Fermentation batch cultivation

A high level of poly(3HB) accumulation is also obtained if the cells are grown under carbon substrate limitation, and the cultivation in the second fermenter is also carried out under carbon limitation. In this case, a substrate flow rate (F2) below that corresponding to the maximum specific poly(3HB) formation rate should be chosen [114]. This cultivation strategy is especially convenient when using toxic substrates like acetic acid. Low substrate concentrations are more conveniently maintained in continuous cultivation than in fed-batch cultivation. The only additional equipment needed is a system to ensure constant working volumes and flow rates. 

Due to the complexity of bioprocesses, and the lack of direct in-process measurements of critical process variables, much work is being done on development of soft sensors and model predictive control of such systems. Soft sensors have long been used to estimate biomass concentration in fed-batch cultivations. The soft sensors can be integrated into automated control structures to control the biomass growth in the fermentation. 

When an NHg solution is used as a nitrogen source of fermentation, the consumption of NHg decreases the pH of the culture broth. A pH stat control utilizes the pH change as a process variable where the NHg addition is manipulated so as to keep the pH value constant during fermentation. The pH stat is often employed in a fed-batch cultivation of industrial glutamic acid production (high NHg consuming fermentation) using molasses as a feedstock. 

Batch cultivation is perhaps the simplest way to operate a fermentor or bioreactor. It is easy to scale up, easy to operate, quick to turn around, and reliable for scale-up. Batch sizes of 15,000 L have been reported for animal cell cultivation [2], and vessels of over 100,000 L for fermentation are also available. Continuous processes can be classified into cell retention and non-cell retention. The devices typically used for cell retention are spin filters, hollow fibers, and decanters. Large-scale operation of continuous processes can reach up to 2,000 L of bioreactor volume. Typically, the process is operated at 1-2 bioreactor volumes... 

Figure 9.19 shows typical cell concentrations reached in the main industrial bioreactors and a comparison of these values with those found in microbial fermentations. As can be observed, batch and fed-batch cultivations attain dry biomass values comparable to those of continuous cultures of microorganisms, so that mass and heat transfer capacities are not limited for these operation modes. However, high cell density cultivation in heterogeneous bioreactors, such as hollow-fiber devices, reaches dry biomass values similar to the maxima observed in microbial cultures. 

Researchers at Ajinomoto have used genetic engineering in a directed fashion to improve yields of amino acids produced by microbial fermentations [31]. For example, by increasing the amount of the enzyme for the step labeled 1 in Fig. 4, they were able to increase the final concentration of threonine in a batch cultivation from 17.5 to 25 g/liter. Subsequent amplification of the enzymatic activity for step 2 in Fig. 4 in concert with the amplification of step 1 gave further yield enhancement to 33 g/liter of culture. 

Ogbonna, J. C., Yada, H. and Tanaka, H. (1995c). Kinetic study on light-limited batch cultivation of photosynthetic cells. J. Ferment. Bioeng. 80,259-264. 

Fed-batch fermentations with P. pastoris ocMnPl-1 were performed using 3.0-1 BioFlo 100 fermentors (New Brunswick Scientific Co. Inc.). The suspended cell, fed-batch cultivation was divided into two stages batch and fed-batch cultivation with glucose. P. pastoris aMnPl-1 inoculum cultures (100 ml) were grown in YPD medium and used to inoculate 1 1 of medium in the bioreactors. 

Figure 22-11. Monitoring of glucose with an enzyme thermistor during a fed-batch cultivation of C. acremonium. Samples were withdrawn from the fermenter via a microfiltration probe. ( Yellow Springs glucose analyzer data, A enzyme thermistor data).

Submerged fermentation can be conducted in different modes of operation. The most traditional is batch fermentation, in which the bioreactor is filled with medium, inoculated and incubated under controlled conditions to the point in which the product (enzyme) has been synthesized to (or nearly to) its maximum level then the cells are harvested for enzyme recovery, if intracellular, or else discarded to recover the medium containing the enzyme, if extracellular. Fed-batch fermentation is a variant of the former in which, after certain time of batch cultivation, the bioreactor is fed with nutrients according to a controlled rate profile and up to a final volume and the product is then recovered as above. This mode of cultivation is particularly appealing for the production of enzymes because it allows the control of the metabolic... 

Fed-batch cultivation is a fermentation strategy whereby one or more nutrients are supplied to the cell culture until the end of the process. The concentration of nutrients that are fed into the bioreactor can be controlled by regulating the feed rate. In fed-batch cultivation the cells are fed continuously as per the required feeding method regime... 

Fed-batch cultivation is one of the best methods to produce high cell density with high PHB content. The important strategy for fed-batch fermentation is to feed the growth limiting substrates at the same rate as the rate of substrate is utilized by the organism.This helps in preventing the formation of by-products that are produced when the substrate is excess and leads to production of product of interest. 

A new fermentation strategy using cell recycle membrane system was developed by Ahn et al. for the efficient production of PHB from whey by recombinant E. coli strain CGSC 4401 harboring the A. latus PHA biosynthesis genes. The working volume of fermentation was constandy maintained by cell recycle and by fed-batch cultivation employing an external membrane module. The PHB concentration and PHB content were 168 g/L and 87%, respectively. 

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