Development of a helical‐ribbon impeller bioreactor for high‐density plant cell suspension culture

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Abstract
A double helical-ribbon impeller (HRI) bioreactor with a 11-L working volume was developed to grow high-density Catharanthus roseus cell suspensions. The rheological behavior of this suspension was found to be shear-thinning for concentrations higher than 12 to 15 g DW · L−1. A granulated agar suspension of similar rheological properties was used as a model fluid for these suspensions. Mixing studies revealed that surface baffling and bottom profiling of the bioreactor and impeller speeds of 60 to 150 rpm ensured uniform mixing of suspensions. The HRI power requirement was found to increase singnificantly for agar suspensions higher than 13 g DW · L−1, in conjunction with the effective viscosity increase. Oxygen transfer studies showed high apparent surface oxygen transfer coefficients (kLa ∼4 to 45 h−1) from agar suspensions of 30 g DW · L−1 to water and for mixing speeds ranging from 120 to 150 rpm. These high surface kIa values were ascribed to the flow pattern of this bioreactor configuration combined with surface bubble generation and entrainment in the liquid phase caused by the presence of the surface baffles. High-density C. roseus cell suspension cultures were successfully grown in this bioreactor without gas sparging. Up to 70% oxygen enrichment of the head space was required to ensure sufficient oxygen supply to the cultures so that dissolved oxygen concentration would remain above the critical level (≥10% air saturation). The best mixing speed was 120 rpm. These cultures grew at the same rate (∼0.4 d−1) and attained the same high biomass concentrations (∼25 to 27 g DW · L−1, 450 to 500 g filtered wet biomass · L−1, and 92% to 100% settled wet biomass volume) as shake flask cultures. The scale-up potential of this bioreactor configuration is discussed.