Continuous cultures

Combining Eqs. (11) and (5) produces the well known relation between growth rate and dilution rate at steady-state (dX/df = 0)  

Therefore, continuous culture offers the opportunity to control the rate of cell growth. From Fig. 4, this allows one to select the for high enzyme production. 

The steady-state relation between cell mass and dilution rate is obtained by combining Eqs. (5-9)  

Combining Eqs. (14) and (15) relates protein concentration and dilution rate 

---

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... 

R. Retovsky, Continuous Cultivation of Algae, Theoretical and Methodological Bases of Continuous Culture of Microorganisms, Academic Press, Inc., New York, 1966. 

This is an old, familiar analysis that applies to any continuous culture with a single growth-limiting nutrient that meets the assumptions of perfect mixing and constant volume. The fundamental mass balance equations are used with the Monod equation, which has no time dependency and should be apphed with caution to transient states where there may be a time lag as [L responds to changing S. At steady state, the rates of change become zero, and [L = D. Substituting ... 

Figure 24-23 is a sketch of continuous culture with recycle. The symbols for flow rates and organism concentrations are F and X, respec tively Assuming perfect mixing and steady state so that the derivatives can be set to zero, mass balances lead to ... 

FIG. 24-23 Continuous culture with recycle. (A. E. Humphrey, Biochemical Engineering in Encyclopedia of Chemical Processing and Design, -ool. 4, July 1977,pp. 359A 94.)... 

Two types of interac tion, competition, and predation are so important that worthwhile insight comes from considering mathematical formulations. Assuming that specific growth-rate coefficients are different, no steady state can be reached in a well-mixed continuous culture with both types present because, if one were at steady state with [L = D, the other would have [L unequal to D and a rate of change unequal to zero. The net effect is that the faster-growing type takes over while the other dechnes to zero. In real systems—even those that approximate well-mixed continuous cultures—there may be profound... 

Continuous culture A method of eultivation in whieh nutrients are supplied and produets are removed eontinuously at volumetrieally equal rates maintaining the eells in a eondition of stable multiplieation and growth. 

Keller, R. and Dunn, I. J., Computer simulation of the biomass production rate of cyclic fed batch continuous culture, J. Appl. Chem., BiotechnoL, 28, 508-514, 1978. 

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 multi-stream multi-stage system is a valuable means for obtaining steady-state growth when, in a simple chemostat, the steady-state is unstable eg when the growth-limiting substrate is also a growth inhibitor. This system can also be used to achieve stable conditions with maximum growth rate, an achievement that is impossible in a simple chemostat (substrate-limited continuous culture). 

Biomass feedback refers to increasing the concentration of biomass in the culture vessel. This is achieved by fitting some device, either internally or externally, to the continuous culture which retains or returns biomass to the vessel. The main advantage of biomass feedback is that the maximum output rate of biomass (and products) in the vessel with a given medium can be increased. This is particularly useful when the growth-limiting substrate is unavoidably dilute, for example if substrate has low solubility or has to be limited because of the formation of an inhibitory product. 

True. Batch cultures give lower overall outputs than continuous cultures, as they suffer from non-productive down-time (the time taken to empty, clean, re-sterilise and re-fill the fermentor). After inoculation, considerable time can be taken for biomass to build up to a level where substrates are effectively utilised. Continuous cultures do not suffer such drawbacks once they are in operation. 

False. Continuous culture systems are more difficult to operate than batch cultures. Medium is continuously added and withdrawn, making the process more prone to contamination. Maintenance cannot be carried out during lengthy culture runs, making the equipment more prone to breakdown. Batch cultures do not suffer such drawbacks. 

False. Continuous cultures operate for lengthy periods. Spontaneous mutations will arise and if they can compete successfully with the parent organism (by virtue of higher growth rate) they can predominate in the culture. Batch cultures have short growth times and so do not suffer such drawbacks. 

False. Batch cultures can convert high proportions of substrates, as growth can be allowed to proceed until substrates are exhausted. In continuous cultures substrates are never fully converted, as medium is continuously removed. In fact, residual substrate concentration increases as the dilution rate increases, until virtually all of the medium remains unused. Continuous cultures usually recycle the medium after biomass removal to increase the efficiency of substrate conversion. 

Continuous cultures can be ru n at different dilution rates. An optimum rate must be chosen, taking account of output, substrate utilisation and aeration and cooling requirement. We will work this out for ourselves later on. 

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. 

Continuous culture is not considered suitable for citric add production the requirement for a multi-tank system to separate growth and product formation would make the process uneconomic. 

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. 

Batch. All waste products (metabolites) accumulate in batch culture in continuous culture much is lost through the outflow. 

Fig. 5.3. Schematic diagram of continuous culture with control units in a constant volume chemostat.

The continuous cultures of chemostat and biostat systems have the following criteria ... 

---