At the present time, very few large-scale continuous-culture processes are being operated. These are primarily for the production of microbial biomass, glucose isomerase, buttermilk souring and yoghurt (Heijnen et al., 1992).
It is appropriate at this stage to compare batch-culture productivity and continuous culture productivity. Wang et al. (1979) derived an equation to quantify this relationship:
Continuous-culture productivity Batch-culture productivity
= (X>n/Xo) + (Xm-X0)/Xm where ¡xm = maximum specific-growth rate, X0 = initial cell density, Xm = maximum cell density, tL = turn-around time, Dc = critical dilution rate, Y = cellular yield coefficient for the limiting nutrient.
In an example they used an inoculum size of 5% (X0/Xm = 0.05), a process turn-around time of 10 hours, a cellular yield of 0.5 g cells per g of substrate and a final cell concentration of 30 g dm"3. Productivity was then calculated for a series of maximum specific growth rates (Table 12.7). It is clear from these data that the faster the growth of the organism, the more favourable is a continuous process over a batch process.
When assessing feasibility of a continuous process for product formation it is necessary to know: volumetric productivity, conversion yield of product from the most expensive substrate in the medium and the product concentration (Wang et al., 1979). In processes for cell biomass, some alcohols and organic acids where the major production cost is at the fermentation stage, volumetric productivity and conversion yield will be most important. A continuous process may be more economic than a batch process if higher productivities at higher efficiencies can be achieved. Unfortunately, in some processes, the final product concentration in the effluent broth from a continuous culture will be less than that obtained in a batch process, which will create a need for greater concentration at the recovery stage.
Heijnen et al. (1992) argue that low dilution rates are favoured if high product concentrations are to be obtained as recovery cost may be 50-80% of total costs. However, the concentration preference is not valid in processes which can start with a simple concentration step (SCP or intracellular glucose isomerase) or with products that need no concentrating (beer, yoghurt and buttermilk).
In a production plant, continuous culture offers the advantages of constant flow, product quality and simple automation and control. Disadvantages are due to specific production facilities that cannot be used for other purposes, lack of continuous recovery techniques and lack of constant market demand (Heijnen et al, 1992).
The continued increase in efficiency of fed-batch culture processes for antibiotics and other non-growth-associated products makes manufacturers reluctant to make radical alterations to established processes such as the introduction of continuous culture (see also Chapter 2).
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