l"r, i , The cyclic fed-batch process gave
Jrcc tun'- llu- hirudin activity of a continuous fermen-
truiwripiion .,I though a genetic explanation of the .¿ulisoHilJ not be offered.
' penicillin lei mentation provides an excellent exam-nle of the use of feed systems in the production of a secondary metabolite. The fermentation may be divided into two phases — the 'rapid-growth' phase, during which the culture grows at /imax, and the 'slow-growth' or -production' phase. Glucose feeds may be used to control the metabolism of the organism during I.01I1 ph.ixeN During the rapid-growth phase an excess of glucose causes an accumulation of acid and a biomass oxygen demand greater than the aeration capacity of the fermenter, whereas glucose starvation may result in the organic nitrogen in the medium being used as a carbon source, resulting in a high pH and inadequate biomass formation (Queener and Swartz, 1979). The accumulation of hexose may be prevented by the use of a slowly hydrolysed carbohydrate such as lactose in simple batch culture (Matelova, 1976). However, according to Oueener and Swartz, considerable increases in productivity have been achieved by the use of computer controlled feeding of glucose during the rapid growth phase, such that the dissolved oxygen or pH is maintained within certain limits. Both control parameters essentially measure the same activity in that both oxygen concentration and pH will fall when glucose is in excess, due to an increased respiration rate and the accumulation of organic acids when the respiration rate exceeds the aeration capacity of the fermenter. Both systems appear to work well in controlling feed rates during the rapid-growth phase.
During the production phase of the penicillin fermentation the feed rates utilized should limit the growth rate and oxygen consumption such that a high rate of penicillin synthesis is achieved, and sufficient dissolved oxygen is available in the medium. The control factor in this phase is normally dissolved oxygen because pH is less responsive to the effect of dissolved oxygen on penicillin synthesis than on growth. As the fed-batch process proceeds then the total biomass, viscosity and oxygen demand increase until, eventually, the fermentation is oxygen limited. However, limitation may be delayed by reducing the feed rate as the fermentation progresses and this may be achieved by the use of computer controlled systems.
Suzuki et al. (1987) developed a pH feedback fed-batch system for the production of thiostrepton from Streptomyces laurentii. When glucose was exhausted in the fermentation the pH rose immediately and this event was used as the signal for the addition of more feed which consisted of a concentrated glucose, corn steep liquor, soy bean meal and mineral mixture. This process maintained a biomass level of 157 g dm-3 and a thiostrepton concentration of 10.5 g dm-3 with a productivity nine times that of a conventional batch culture.
Many enzymes are subject to catabolite repression, where enzyme synthesis is prevented by the presence of rapidly utilized carbon sources (Aunstrup et al., 1979). It is obvious that this phenomenon must be avoided in enzyme fermentations and fed-batch culture is the major technique used to achieve this. Concentrated medium is fed to the culture such that the carbon source does not reach the threshold for catabolite repression. For example, Waki et al. (1982) controlled the production of cellulase by Trichoderma reesei in fed-batch culture utilizing C02 production as the control factor and Suzuki et al. (1988) achieved high lipase production from Pseudomonas fluorescens also using C02 production to control the addition of an oil feed.
Shioya (1990) developed a method for the optimization of fed-batch systems based on the relationship between ¡x and qp, the product specific production rate. Once the relationship between the two parameters was established a computer control system was used to maintain the fed-batch at the optimum specific growth rate (feed rate). The system was tested for a number of fermentations including histidine (Brevibac-terium flavum), acid phosphatase and glutathione (S. cerevisiae), and lysine (Corynebacterium glutamicum). Specific growth rate was maintained constant using a feed-forward control profile which was updated throughout the fermentation as data were collected. Very promising results were obtained but difficulties were experienced in generating sufficiently accurate data to up-date the control system and maintain the specific growth rate constant.
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