Cell Damage and Oxygen Mass Transfer during Cultivation of Nicotiana tabacum in a Stirred‐Tank Bioreactor

Abstract

AbstractCell damage and oxygen mass transfer were investigated during the batch culture of Nicotiana tabacum (tobacco) cells in a 3‐L Applikon stirred‐tank bioreactor equipped with a marine‐blade impeller. Total cell density, % culture viability, viable cell density, and kLa in the culture were measured over a 24‐day cultivation period at impeller speeds of 100–325 rpm under a fixed air flow of 0.43 wm and a temperature of 27 °C. The maximum total biomass density decreased from 11.8 g of dry cell weight (DCW)/L at 100 rpm to 8.6 g of DCW/L at 325 rpm. The net specific biomass production rate of viable cells (μv) during the exponential phase and the viable cell death rate (kd) during the stationary phase were estimated from semilog plots of viable cell density vs cultivation time. Values for μv decreased from 0.175 to 0.077 day−1 whereas values for kd increased from 0.042 to 0.109 day−1 with increasing impeller tip speed (from 24 to 77 cm/s), clearly showing that increasing agitation intensity increased the rate of cell damage. The effect was most pronounced at tip speeds of 24–60 cm/s. With respect to oxygen mass transfer, culture kLa values were about 10–20% higher than their corresponding initial kLa° values, but as the cell density increased, the values for kLa ultimately decreased. Despite the reduction in oxygen mass transfer rates, at impeller speeds of 100–325 rpm and an aeration rate of 0.43 wm, oxygen starvation was not observed. Therefore, the reduction in biomass productivity can be attributed to cell damage by hydrodynamic forces and not by inadequate oxygen mass transfer.