Modelling and control of microbial fed-batch and pH-auxostat processes

Glucose and energy metabolism ofSaccharomyces cerevisiaehas been studied for decades,yet the exact location of the so called "bottleneck" thatcauses overflow metabolism resulting in aerobic ethanolformation remains unknown. However, this does not completelyhamper the modelling, simulation and development of methods tocontrol the aerobic ethanol formation under processconditions. A metabolically structured model of the glucose flux toenergy metabolism and growth ofS. cerevisiaewas developed. The model can be used tosimulate growth, respiration and ethanolproduction/consumption. It could predict these parameters insimulations of fed-batch and pH-auxostat cultures ofS. cerevisiae. Also published data from chemostatcultures could be predicted by this model. The model is characterised by the parameterqOmaxthat describes the maximum respiration rate (about0.3 g/(gh)). The glycolysis rate and concomitant respirationrate increase with increasing glucose concentration accordingto the Monod model, but only up to a critical value, about 30mg/L, above which no further increased oxygen consumption rateis observed, while the sugar uptake rate and glycolyticconversion to pyruvate, continue to increase. This surplus ofpyruvate is mainly converted to ethanol. It was shown that theqOmaxwas subjected to ethanol inhibition in anon-competitive way, with a inhibition constant of 10 g/L. Thelocation of the so called metabolic bottleneck that causes themaximum rate in the electron flux from pyruvate to oxygen inthe respiration is subject to much discussion in theliterature. By using a fed-batch technique with oscillatingsugar feed rate it was shown that pyruvate dehydrogenase, oneof the candidates of the bottleneck, is not a limiting factorfor the respiration rate. Simulations of fed-batch cultures of microorganismsexhibiting over-flow metabolism are very sensitive to theinitial conditions. To improve the reproducibility of theinitial phase of such cultures a pre-cultivation technique wasdeveloped, based on exponential feeding of a diluted completemedium. In this way the biomass concentration and metabolicstate of the cells at the start of the main fermentation couldbe controlled. The pH-auxostat principle for continuous cultivation wasapplied toEscherichia coliandS. cerevisiae. To make the concentrations of biomass andethanol readily controllable, a system with parallel pHcontrolled flows of medium and titrant was usually applied. Itwas shown that the pH-auxostat can only be stable at, or closeto the maximum specific growth rate. If conditions do notfulfil this criterion, the pH-auxostat becomes very sensitiveto variations in the two flows. A boundary condition, expressedas function of the ratio between the two flows and thesubstrate concentration, was identified, below which thepH-auxostat substrate becomes exhausted and the flow ceaseswithout wash-out of the cells. The pH-auxostat complements thechemostat in that it permits continuous processes at maximumspecific growth rate. Key words:S. cerevisiae;E. coli; fed-batch cultivation; pre-cultivation;pH-auxostat; overflow metabolism; metabolic bottleneck; aerobicethanol formation; ethanol inhibition; respiration; kineticsmodel; simulationNR 2014080