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A novel stirrer design and its application in submergedfermentation of the edible fungus Pleurotus ostreatusAbstract In this study, a straight diagonal-pitched bladestirrer was designed, built and characterized in a 5-L fer-menter. Compared with the six straight blade Rushtonturbine, the power consumption of the new stirrer is lowerat a given speed under conditions of no ventilation. 30579
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Theoxygen transference is poorer at the same agitation speed inthe cultivation conditions and scales investigated, whichconfirms that the shear stress of the new stirrer is lower andthe gas dispersion is weaker. The new stirrer was installedin a 5-L bioreactor and evaluated in submerged fermenta-tion of the edible fungus Pleurotus ostreatus. The resultsshowed that the maximum dry weight of mycelium isincreased by 47 % and reached 7.47 g/L, and the maxi-mum laccase activity is increased by 15 % up to 2,277 U/L. Glucose consumption was also found to be relativelyfaster. The power consumption is 2.8 % lower than that ofthe Rushton turbine.Keywords Growth rate Laccase Pleurotus ostreatus Six straight blade Rushton turbine Straight diagonal-pitched blade stirrerAbbreviationsRushton turbine Six straight blade Rushton turbineDO Dissolved oxygenPo The mixing powerQ Medium densityN The stirring speedNp The power numberD The agitator diameterKLa Volumetric mass transfer coefficientCL* Saturated oxygen concentrationCL Dissolved oxygen concentration at time(t)C0 Initial dissolved oxygen concentrationl The apparent viscosityc The kinematic viscosityut The impeller tip speedcmax The maximal shear stresscave The average shear stressRe Reynolds numberV VoltageI CurrentABTS 2,2-azinobis-3-ethylbenzthiazoline-6-sulfonatePDA Potato dextrose agarMDW Mycelium dry weightIntroductionThe stirrer, capable of dispersing gas in a broth therein, isan important component of the bioreactors that are widelyused in the submerged fermentation of higher fungi [1].The Rushton turbine, which is provided on a verticallyextending shaft in bioreactors, has radial dispersion, andthe disc prevents the dispersion of overflowing liquid bythe gas. It also has a simple structure and is easily manu-factured. Therefore, most bioreactors are usually equippedwith a Rushton turbine to meet the production requirementsof the fermentation process [2]. However, there are somedrawbacks of this stirring apparatus, such as its high powerconsumption, destructive effects of high shear stress, andlow mixing efficiency, which have become increasinglyevident. Milly et al. found that it is easy to form cavitationsin the rear of the high-speed rotating blades, which not onlyreduces the mixing efficiency, but also reduces the gas–liquid mass transfer capacity [3]. Therefore, the design anddevelopment of a new stirrer with low shear stress, lowpower consumption, and good mixing is of significantinterest.Some researchers found that curved blades were supe-rior to straight blades in the mixing process. Many equip-ment companies have therefore designed and developedcurved blade stirrers such as the semicircular tube CD-6stirrer, the deep depression ICI stirrer, and the curvedasymmetric blade BT-6 stirrer [4]. Experiments haveconfirmed that these new stirrers reduce power consump-tion and improve the gas–liquid mass transfer rate. Xiaet al. [5] found that the flow field generated by the com-bination of a six arrow leaf disk turbine paddle in thebottom and a three pitched blade axial flow propeller canprovide a good mix of high oxygen transfer rate andmoderate shear environment in a 50-L bioreactor, signifi-cantly increasing the yield of fermentation products.
Wanget al. [6, 7] also designed a new type of centrifugal stirrer,which produces a circular flow in the entire mixing tank,shortens the mixing time compared with traditional diskturbine paddles, increases liquid lift capacity and has asmall shear stress [8]. All of these are very suitable forsubmerged fermentation of microbial systems [9].In the present study, a straight diagonal-pitched bladestirrer with low power consumption, small shear stress, andgood mixing performance was designed and built. Thiscombines the advantages of the radial flow and axial flowstirrers [6, 7]. Tests were conducted with the apparatus, inthe course of which the complex mixing system—inrespect of the characteristic hydromechanical parameters,time of homogenization, dispersion capacity and ‘‘hold up’’of the gas—was found to be more favorable as comparedwith the traditional Rushton turbomixers. This new stirrernot only reduced energy consumption in the productionprocess, but also improved the efficiency of mass transferand reduced the shear stress on biological cells, therebyincreasing the production of microbial metabolites. At thesame time, a bench scale fungi culture model representa-tive of manufacturing scale was also developed based onoxygen mass transfer principles. Furthermore, this finalbench scale model generated similar growth and metabolicprofiles as manufacturing, which also demonstrated thevalidity of this straight diagonal-pitched blade stirrer. Weexpect that this new stirrer will replace some or all radialflow stirrers such as A315, HPM, and Maxflo stirrers. Theresearch will also provide a reference for other new stirrerdesigns.Materials and methodsDesign of the straight diagonal-pitched blade stirrerThe straight diagonal-pitched blade stirrer is shown inFig. 1, which was designed and processed after consider-ation of power consumption, shear stress, mixing effects,and biological characteristics of higher fungi.The new stirrer has a simple structure, low shear, lowpower consumption, and high efficiency mixing charac-teristics. The structural characteristics of the new stirrer areas follows:1. The three wheels of the stirrer are co-linear for easyinstallation;2. The top and bottom of the paddle are tilted 45 to theright and the upper and lower paddles are vertical. Thisreduces the potential for direct collision of mixingblades and reduces the cell area by half, significantlyreducing damage to the fungal mycelium. It alsopromotes uniform mixing of the entire tank andprevents material splashing;3. The paddles are welded together in the middle of thewheel and their other ends are welded to the edges ofthe upper and lower leaves. This reduces shear stressand collisions with the mycelium. The middle twopaddles are vertical and at 90 to each other. Thisgenerates simultaneous axial and radial flow andimproves mass transfer effects [4].The mixing powerExperiments were performed in a 5-L stirred-tank biore-actor using water and air as media at the appropriate stir-ring speed. The parameters of vessel in this bioreactorequipped with three baffles are summarized as follows: theinner diameter of vessel is 16 cm; the height of vessel is25 cm; the cylinder volume is 5 L; the gross volume is5.8 L; the impeller diameter is 5.6 cm; the stirring shaftdiameter is 1.25 cm.