Optimization of fermentation medium composition in substrate-controlled continuous stirred tank reactors

A ‘carbon source controlled shift technique’ was developed for fermentation medium optimization in continuous culture with the objective to maximize growth rate and growth‐linked product formation of a biological system. An automatic culture medium preparation system was operated together with 2 parallel stirred tank reactors and a HPLC system for on‐line analysis of the carbon source concentration in the reactors. A genetic algorithm was applied for experimental design. The concentrations of 7 medium components (mineral salts and vitamins) were optimized automatically within 40 continuous experiments to result in a maximum growth rate of the methylotrophic yeast Candida boidinii and growth‐linked production of the formate dehydrogenase enzyme (FDH). The specific growth rate of Candida boidinii and the specific activity of the FDH enzyme at a set‐point of 420 mM methanol in the reactor were, thus, improved by 19% to 0.16 h−1 and 26% to 164 U g−1, respectively, compared to the previously used medium, which has already been previously optimized in shake flask experiments. The results of the continuous medium optimization were evaluated with a full second order seven‐dimensional polynomial model (regression coefficient 96.8%).