Continuous culture fermentation

Cell immobilization appears to offer several advantages in comparison with batch or continuous culture fermentations where free cells are used (1). Polysaccharide-based gels and reticulate foams, polyvinyl(PV) and polyurethane(PU), are the preferred matrices for photosynthetic systems. When cyanobacteria are grown in foam pieces, immobilization is achieved by adsorption on the foam surface as well as by entrapment of the cells in the foam cavities. 

Fermenter Type of traditional bioreactor (involving either stirred or nonstirred tanks) in which cell fermentation takes place in continuous-culture fermenters, nutrients are continuously fed into the fermentation vessel so that cells can ferment indehnitely, whereas in batch fermenters, nutrients are added in batches. 

Floetenmeyer, M.D., Glatz, B.A. and Hammond, E.G. (1985) Continuous culture fermentation of whey permeate to produce microbial oil. J. Dairy Sci. 68, 633-637. 

This is a set of simultaneous ordinary differential equations, which describe the dynamics of a continuous culture fermentation. 

Commercial-scale operations are conducted in batch, fed-batch, or continuous culture systems. Fermentation vessels include the conventional baffled aerated tank, with or without impeller agitation, and the ak-lift tower fermentors in which ak is sparged into an annular space between the... 

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).

The production-scale fermentation unit, with a projected annual capacity of over50,000 tonnes was fully commissioned in 1980. The bioreactor (Figure 4.8) is 60 m high, with a 7 m base diameter and working volume 1,500 m3. There are two downcomers and cooling bundles at the base. Initial sterilisation is with saturated steam at 140°C followed by displacement with heat sterilised water. Air and ammonia are filter sterilised as a mixture, methanol filter sterilised and other nutrients heat sterilised. Methanol is added through many nozzles, placed two per square metre. For start-up, 20 litres of inoculum is used and the system is operated as a batch culture for about 30 h. After this time the system is operated as a chemostat continuous culture, with methanol limitation, at 37°C and pH 6.7. Run lengths are normally 100 days, with contamination the usual cause of failure. 

Despite the advantages of continuous cultures, the technique has found little application in the fermentation industry. A multi-stage system is the most common continuous fermentation and has been used in the fermentation of glutamic add. The start-up of a multi-stage continuous system proceeds as follows. Initially, batch fermentation is commenced in each vessel. Fresh medium is introduced in the first vessel, and the outflow from this proceeds into the next vessel. The overall flow rate is then adjusted so that the substrate is completely consumed in the last vessel, and the intended product accumulated. The concentration of cells, products and substrate will then reach a steady state. The optimum number of vessels and rate of medium input can be calculated from simple batch experiments. 

Exponential growth in a batch culture may be prolonged by addition of fresh medium to the fermentation vessel. In a continuous culture the fresh medium has to be displaced by an equal volume of old culture, then continuous cell production can be achieved. 

Najfpour, G.D., Propionic and Acetic Acid Fermentation using Propionibacterium acidipropionici in Batch and Continuous Culture , Ph.D. thesis, University of Arkansas, Fayetteville, AR, 1983. 

The medium composition used in the fed-batch process was optimized, resulting in cell densities near 100 g l-1. By using an exponential feed rate resulting in a growth rate of 0.05 h-1, a maximum biomass concentration of 112 g 1 1 was attained, with a biomass productivity of 1.8 g 1 1 h. The poly(3HAMCL) productivity however was low, 0.34 g 1 1 h, caused by a steady decrease of the poly(3HAMCL) content during the last part of the fermentation [51]. When this optimized medium composition was used in the continuous culture system described above, a maximum biomass concentration of 18 g 1 1 was reached. The PHA content however remained low at approximately 10% [51]. It is still unclear what causes these low PHA contents. 

When a continuous culture is fed with substrate of concentration 1.00 g/1, the critical dilution rate for washout is 0.2857 h-1. This changes to 0.0983 h-1 if the same organism is used but the feed concentration is 3.00 g/1. Calculate the effluent substrate concentration when, in each case, the fermenter is operated at its maximum productivity. 

Odour will return in treated slurry as a result of post treatment fermentation. The concentration of readily fermentable substrates, measured as BOD5, provide an indicator of this problem. In continuous culture without oxygen limitation the BOD5 can be described by a model derived from the Monod (13) model of microbial growth (14). The supernatant BOD5 (g/1) from treatment at 15 to 45°C, was described by equation 3 and the whole BOD5 by equations 4 and 5(15). 

Figure 5.16 Typical laboratory scale fermentor for batch or continuous culture studies (Courtesy of L.H. Fermentation Ltd).

Microbial populations can be maintained in a state of exponential growth over a long period of time by using a system of continuous culture. Figure 6.7 shows the block diagram for a continuous stirred-tank fermenter (CSTF). The growth chamber is connected to a... 

I. Jobses, G. Egberts, K. Luyben, J.A. Roels, Fermentation kinetics of zymomonas mo-bilis at high ethanol concentrations oscillations in continuous cultures, Biotechnology and Bioengineering, 28, 868-877, 1986... 

While enzymes can be prepared from cells grown in either batch or continuous culture, we will cover only batch fermentation here as, even for pilot-scale biocatalysis runs, the amounts of enzyme obtainable in a batch fermentation are sufficient. 

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