Microbial fermentation schemes

RESEARCH DESIGN AND METHODS Microbial Fermentation Schemes... 

Glycerol could offer a significant advantage in several microbial fermentations as compared to glucose, because in certain cases it could lead to higher production yields and less by-product formation (Lee et al., 2001 Bories et al., 2004 Dharmadi et al., 2006). However, intensive research is still required in order to develop bioprocessing schemes for viable chemical production from glycerol. 

In microbial fermentation the oxidation of organic matter would result in the reduction of a number of oxidation-reduction systems. Glass (38) has proposed that these systems could provide an electron for the dissociation of DDT to form a free radical. A generalized scheme, using this electron source in place of the photoinduced charge transfer, is illustrated in Figure 7. Reactions analogous to those proposed for photolysis (34) could produce other products. 

Study the unlabelled block diagram, and then replace the question marks with the words and phrases to give a generalised scheme of an industrial fermentation. Assume in this example that the product is excreted from the microbial cells. 

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. 

Gaden(54) proposed a scheme which grouped fermentations according to the manner in which the microbial product appeared in the broth (Table 5.18). Whilst this is simpler than that put forward later by Deindoerfer<55), it does form an useful basis from which to develop a quantitative description of the process. 

The yeast-mediated condensation of benzaldehyde with acetaldehyde is of particular interest since it represents one of the first industrially useful microbial transformations, with the acyloin produced being subsequently converted chemically to o-ephidrine. Other illustrations of synthetic value are the yeast-induced condensation of aldehyde (4a,b) with fermentatively generated acetaldehyde. The initially formed acyloins (5a,b) are not isolated but are further reduced, again with enantiotopic specificity, to give the pheromone synthon (6a R = 3-styryl) and the a-tocopherol chromanyl moiety precursor (6b R = 2-propenylfuran) respectively (Scheme 3). ... 

The microbial glycosidation of (8S)-8-fluoro erythronolides were investigated by the same research group. Fluorinated erythronolides A (46) and B (47) were added into the fermentation broth of Streptomyces erythraeus ATCC 31772, a blocked mutant in EM biosynthesis (Scheme 6). In this procedure, (85 )-8-fluoro... 

Starting with bench scale equipment (up to 100 L in the so-called kilo lab) or pilot scale fermentors (up to 5000 L when titer is low), this early preparative work uses whatever synthetic method or fermentation conditions (the microorganism and the nutrients) are immediately available. In most cases of synthesis, the route may be a somewhat streamlined version of the discovery route or a temporary route that may or may not include parts of synthesis schemes being considered for eventual development. In most cases of biosynthesis, the microorganism is that from the discovery stage, but taken from whatever stage of microbial strain improvement is amenable to scale up from shake flasks or bench scale fermentors. 

Tableii shows a comparison between fermentability values utilizing rumen and human inocula on the same substrates. A comparison of this nature may have inherent inequalities due to differences in inoculum source and collection procedures. These parameters have been evaluated by a number of workers (11, 17, 18). The differences in nutritional schemes between animal species have an effect on microbial populations. Fermentability estimates of a particular substrate are biologically acceptable with respect to the inoculum source.

Most studies on microbial exopolysaccharides production have been performed so far using batch fermentation conditions and polymer macromolecules are recovered from fermentation broths by simple chemical and physical techniques, e.g. precipitation and centrifugation. In Scheme 7.2 the route of production of alginate is presented [8]. Some attempts have been made to apply immobilized-cell cultures to the production of alginate and other bacterial polysaccharides. Immobilization techniques are likely to allow the permanent separation of microbial cells from the incubation broth. In the last few years, however, membrane processes have been increasingly used to separate microbial cells from the production medium. A number of studies have therefore focused on the microfiltration of fermentation broths after batch incubation and the mechanisms of membrane fouling by cells, debris, colloidal particles and macromolecules, e.g. for recovery of polysaccharides from fermentation broths [2]. 

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. 

Another microbial production of GDP-Fuc has been reported by a Japanese group [90]. GDP-Man obtained by fermentation was converted to GDP-Fuc by A. radiobacter. A crude extract from hog submaxillary glands was also found to convert fucose directly into GDP-Fuc by a two-step enzymatic process (see Scheme 14) [91]. 

Prior to the widespread awdlabdity of recombiant carbonyl reductases enzymes, the use of microbial reductions using either actively growing or dormant cells was commonplace Bakers yeast in particular, was a readily available source of stereoselective carbonyl reductases enzymes. Even with the widespread knowledge of the power of recombinant CRED biocatalysts, the literature is still rife with wild-type whole-cell microbial reductions. The reductions presented have advanced well beyond the early Bakers yeast reduction and have an apphcation even today. When the whole-cell fermentation is developed and finely tuned, high titers of product alcohol are possible and Scheme 6.4 shows m example of a keto-amide 12 bioreduction performing at 100 g/L with more than 98% ee with multi-kg isolation [12]. The bioprocess was performed over 8 days at pH 7 using the yeast Candida sorbophila. 

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