Design of estimators for specific growth rate control in a fed-batch E. coli fermentation

Abstract

The specific growth rate is one of the most important process variables characterizing the state of microorganisms during fermentations mainly because the biosynthesis of many products of interest is often related with the values assumed by this parameter. In the particular case of the fed-batch operation of Escherichia coli for the production of recombinant proteins, it is important to maintain the specific growth rate below a certain threshold in order to avoid the accumulation of acetic acid throughout the fermentation and, additionally, it is often argued that both pre- and the post-induction specific growth rates should be closely controlled in order to achieve maximum productivities on the desired recombinant protein. The main purpose of this work was to develop and implement reliable algorithms for the estimation of the specific growth rates in E. coli fermentation that can be used for the feedback control of this variable using a developed mathematical model of the process, together with common on-line measurements of dissolved and exhausted oxygen and carbon dioxide. For that purpose, a biomass asymptotic observer is derived for the on-line estimation of biomass. Using estimated biomass concentration values together with on-line process data, the determination of the specific growth rates is performed using observer-based estimators. Finally, a feedbackfeedforward control algorithm was developed that uses estimated values of the specific growth rates and biomass concentration for the calculation of the feeding rate. The developed algorithms were validated by simulations where on-line variables used for the calculation of the estimators were corrupted with white noise.