Fluid mixing and oxygen transfer in cell suspensions of Taxus chinensis in a novel stirred bioreactor

In high-density plant cell cultures, mixing and mass transfer are two key issues, which should be emphasized for process optimization. In this work, both mixing and oxygen transfer characteristics of cell suspensions of Taxus chinensis were studied in a new centrifugal impeller bioreactor with a working volume of 1.2 L. The mixing time (tM) and the volumetric oxygen transfer coefficient (KLα) under different operational conditions were determined in both tap water and cell suspensions of 100-400 g fresh weight/L (i.e., 5.65-23.1 g DW/L). At an aeration rate of 0.1 L/min, tMdecreased from 10.6 s at 30 rpm to 2.89 s at 200 rpm under 100 g FW/L, and from 9.63 s (120 rpm) to 4.05 s (300 rpm) under 400 g FW/L. Compared with the effect of agitation, aeration was less significant to the suspension mixing. At a relatively high agitation speed (e.g., 200 rpm), tMremained almost the same even though aeration rate was changed from 0.1 to 0.4 L/min. The tMvalue increased slowly from 3.98 to 5.26 s at 120 rpm when the cell density was raised from 100 to 250 g FW/L. A rapid increase of both tMand the suspension viscosity was observed at a cell density above 300 g FW/L. As expected, the KLα value increased with an increase of aeration rate and agitation speed, but decreased with an increase of cell density. The quantitative data obtained here are useful to investigate the effect of mixing stress on the cell physiology and metabolism of Taxus chinensis in the bioreactor.