The computational fluid dynamics (CFD) software package Fluent was utilized to simulate the flow field of Escherichia coli (E. coli) BL21 fermentation in a 50 L automatic bioreactor for producing α-cyclodextrin glycosyltransferase (α-CGTase) in this study. 4-down-pumping propeller (4DPP), 6-curved-blade disc turbine (6CBDT), and Rushton turbine (RT) were assembled to form eight impeller combinations (C1-C8). Through flow field simulating, four referential impeller combinations, in which C6, C7, and C8 were three layers stirring blades and C1 as a control, were selected to carry out batch fermentation experiments (TC1, TC6, TC7, and TC8) for validation. The correlation analysis between simulation results and experimental measurements indicated that TC6 (tank equipped with C6 impeller combination) exhibited lower enzymatic activity though it had the better mixing effect, fastest oxygen uptake rate (OUR), and maximum specific growth rate (µ) in the initial stage, which was just to the contrary in TC8. It was revealed by next fed-batch fermentation experiments in TC6 and TC8 that TC6 was considered as excellent flow field properties brought about the higher µ of E. coli BL21 and fast acetic acid (HAc) accumulation, which resulting in a serious inhibition on α-CGTase expression and this negative effect could not be removed. As a result, there should be a threshold of HAc accumulation rate which brought about a terrible inhibitory effect on α-CGTase expression. Moreover, the yield of α-CGTase activity reached 231.38 U mL- 1 in TC8, which elevated 31.74% compared to that obtained in TC1.
Keywords: Computational fluid dynamics; Flow field simulation; α-Cyclodextrin glycosyltransferase.