Optimization And Application Of Various Impeller Combination System On Curdlan Fermentation

Curdlan has wide applications in food,pharmaceutical and aquaculture fields due to its unique gelation and immune-enhancing properties.The curdlan fermentation broth is a kind of highly viscous fluid,and it increases significantly with the curdlan accumulation in fermentation process,which always results in poor mixing and mass transfer efficiency and subsequently limit curdlan biosynthesis.To improve this issue,the cold and hot model experiments on different impeller combinations were investigated and then the optimized mode was applied for curdlan fermentation.Firstly,the cold model experiments on mass transfer and mixing characteristics with 5 kinds of impeller combinations,including 3RT(triple Rushton turbines),RT+E(bottom Rushton turbines combined with upper Ellipse),RT+2WHD(bottom Rushton turbines combined with upper two wide-blade hydrofoil impeller pumping down),RT+DHR(bottom Rushton turbines combined with upper double helical ribbon)and EG(bottom Gate combined with upper Ellipse),were systematically conducted under various viscosity conditions of curdlan solutions.The results indicated that in the low-viscosity curdlan solutions with the same power consumption,the gas holdup,generated by the small diameter impeller combinations(3RT,RT+2WHD)and RT+E impeller combination were higher than that generated by the large diameter impeller combinations(EG,RT+DHR),but this discrepancy trended to be negligible with the viscosity increase.On the other hand,although the impeller combinations of 3RT and RT+2WHD had better mass transfer efficiency,they presented relative worse mixing ability,compared with the large diameter impeller combinations.In contrast,the impeller combinations of RT+E has been proved to be more efficient both in mixing and mass transfer performances.Secondly,curdlan fermentation were carried out with different impeller combinations.The results showed that the good mixing and mass transfer abilities and highest yield(37.02 g/L)was obtained with the RT+E impeller combination,which was consistent with the results of cold model experiments.In addition,it was found that the curdlan biosynthesis rate decreased at the late phase of curdlan fermentation under 3RT as well as RT+2WHD and EG impeller conditions,which could be ascribed to the poor mixing efficiency at the higher viscosity condition.The lowest yield of curdlan(23.97 g/L)was observed with the RT+DHR.Furthermore,the effect of agitation speed on the mass transfer performance was studied.The mass transfer conditions could well meet the demand when the agitation speed reached 500 r/min under 3RT.With comparison,the mixing and mass transfer efficiency were significantly enhanced,when the agitation speed increased from 400 r/min to 500 r/min of RT+E condition,and correspondingly,the curdlan yield is increased about 13%.In terms of RT+2WHD mode,although the agitation speed increased from 550 r/min to 650 r/min,it remained at oxygen limiting condition the final curdlan yield could increase about 9.6%.Finally,on the basis of former results,the curdlan fermentation process adopting RT+E mode to improving mixing and mass transfer efficiency with glucose as substrate was further optimized.The glucose continuous feeding strategy to maintain the concentration not higher than 20 g/L could alleviate inhibition by the high glucose concentration,the final curdlan yield improved approximate 44.4%compared with that of batch fermentation.It was believed that this work would be available to provide a theoretical basis for further industrial production.