Fermentation processes, modes

Lor example, in a fermentation process, tlie modeler may be interested in various controlled parameters such as the feed rate, rate and mode... 

Figure 5.4 summarises the changes occuring in A. niger in citric add production mode when compared to conventional metabolism. It is worth studying the Figure for some time because it explains some of the features necessary for a successful fermentation process. 

The ability of Monod s empirical relation to fit kinetic data for biochemical reactions has its foundations in generalizations of two phenomena frequently observed for fermentation processes (1) nature places a cap on the quantity of microorganism that can be achieved during the exponential phase of growth in a bioreactor operating in a batch mode and (2) as the concentration of the limiting substrate approaches zero, the rate laws for biochemical reactions approach pseudo-first-order behavior with respect to that substrate. The cap indicated on the cell growth rate has been associated with the natural limit on the maximum rate at which replication of DNA can be achieved. 

Decisions as to whether a chemostat or a batch reactor is preferable for use in a particular application depend on a number of factors. However, an important consideration is whether or not the enhancement in productivity associated with the use of a continuous fermentation process is sufficient to offset the difficulties associated with this mode of operation the need for continuously feeding sterile supplies of the growth medium and (if appropriate) either air or oxygen, the potential for other modes of contamination of the fermentation broth, the potential for mutation of the microorganism, and so on. As an introduction to these considerations, let us consider the relative productivities of... 

Most fermentation processes are operated in either batch or fed-batch mode. The fed-batch mode has been introduced ia an increasing number of fermentation processes. This is because theoretical analysis as well as e q)erimental data of many fermentation processes have revealed that controlled feeding during a fermentation... 

Various modes of fermentation processes for the production of biobutanol have been examined, e.g., batch, fed-batch, and continuous processes (40). 

If, for example, one is dealing with a microbial growth process in a CSTR, the Xj = 0, and one may apply Equ. 2.5a for the rate of growth and thereby obtain the well-known relations for the equilibrium state. This is the basis for the CSTR, the so-called chemostat" or realstat mode of operation of a continuous fermentation process (see Sect. 6.1). 

The main tank fermentation process is carried out as a fed batch process, since this mode of operation is ideal for many microbial processes. The feed substrate is in most cases the carbon source, which is added continuously during the entire batch. This allows the process engineer to control the growth rate of the organism and thus to keep the culture at its optimal conditions for product... 

Given the diversity of bioconversion/fermentation processes, different reactor configurations and modes of operation have been developed, so as to provide the more adequate... 

The fermentation process and downstream processing are usually unlinked. The fermentation broth is harvested and stored, and the recovery is carried out later. In rare cases, when the product is not stable, purification is carried out immediately after fermentation. The construct of the expression plasmid also influences the purification strategy. It is important to take this fact into account when an efficient and sustainable process must be designed. The expression plasmid determines whether a protein is secreted into the culture medium, into the periplasm, or stays in the cytoplasm. It also plays a pivotal role in the half-life of a protein. Although it may sound odd, the N-terminal amino acid is responsible for the half-life of a protein in the cell [26,27]. The most important features determining the functionality of an expression system are listed in Table 2. The expression system, together with the mode of operation, produces a characteristic product concentration profile with time (Fig. 3). 

Fermentation processes can be conducted in batch, fed-batch or continuous modes. However, many fermentation processes are operated in batch or fed-batch modes due to the advantages of these schemes compared to continuous operation. These advantages include easier set-up and operation, less complication in process control, less susceptibility to microbial contamination, and high final product titer and yields. On the other hand, continuous bioreactors have advantages such as consistent quality of product, high productivity and flexibility in system investigation and analysis. 

A fed batch process using suspended Rhodotorula rubra cells in an aqueous medium at pH 10.6 was developed [263]. Since PAL from R. rubra is oxygen-sensitive, the biotransformation is performed under anaerobic, static condition in fed batch mode with periodic addition of concentrated ammonium dnnamate solution and pH maintenance by CO addition [263]. The fermentative process was improved by starting the process direcdy from glucose as a substrate and L-phenylalanine is now manufactured by fermentation on a scale of 8-10,000 tons per year [264]. 

Firing. A hot-air oven having forced circulation in a countercurrent mode is used to dry the fermented tea leaves and inactivates the key enzymes required for fermentation. The firing process generally occurs over an 18—20-min period, which is optimum for normal process efficiencies. 

We can see that for type 1 processes, high growth rate is obligately linked to a high rate of product formation. Indeed, this is the case for all products produced by a fermentative mode of metabolism, eg ethanol, lactic add, acetone. Chemostat studies have shown that for most aerobic processes when growth is limited by some nutrient other than the carbon source, the yield of product decreases with increase in spedfic growth rate (p or D p = dilution rate (D) in chemostat culture). Conversely, both the biomass yield and the spedfic rate of substrate utilisation (qs g substrate g biomass-1 h-1) increase with spedfic growth rate. 

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