The use of fed-batch culture by the fermentation industry takes advantage of the fact that the concentration of the limiting substrate may be maintained at a very low level, thus avoiding the repressive effects of high substrate concentration. Furthermore, the fed-batch system also gives some control over the organisms' growth rate, which is also related to the specific oxygen uptake rate giving some control over the oxygen demand of the fermentation (see Chapter 9). Both variable and fixed volume systems result in low limiting substrate concentrations, but the quasi steady-state of the variable volume system has the advantage of maintaining the concentrations of both the biomass and the non-limiting nutrients constant. Pirt (1979) cites this as an important feature because the concentrations of substrates other than those which limit growth can have a significant effect on biomass composition and product formation.
The obvious advantage of cyclic fed-batch culture is that the productive phase of a process may be extended under controlled conditions. However, a further advantage lies in the controlled periodic shifts in growth rate which may provide an opportunity to optimize product synthesis. Dunn and Mor (1975) pointed out that changes in the rates of chemical processes can give rise to increases in intermediate concentrations and similar effects may be possible in microbial systems. This observation is particularly relevant to secondary metabolite production which is maximal in batch culture during the deceleration phase. Bushell (1989) suggested that optimum conditions for secondary metabolite synthesis may occur during the transition phase after the withdrawal of a volume of broth from the vessel and before the re-establishment of the steady-state following the resumption of the nutrient feed. During this period the dilution rate will be greater than the growth rate but, according to Bushell, the rate of uptake of the growth-limiting substrate should respond immediately to the increased substrate concentration. Thus, an imbalance results between the substrate uptake rate and the specific growth rate — this imbalance then contributing to a diversion of intermediates into secondary metabolism.
The advantages of cyclic fed-batch culture must be weighed against difficulties inherent in the system. Care has to be taken in the design of cyclic fed-batch processes to ensure that toxins do not accumulate to inhibitory levels and that nutrients other than those incorporated into the feed medium become limiting (Queener and Swartz, 1979). Also, a prolonged series of fed-batch cycles may result in the accumulation of non-producing or low-producing variants.
The early fed-batch systems that were developed did not incorporate any form of feedback control and relied on the inherent quasi steady state to maintain process stability. However, the use of concentrated feeds resulting in very high biomass concentrations ■ the development of more sophisticated feeding grammes has necessitated the introduction of feed^t control techniques. In such feedback controlled mentations a process parameter directly related to th" organism's physiological state is monitored con ously by an on-line sensor. The signal generated b sensor is then used in a control loop (see Chapter control the medium feed rate. Parameters which been utilized in this way include dissolved oxygen con centration, pH, effluent gas composition and limitir" substrate concentration. Examples of these control sys tems are included in the next section and in Chapter 9
Fed-batch culture was used in the production of bakers' yeast as early as 1915. It was recognized that an excess of malt in the medium would lead to too high a growth rate resulting in an oxygen demand in excess of that which could be met by the equipment. This resulted in the development of anaerobic conditions and the formation of ethanol at the expense of biomass production (Reed and Nagodawithana, 1991). Thus, the organism was grown in an initially weak medium to which additional medium was added at a rate less than the maximum rate at which the organism could use it. Thus, this process fulfils the criteria stipulated in Pirt's (1975) kinetic description for the establishment of a quasi steady state, that is, a substrate limited culture and the use of a feed rate equivalent to a dilution rate less than /imax. It is now recognized that bakers' yeast is very sensitive to free glucose and respiratory activity may be repressed at a concentration of about 5 /¿g dm-3 (Crabtree, 1929). Thus, a high glucose concentration represses respiratory activity as well as giving use to a high growth rate, the oxygen demand of which cannot be met. In modern fed-batch processes for yeast production the feed of molasses is under strict control based on the automatic measurement of traces of ethanol in the exhaust gas of the fermenter. Although such systems may result in low growth rates, the biomass yield is near the theoretical maximum obtainable (Fiechter, 1982).
It is interesting to note that the production of recombinant proteins from yeast may be achieved using fed-batch culture techniques very similar to that developed for the bakers' yeast fermentation. Gu et ai (1991) reported the production of hepatitis B surface antigen (HBsAg) in a 0.9 dm3 fed-batch reactor where the feed rate was increased exponentially to maintain a
, ■ |V hi-ll g'"Wth nltC- HBSAg Pr°dUCti0n WaS
relativciv _ * . ^ ^ strain and good productivity i ' , 'i iK'd in this strain ai gr„»lh a- , ;;m;ijntajnin„ a high growth rate whilst
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keeping thcj, ^^ activity. Ibba # al. (1993) yclic fed-batch process for recombinant S cerevisiae, under the control of a con-loter. The cyclic fed-batch process gave ie hirudin activity of a continuous fermen-,.,„„„. the Miperior productivity being due to increased re-purled
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