CELL-DAMAGE AND OXYGEN MASS-TRANSFER DURING CULTIVATION OF NICOTIANA-TABACUM IN A STIRRED-TANK BIOREACTOR

Cell damage and oxygen mass transfer were investigated during the batc h culture of Nicotiana tabacum (tobacco) cells in a 3-L Applikon stirr ed-tank bioreactor equipped with a marine-blade impeller. Total cell d ensity, % culture viability, viable cell density, and k(L)a in the cul ture were measured over a 24-day cultivation period at impeller speeds of 100-325 rpm under a fixed air flow of 0.43 vvm and a temperature o f 27 degrees 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 (mu(v)) durin g the exponential phase and the viable cell death rate (k(d)) during t he stationary phase were estimated from semilog plots of viable cell d ensity vs cultivation time. Values for mu(v), decreased from 0.175 to 0.077 day(-1) whereas values for hd increased from 0.042 to 0.109 day( -1) with increasing impeller tip speed (from 24 to 77 cm/s), clearly s howing that increasing agitation intensity increased the rate of cell damage. The effect was most pronounced at tip speeds of 24-60 cm/s. Wi th respect to oxygen mass transfer, culture k(L)a values were about 10 -20% higher than their corresponding initial k(L)a(0) values, but as t he cell density increased, the values for k(L)a ultimately decreased. Despite the reduction in oxygen mass transfer rates, at impeller speed s of 100-325 rpm and an aeration rate of 0.43 vvm, oxygen starvation w as not observed. Therefore, the reduction in biomass productivity can be attributed to cell damage by hydrodynamic forces and not by inadequ ate oxygen mass transfer.