Fermentors microbial cells

Gas-liquid mass transfer plays a very important role in aerobic fermentation. The rate of oxygen transfer from the sparged air to the microbial cells suspended in the broth or the rate of transfer of carbon dioxide (produced by respiration) from the cells to the air often controls the rate of aerobic fermentation. Thus, a correct knowledge of such gas-liquid mass transfer is required when designing and/or operating an aerobic fermentor. 

Fermentation broths are suspensions of microbial cells in a culture media. Although we need not consider the enhancement factor E for respiration reactions (as noted above), the physical presence per se of microbial cells in the broth will affect the k a values in bubbling-type fermentors. The rates of oxygen absorption into aqueous suspensions of sterilized yeast cells were measured in (i) an unaerated stirred tank with a known free gas-liquid interfacial area (ii) a bubble column and (iii) an aerated stirred tank [6]. Data acquired with scheme (i) showed that the A l values were only minimally affected by the presence of cells, whereas for schemes (ii) and (iii), the gas holdup and k a values were decreased somewhat with increasing cell concentrations, because of smaller a due to increased bubble sizes. 

Bioreactors that use enzymes but not microbial cells could be regarded as fermentors in the broadest sense. Although their modes of operation are similar to those of microbial fermentors, fed-batch operation is seldom practiced for enzyme reactors. The basic equations for batch and continuous reactors for... 

What has been previously said about the advantages of continuous membrane fermentors also applies to such complex systems. UF membranes can in fact be used to recycle both enzymes and microbial cells thus increasing overall system productivity.98... 

With regard to the fermenters, the most common configurations are membrane recycle fermentor (MRF) and hollow fibre fermentor (HFF). In the MRF the membrane module forms a semi-closed loop with a conventional fermentation vessel the MRF gives much better performance than the HFF, where the microbial cells are loaded onto the shell-side and the feed is pumped through the lumen side. Further advantages of this system are a cell/particulate-free product stream and the reduction of capital costs. Furthermore, in these systems cell growth is a caitical point. 

As most biochemical reactions occur in the liquid phase, bioreactors usually handle liquids. Processes in bioreactors often also involve a gas phase, as in cases of aerobic fermentors. Some bioreactors must handle particles, such as immobilized enzymes or cells, either suspended or fixed in a liquid phase. With regard to mass transfer, microbial or biological cells may be regarded as minute particles. 

This laboratory long ago devised [120] the use of radio-frequency dielectric spectroscopy [121, 122] for the on-line and real-time estimation of microbial and other cellular biomass during laboratory and industrial fermentations. The principle of operation is that only intact cells (see [123] for what is meant in this context by the word viable ), and nothing else likely to be in a fermentor, have intact plasma membranes and that the measurement of the electrical properties of these membranes allows the direct estimation of cellular biomass (Fig. 4). 

The use of air lift fermentors has been advocated as a means of overcoming the sensitivity to shear while still maintaining adequate oxygen and mixing characteristics (24). Scale-up of such fermentors has shown mixed results in that the productivity has either decreased (22-25) or increased (24) in going to larger sizes. The hydrodynamics of such systems are extremely complex to model. Some work on modeling of air lift systems for microbial fermentations has been carried out (26-29) however, none of the models so far proposed has been tested with plant cell systems. 

Currently, there are many examples of cell processing in the industrial environment using tangential flow filtration. To illustrate the breadth of microbial types which may be processed by this technology, we will discuss three applications which have been in routine operation under production conditions. The applications include cell/growth medium separations directly from fermentors (Escherichia coli and Mycoplasma species) and the concentration/washing of influenza virus used in the production of flu vaccines. 

Cell recycle fermentors consist of two main units a vessel where the biomass is allowed to grow, and a membrane separation unit (as in Figure 7.40). Vessels are usually designed to insure a uniform concentration of nutrients and pH throughout the whole volume. Due to complete mixing, process control and stability of the microbial slurry are not difficult to achieve.88 After anaerobic stabilization, when the biomass is well developed, the reactor biomass is pumped to the UF unit where solid-liquid separation occurs. The sludge is flushed back to the reactor. In most cases, the flow rate of nutrient feed is kept equal to the permeate flow rate thus keeping a constant liquid level in the anaerobic reactor. 

The microbial populations propagated In reactors ranging from Petri dishes to fermentors to natural ecosystems are typically heterogeneous with respect to age, size, and biochemical activity. The overall rates of substrate utilization, biosynthesis and product formation in these reactors are sums, taken over the cell population, of the rates of the corresponding processes In the individual cells present (X, ) Viewed from this perspective, optimizing the performance of a microbial process requires determination of the cells genotype and of a reactor design such that the optimal distribution of states Is achieved In the process s microbial population. 

Microbial Reduction of 4-Benzyloxy-3-Methanesulfonyl-amino-2 -Bromoacetophenone. The microbial reduction of 4-benzyloxy-3-methanesulfonylamino-2 -bromoacetophe-none (20) (Fig. 5B) to the corresponding (/ )-alcohol (21) has been demonstrated (32) using Sphingomonas paucimo-bilis SC 16113. The growth of S. paucimobilis SC 16113 was carried out in a 750-L fermentor cells (60 kg) harvested from the fermentor were used to conduct the biotransformation in 10- and 200-L preparative batches. The cells were... 

PHA production uses strain development, shake flask optimization, lab and pilot fermentor studies, and industrial scale up (Figure 3.6). Effective microbial production of PHAs is dependent on a variety of factors, which include the final cell density, bacterial growth rate, percentage of PH A in cell dry weight, time taken to reach high final cell density, substrate to product transformation efficiency, price of substrates and a convenient and cheap method to extract and purify the PHAs (Figure 3.6). 

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