Cell recycle batch fermentation

FIGURE 8.4 Ten cycles of cell recycle batch fermentation from hydrothermally pretreated rice straw in the presence of 10 FPU/g-biomass cellulase using wild-type (open circle) and cellulase-displaying Saccharomyces cerevisiae strain, NBRC1440/B-EC3 (closed square). 

Matano Y, Hasunuma T, Kondo A. (2013a). Cell recycle batch fermentation of high-solid lignocellulose using a recombinant cellulase-displaying yeast strain for high yield ethanol production in consolidated bioprocessing. Bioresour Technol, 135,403 09. 

L. casei in a repeated batch fermentation showed excellent biofilm stability for up to 72 days (Demirci and Pometto 1995). Using a different bioreactor configuration, L. casei cells on polyethyleneimine-coated foam glass beads yielded excellent results in a recycle batch fermentation. Both systems can produce an average of 4.3—4.6 g of lactic acid/l-h (Ho et al. 1997). 

Figure 2.5 Possible technological solutions to bioprocess problems a) Fed-batch culture b) Continuous product removal (eg dialysis, vacuum fermentation, solvent extraction, ion exchange etc) c) Two-phase system combined with extractive fermentation (liquid-impelled loop reactor) d) Continuous culture, internal multi-stage reactor e) Continuous culture, dual-stream multi-stage reactor f) Continuous culture with biomass feedback (cell recycling). (See text for further details).

It appears that dCx/dt and rx are always the same, but this is not true. The former is the change of the cell concentration in a fermenter, which may include the effect of the input and output flow rates, cell recycling, and other operating conditions of a fermenter. The latter is the actual growth rate of the cells. The two quantities are the same only for batch operation. 

A PFF requires the initial presence of microorganisms in the inlet stream as a batch fermenter requires initial inoculum. The most economical way to provide cells in the inlet stream is to recycle the part of the outlet stream back to the inlet with or without a cell separation device. Figure 6.17 shows the schematic diagram of a PFF with cell recycling. Unlike the CSTF, the PFF does not require the cell separator in order to recycle, though its presence increases the productivity of the fermenter slightly as will be shown later. The performance equation of the PFF with Monod kintics can be written as ... 

Batch fermentations result in high product concentration (120-150 kg/m3) but in low productivity (2 kg m Iff1). Dramatic improvements in productivity (20-80 kg m Iff1 in laboratory- and pilot-plant experiments) were obtained by using cell recycle via MF or immobilized cells, at the expense of lactate concentration in the effluent (usually lower than 50 kg/m3). 

The production of substances that preserve the food from contamination or from oxidation is another important field of membrane bioreactor. For example, the production of high amounts of propionic acid, commonly used as antifungal substance, was carried out by a continuous stirred-tank reactor associated with ultrafiltration cell recycle and a nanofiltration membrane [51] or the production of gluconic acid by the use of glucose oxidase in a bioreactor using P E S membranes [52]. Lactic acid is widely used as an acidulant, flavor additive, and preservative in the food, pharmaceutical, leather, and textile industries. As an intermediate product in mammalian metabolism, L( +) lactic acid is more important in the food industry than the D(—) isomer. The performance of an improved fermentation system, that is, a membrane cell-recycle bioreactors MCRB was studied [53, 54], the maximum productivity of 31.5 g/Lh was recorded, 10 times greater than the counterpart of the batch-fed fermentation [54]. 

It may also be economical to remove the inhibitory product directly from the ongoing fermentation by extraction, membranes, or sorption. The use of sorption with simultaneous fermentation and separation for succinic acid has not been investigated. Separation has been used to enhance other organic acid fermentations through in situ separation or separation from a recycled side stream. Solid sorbents have been added directly to batch fermentations (18,19). Seevarantnam et al. (20) tested a sorbent in the solvent phase to enhance recovery of lactic acid from free cell batch culture. A sorption column was also used to remove lactate from a recycled side stream in a free-cell continuously stirred tank reactor (21). Continuous sorption for in situ separation in a biparticle fermentor was successful in enhancing the production of lactic acid (16,22). Recovery in this system was tested with hot water (16). 

The different operation modes used in microbial fermentations are employed also in animal cell cultivation. Although many different classifications can be adopted, the most general is the one that considers the following operation modes batch, fed-batch, continuous, and perfusion, which is a continuous mode with cell recycle/retention (Castilho and Medronho, 2002). 

Kluyveromices fragilis. The behavior of the fermentor is similar to that of a cell recycle reactor with the same fermentation system. Higher productivity and yields than with a batch fermentation are obtained (Tables 7.1, 7.2). The long term cell stability in an HFF (Figure 7.46) is also better than in a batch fermentor. However, even with HFF, reactor productivity is increased at the expense of low substrate conversions, at least at low dilution rates. 

Bae SM, Park YC, Lee TH, Kweon DH, Choi JH, Kim SK, Ryu YW, Seo JH. (2004). Production of xylitol by recombinant Saccharomyces cerevisiae containing xylose reductase gene in repeated fed-batch and cell-recycle fermentations. Enzyme Microb Tech, 35, 545-549. 

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. 

For example, different fermentation schemes have been developed for the production of ethanol. Conventional batch, continuous, cell recycle and immobilized cell processes, as well as membrane, extraction and vacuum processes, which selectively remove ethanol from the fermentation medium as it is formed, were compared on identical bases using a consistent model for yeast metabolism (Maiorella et al., 1984). The continuous flow stirred tank reactor (CSTR) with cell recycle, tower and plug flow reactors all showed similar cost savings of about 10% compared to batch fermentation. Cell recycle increases cell density inside the fermentor, which is important in reducing fermentation cost. 

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