Effect of bottom clearance on performance of airlift bioreactor in high-density culture of Panax notoginseng cells

A fed-batch cultivation of Panax notoginseng cells in a concentric-tube airlift reactor was performed to study the effects of bottom clearance on cell growth and the production of ginseng saponin and polysaccharide. At a bottom clearance of 4.0 cm, the highest cell density of 29.1+/-1.6 g/l by dry weight was obtained, and the accumulation of saponin and polysaccharide also reached a maximum, i.e., 2.39+/-0.43 and 2.73+/-0.40 g/l, respectively. Cell growth and metabolite production were limited at a small (2.5 cm) or large (5.0 cm) bottom clearance. By analyzing the time constants of mixing, mass transfer and oxygen consumption, bulk gas-liquid oxygen transfer was found to be responsible for the growth limitation at a small bottom clearance (2.5 cm). The decrease in cell density at a large bottom clearance (5.0 cm) was related to cell sedimentation at the reactor bottom. This work is beneficial for the scale-up and efficient operation of the airlift reactor in cell cultures.