Fermentor, microbial

Biotransformation with flasks can be used to make gram quantities of a desired product, as shown for the 21 -hydroxylation of epothilone B [75]. In cases when greater quantities of a metabolite are needed, microbial biotransformations can be carried out in a fermentor, which will allow better monitoring and control of fermentation conditions (such as pH, oxygen and glucose levels, etc.) for reaction optimization [76]. 

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. 

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. 

Some fermentation broths are non-Newtonian due to the presence of microbial mycelia or fermentation products, such as polysaccharides. In some cases, a small amount of water-soluble polymer may be added to the broth to reduce stirrer power requirements, or to protect the microbes against excessive shear forces. These additives may develop non-Newtonian viscosity or even viscoelasticity of the broth, which in turn will affect the aeration characteristics of the fermentor. Viscoelastic liquids exhibit elasticity superimposed on viscosity. The elastic constant, an index of elasticity, is defined as the ratio of stress (Pa) to strain (—), while viscosity is shear stress divided by shear rate (Equation 2.4). The relaxation time (s) is viscosity (Pa s) divided by the elastic constant (Pa). 

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... 

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. 

Microbial fermentations must be tightly controlled to ensure optimal growth of micro-organisms and efficient production of enzymes. As shown in Fig. 4, a modern fermentor will allow operators to control the temperature, pH, redox potential, and dissolved... 

Once an effective microorganism has been identified from screening studies, scale-up can be undertaken first in laboratory fermentors which have efficient stirring and aeration capabilities. The capacity of these fermentors typically range from 3-10 liters. Here one studies additional operating parameters which can affect microbial metabolism and the production of volatile chemicals. 

Intermediates in the biosynthetic route are 3-dehydroshikimic acid, protocate-chuic acid and catechol (Scheme 7.16). Optimization of microbial ds,ds-muconic acid synthesis required the expression of three enzymes not typically found in Escherichia coli [42cj. E. coli WNl/pWN2.248 was developed that synthesized 36.8 g of ds,ds-muconic acid in a 22% (mol/mol) yield from glucose after 48 h of culturing under fed-batch fermentor conditions. Optimization of the carbon flow directed into ds,ds-muconic acid biosynthesis and manipulation of enzyme activities were aimed at avoiding accumulation of biosynthetic intermediates. 

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. 

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