of 85 m. ... 

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