Fed-batch fermentations

The fed-batch mode starts the same as batch operation. However, concentrated medium is at some time continuously or intermittently added into the fermentor until the desired working volume is reached. The volume of the medium inside the fermentor changes in the fed-batch mode due to the frequent additions of substrate and nutrients. This is done to keep the substrate concentration low to minimize substrate inhibition to microorganisms. Higher product concentration and yield can be obtained in fed-batch compared to batch operation. Both batch and fed-batch fermentations are run until completion and no products are taken out from the fermentor until the end of fermentation. 

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Fed-batch fermentations are batch fermentations which are fed continuously, or intermitantly, with medium without the removal of fluid. In this way the volume of the culture increases with time. 

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 culture can be used directly for the conversion of phenylpyruvic add to resting cells L-phenylalanine. Therefore, a batch process with resting cells can be carried out, with some glucose added for maintenance (fed-batch fermentation). Another approach is to harvest the cells from the fermentation broth and to use them in a separate bioreactor in higher concentrations than the ones obtained in the cell cultivation. An advantage of the last method can be that the concentration of compounds other than L-phenylalanine is lower, so that downstream processing may be cheaper. 

Figure 8.7 Fed-batch fermentation of phenylpyruvic acid to L-phenylalanine.

Fed-batch fermentation process is a production technique between batch and continuous fermentation. A proper medium feed rate is required to add sequentially into the fermentor during the process and the product is harvested at the end of fermentation just like a batch type. 

Fig. 6 shows a fed batch fermentation of sweet sorghum juice (SSJ) by Bacillus aryabhattai in 3 L fermentor under cultivating condition with agitation rate at 200 rpm, air rate of 1.5 1/min, at 30° C and feeding time at 18 and 24 hr during log phase of the culture. It was found that the cell could continuously produce both biomass and PHAs. Maximum cells were obtained at about 14.20 g/1 at 54 hr when PHAs content reached 4.84 g/1 after 66 hr (Tanamool et al., 2011). In addition, in Table 2, fed batch fermentation by A, latus was used for the production of PHAs (Yamane et al, 1996 Wang Lee, 1997). It could yield high productivity with the use of cheap carbon sources. 

Fig. 6. Growth monitoring and PHAs production during fed-batch fermentation of sweet sorghum by Bacillus arybhattai in 3 L fermentor (Tanamool et al., 2011)...

Fig. 21. Generalized description of fed-batch fermentation process data.

Bakshi and Stephanopoulos (1994b) have applied the above procedure to a fed-batch fermentation process. The problem involved 41 sets of batch records on 24 measured variables. Of these variables only very few were found by the decision tree to be relevant, and yield rules such as the following for guiding the diagnosis or control of a fermentor. 

The process developed on lab scale was transferred to pilot plant scale. The process is a fed-batch fermentation with a growth-phase (24h) on complex sugars and a production phase (48h). During production phase a linear feed is added containing various carbohydrates. 

Tuah PBM (2006) The performance of phenol biodegradation by Candida tropicalis Retl-Cr-1 using batch and fed-batch fermentation techniques. Ph.D. Thesis, Universiti Teknologi Malaysia... 

Ikeda, S., Nikaido, K., Araki, F K. et al. (2004) Production of recombinant human lysosomal acid lipase in Schizosaccharomyces pombe. development of a fed-batch fermentation and purification process. Journal of... 

Fed-batch fermentations with P. oleovorans have been carried out using octa-nol and octanoate as substrate [53]. To ensure high oxygen transfer rates pure oxygen was used. With octanoate as substrate 41.8 g 1 1 biomass with a cellular PHA content of 37 % and a productivity of 0.34 g l"1 were reached. Higher biomass concentrations could not be achieved due to accumulation of the toxic octanoate. 

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

For fed-batch fermentation, the model equations need to include the continuous feeding of sterile substrate to the fermenter, but zero outflow. The increase in volume (total accumulation of mass) that occurs in the fermenter due to the feeding is represented by a total mass balance relationship. 

Walker and Dhurjati1421 have used culture fluorescence for on-line discrimination of host and plasmid-carrying strains of Escherichia coll In addition, culture fluorescence has also been used in the control of fed-batch fermentation on yeast cell production/29, 431... 

J.F. van Impe and G. Bastin. Advanced Instrumentation, Data Interpretation, and Control of Biotechnological Processes, chapter Optimal Adaptive Control of Fed-Batch Fermentation Process, pages 401-435. Kluwer Academic Publishers,... 

H. Zhang and B. Lennox, Integrated condition monitoring and control of fed-batch fermentation processes, J. Process Control, 14, 41-50 (2004). 

These optical probes are the most universally applicable in situ devices for on-line biomass monitoring up to now [15,16]. Konstaninov et al. [17] tested several absorbance and scattering sensors for real-time biomass concentration monitoring in mammalian cell cultivation processes and Hatch and Veilleux [18] compared optical density probes with oxygen uptake rates, packed cell volume, and off-line cell mass monitoring in commercial fed-batch fermentations of Saccharomyces cerevisiae [19]. In order to minimize influencing effects, special chemometric data treatment is necessary [20]. 

PD is the least toxic product in the glycerol fermentation, but nevertheless determines the achievable final concentration. A final concentration of propanediol around 60-70 g/1 is usually achieved with wild-type strains. More than 85 g/1 1,3-PD can be produced with these microorganisms in special fed-batch fermentations (unpublished results). With externally added 1,3-PD,... 

Fig. 14. Time course of baker s yeast fermentation without (curves 1) and with (E = 2.9) dispersed organic phase (curves 2) simulation of fed-batch fermenter with constant liquid feed rate using data of Table 2...

Fig. S.70. The variation of total substrate and biomass in a fed-batch fermentation...

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

The PLS model for fed-batch fermentation performs well when the fermentation is operating within conditions that are represented in the 20 training batches used in developing the model. As with many model-based systems, model performance is poor when extrapolating outside of the operating conditions of the training set. 

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). 

Importantly, both X and S can be kept at constant values under such conditions during fed-batch fermentations. This can be demonstrated by inserting Eq. (8.20) into Eq. (8.14) to yield Eq. (8.23). 

Figure 20 shows a schematic of a novel membrane-integrated process for citric acid production from glucose syrups by Yarrowia lypolitica ATCC 20346, based on prolonged fed-batch fermentation carried out in a stirred bioreactor coupled to a MF unit equipped with tubular ceramic membranes, and disodium citrate recovery from MF permeates by ED (Moresi, 1995). 

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