Dynamics and optimization of a L-lysine fermentation process

An engineering study of biochemical reaction kinetics and optimization of a fermentation process have been performed on a model culture system. The production of L-lysine from glucose by an auxotrophic mutant, Corynebacterium glutamicum (ATCC 13286) was used as the model system for engineering studies of biochemical kinetics and optimization. Batch, fed-batch and continuous modes of operation of the bioreactor were investigated by free and immobilized cell cultures. Unsteady state operation of either fed-batch or continuous fermentation by free suspension culture was shown to be beneficial to maximizing productivity. Cell immobilization was achieved by occlusion in polyurethane foam. The immobilized cell system was regarded as one option for overcoming threonine inhibition and cell washout in continuous production of L-lysine in this model system.The continuous production of L-lysine with both freely suspended cells and immobilized cells was investigated and compared with batch and fed-batch cell suspension processes. The productivity with freely suspended cells in the CSTR mode was low (0.23 g/L/h) the highest productivity was achieved with immobilized cells in the CSTR (1.6 g/L/h). In terms of productivity and lysine yields, the medium should contain: 80.0 g/L glucose and 0.6 g/L threonine and other nutrients present in the basal medium at an optimal dilution rate of 0.04-0.05 1/h.A simplified metabolic reaction network approach (MRN) was developed for on-line process monitoring and identification for the L-lysine fermentation process. The MRN approach made full use of existing knowledge about metabolic reactions within cells. The rates of cell growth, product formation, and substrate consumption were estimated from on-line measurements of some of the exchangeable metabolites (oxygen and carbon dioxide). Such information would be essential for process identification and control in the absence of reliable on-line biosensors.The development and evaluation of appropriate kinetic models for characterizing cell growth and product synthesis have been performed both theoretically and experimentally. The effects of the growth factors on the fermentation process were established by experimental investigations of the dynamic response of the system and theoretical studies involving kinetic models coupled with algorithms for process identification. Optimization of fed-batch fermentation was performed it was shown that the pulse fed-batch operation mode gave a higher productivity (0.3 g/L/h) for prolonged periods (up to 450 hours). It is demonstrated that the unsteady state nature of a pulse fed-batch process can be used to maximize the productivity, especially when both substrate and product are inhibitory to product synthesis.