Microbial biomass which is produced for human or animal consumption is referred to as single cell protein (SCP). Although yeast was produced as food on a large scale in Germany during the First World War (Laskin, 1977) the concept of utilizing microbial biomass as food was not thoroughly investigated until the 1960s. Since the 1960s, a large number of industrial companies have explored the potential of producing SCP from a wide range of carbon sources. Almost without exception, these investigations have been based on the use of continuous culture as the growth technique.
As previously discussed, continuous culture is the ideal method for the production of microbial biomass. The superior productivity of the technique, compared with that of batch culture, may be exploited fully and the problem of strain degeneration is not as significant as in the production of microbial metabolites. The selective pressure in the chemostat would tend to work to the advantage of the industrialist producing SCP, in that the most efficient strain of the organism would be selected, although this is not necessarily the case for mycelial processes. The development of SCP processes generated considerable research into large-scale chemostat design and the behaviour of the production organism in these veiy large vessels. Many 'novel' fermenters have been designed for SCP processes and these are considered in more detail in Chapter 7.
A very wide range of carbon sources have been investigated for the production of SCP. Whey has been used as a carbon source for biomass production since the 1940s and such fermentations have been shown to be economic in that they provide a high-grade feed product, whilst removing an, otherwise, troublesome waste product of the cheese industry (Meyrath and Bayer, 1979). Cellulose has been investigated extensively as a potential carbon source for SCP production and this work has been reviewed by Callihan et al. (1979) and Woodward (1987). The major difficulty associated with the use of cellulose as a substrate is its recalcitrant nature.
An enormous amount of research has been conducted into the use of hydrocarbons as sources of carbon for biomass processes; the hydrocarbons investigated being methane, methanol and n-alkanes. A large number of commercial firms were involved in this research field but very few created viable, commercial processes based on SCP production from hydrocarbons because of the economic difficulties involved (Sharp 1989). At the start of this research, hydrocarbons were relatively cheap but, following the 1973 Middle East War, oil prices escalated and transformed the economic basis of biomass production from petroleum sources. ICI were successful in developing a commercial process for production of bacterial biomass (Pruteen) from methanol at an annual rate of 54,000 to 70,000 tonnes. The process utilized a novel air-lift, pressure cycle fermenter, of 3000 m3 capacity, and was the first commercial process to produce SCP from methanol (King, 1982). The fermentation was run successfully for periods in excess of 100 days without contamination (Howells, 1982). Regrettably, the economics of the process were such that when the price of soya and fishmeal declined Pruteen could not compete as an animal feed. Selling in bulk ceased in 1985 < Shar 1989) and the 3000 m3 vessel was eventuallv molished. "
The expertise developed by ICI during the Prutee project and RHM's research into the use of a fungu™ Fusarium graminearum, for the production of human food formed the basis of a joint venture between the two companies. The ICI pressure cycle pilot-plant was used to produce the fungal biomass (Myco-protei'n marketed as Quorn) in continuous culture. The advantage of fungal biomass is that it may be processed' t0 give a textured protein which is acceptable for human consumption. The low shear properties of the air-lift vessel conserve the desirable morphology of the fungus The process is operated at a dilution rate of between 0.17 and 0.20 h1 (/xmax is 0.28 h"1). The phenomenon of mutation and intense selection in the chemostat has proved to be problematical in Myco-protein fermentation, because highly branched mutants have arisen in the vessel resulting in the loss of the desirable morphology. However, the process may still be operated in chemostat culture for 1000 hours on the full scale (Trinci, 1992).
Comparison of batch and continuous culture as investigative tools
Although the use of continuous culture on an industrial scale is very limited it is an invaluable investigative technique. The principle characteristic of batch culture is change. Even during the log phase cultural conditions are not constant and it is only the constant maximum specific growth rate which gives the semblance of stability — biomass concentration, substrate concentration and microbial products all change exponentially. During the deceleration phase the onset of nutrient limitation causes the growth rate to decline from its maximum to zero in a very short time, so it is virtually impossible to study the physiological effects of nutrient limitation in batch culture. As Trilli (1990) pointed out, adaptation of an organism to change is not instantaneous, so that the activity of a batch culture is not in equilibrium with the composition of its environment. Physiological events in a batch culture may have been initiated by a change in the environment which took place some significant time before the change was observed. Thus, it is very difficult to relate 'cause and effect'. The major feature of continuous culture, on the other hand, is 'the steady state' — biomass, substrate and product concentration should remain constant over
Idiin .vri.Kls of time. Specific growth rate is conned hv dilution rate and growth is nutrient limited. , -r" il i- important not to exaggerate the sigmfi-r'lh ■ -I -idy localise a constant biomass f1"?' t,H- not necessarily indicate that the culture is ^,oLk-,IIv .liable (Malek <</ al., 1988). It is possible m scpaiate .he effects of growth rate and other envi-r liinieiit.il conditions, for example temperature, pH ll dissolved oxygen concentration. Furthermore be-o[ u icle range of substrates may be used to 1,'mii -rovwh in the chemostat the effects of fx and substiatc concentration may be distinguished.
Continuous cultures may generate valuable physiological information on an industrial strain which may lv used in the optimization of the commercial process. An excellent example is the effect of growth rate and limituu: suhMK.io on metabolite formation. The inter-pielation »1 secondary metabolites as compounds produced in tin1 Uniphase of batch culture may lead one to suppose that the specific production rates (qp) of micIi compounds are inversely linked to specific growth T;,ie ()iI lie testing of this supposition may be achieved in chemostat culture and it has been shown to be correct for cephamycin and thienamycin synthesis by Streptomyces cattleya (Lilley et al., 1981) and .jilik icllm b> <iibberella fujikuroi (Bu'Lock etal.,\91A). However, different relationships have been demonstrated in other systems. Pirt and Righelato (1967) and Kvu and I ln-pndka (1980) showed that the qp of penicillin i-> piMiiw-lv correlated with (jl up to a specific growth rate of 0.013 h~', after which it is independent of fi. Pirt (1990) suggests that the apparent negative correlation may be related to penicillin degradation. These observations suggest that the growth rate in a commercial penicillin process should not decline below 0.013 h '. Positive correlations between p, and qp have been obtained for chlortetracycline production by Streptomyces aureofaciens (Sikyta et al., 1961), oxytetra-cu-liik- K Su<.\»omyces rimosus (Rhodes, 1984) and LTUhinmwm \ hy Streptomyces erythraeus (Trilli et al., 1987).
The fiercely selective nature of the chemostat, which is its major disadvantage for industrial production, mako ii ,,n .■v.illent tool for the isolation and improvement «1 micro-organisms. The use of continuous culture in this context is considered in Chapter 3, from which it may be seen that continuous enrichment culture offers considerable advantages over batch enrichment techniques and that continuous culture may be used very successfully to select strains producing higher yields of certain microbial enzymes.
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