The Response Of Metabolic Activity To Shear Stress Environment And A Device Designed For Shear Stress Research

The morphology and production rate of filamentous fungi can be influenced by shear stress produced by reactors.In this study,on-line and off-line methods were applied to study the influence of shear stress on viable biomass concentration and metabolic activity during penicillin fermentation process.The results showed that the viable biomass concentration and the specific production rate of penicillin(qp)decreased under high shear stress.Then a device which can produce a relatively homogeneous shear stress was designed and applied to investigate the effect of shear stress on viable biomass concentration and qp in the chemostat cultivation of a high-producing Penicillium chrysogenum strain,DS17690.The viable biomass concentration can be monitored by the on-line capacitance which was detected by the viable cell sensor.The capacitance was produced by viable cells in alternating electric field.Furthermore,an assay for quantification of overall metabolic activity was applied.2,3-bis(2-methoxy-4-nitro-5-sulfophenly)-5-[(phenylamino)carbonyl]-2H-tetrazolium hydrox-ide(XTT),which in the presence of active mitochondria can be converted to a water-soluble formazan derivative that have absorbance value in the visible spectrum(430-490 nm).Thes-two methods were utilized in penicillin fermentation experiments which were carried out in a 5L stirred tank reactor.In batch phase(stirring speed is 400 rpm),both XTT absorbance and capacitance showed good linear relation with the dry cell weight.The results of chemostat phase show a big difference of capacitance between two stirring speeds,which was probably caused by the increased gas hold-up under higher speed;the XTT absorbance per gram dry cell weight gradually increased during the whole process under 400 rpm.however,under 600 rpm it increased first and then kept constant,which means that a high shear stress can induce the metabolic activity.In addition,under 600 rpm the dry cell weight was reduced by 12.9%and there was a 9.6%decrease in specific penicillin production rate,moreover,degeneration of penicillin production in Penicillium chrysogenum strain during chemostat cultivations was observed.Based on results of 5L stirred tank reactor(STR),a cylindrical bioreactor which can produce a relatively homogeneous shear stress was designed on the basis of a stirred tank bioreactor with working volume of 1L.Different shear stress can be produced by different stirring speeds.Power number was obtained from computational fluid dynamics(CFD)simulation.Average shear stress appeared on different fermentation scale can be reproduced by controlling power input per volume based on power number.The device was applied to penicillin fermentation process to study the effect of shear stress.The results indicated that the dry cell weight and specific penicillin production rate decreased with the increase of shear stress.In addition,result of this device was compared with that of 5L STR reactor based on the same power input per volume,there existed difference of dry cell weight and specific penicillin production rate,dry cell weight was 9%higher in cylindrical bioreactor,however,the production rate was 20%lower than 5L STR bioreactor.