Effects of rheological properties and mass transfer on plant cell bioreactor performance: Production of tropane alkaloids

Abstract

Volumetric mass transfer coefficients, K(L)a were measured over an aeration rate range from 0.1 to 1.0 vvm in a 1.2-L draft-tube-type airlift bioreactor for different Datura stramonium cell concentrations and correlated with superficial air velocity and rheological properties of the cell suspension. The measured K(L)a values (17-40 h(-1)) for a cell volume fraction of 0.2 (v/v) were approximately 2 times higher than those for the highest cell concentrations tested (cell volume fraction 0.7-0.8 v/v). Cell suspensions exhibited yield stress and pseudoplastic behavior. This behavior was described by the Casson model. The estimated yield stress values depended upon cell concentration with an exponent of 4.0. An empirical correlation based on the data for plant cell suspensions exhibiting yield stress was developed in order to determine aeration strategy for the plant cell cultivation in draft-tube-type airlift bioreactors: K(L)a = A(U(gr))(0.3(eta(eff))(-0.4). Aeration rates above 1.0 vvm caused a significant drop in cell yield and product content. Maximum growth and production were obtained at 0.6 vvm aeration. The cell and product yields obtained at 1.7 vvm were 2.8 times lower than the maximum values (25 g cell DW/L and 73.8 mg tropane alkaloid/L). The effects of the increased aeration rates on cell yield were also evaluated in terms of Reynolds stress. It was found that there was a relation between cell damage and the estimated Reynolds stress. The Reynolds stress estimated for the same aeration rate decreased with increasing cell concentration, suggesting that cells in the cultures at low cell concentrations are subjected to hydrodynamic damage. In the experiments with the cell cultures having a cell concentration of 0.3 (v/v), approximately 70% reduction in cell concentration was observed when the Reynolds stress was increased from 10 to 50 dyn/cm(2).