When the new stirrer was installedand used in a 5-L fermenter, P. ostreatus grew better thanwith the Rushton turbine. Glucose consumption was rela-tively faster, and the maximum dry weight of myceliumwas increased by 47 % to 7.4 g/L relative to the Rushton turbine. The maximum laccase activity is also increased by15 % up to 2,277 U/L.Analysis of the viscosityThe viscosity of the fermentation broth has some impact onthe metabolism of fermentation products. In most cases, ahigh viscosity of fermentation broth is detrimental to theproduction process of target metabolites from microor-ganism. As shown in Figs. 5 and 6, the viscosity changesare corresponding to the accelerated growth. The viscosityof the culture medium was increased exponentially duringthe cultivation time from 25 to 125 h. At the same time, thecell growth rate was also increased exponentially. There-fore, the relationship between cell growth and viscosityvalues is very regular. However, compared to the Rushtonturbine, the new stirrer can reduce the shear stress onbiological cells, which is beneficial to the accumulationand secretion of laccase, but is disadvantageous to accu-mulation and secretion of exopolysaccharides. Therefore,the viscosity of the fermentation broth of the new paddlewas lower than the conventional paddle throughout theprocess. In the logarithmic growth phase, the viscosity wasslightly higher, which may be due to the rapid myceliumgrowth. The lower viscosity of the fermentation promotesthe transfer of dissolved oxygen and complete mixing ofnutrients, all of which facilitate cell growth, and lead tomore products. Therefore, when the cellular activity andbiomass concentrations were getting similar in two fer-mentation runs, the viscosity courses were also different inthe late fermentation phase.Analysis of the pHIn general, the fermentation broth pH gradually increasedfor the secretion of alkaline metabolites. The pH gap isFig. 6 Time profiles of vsicosity during fermentation with differentstirrers. Rushton turbine (open triangle), straight diagonal-pitchedblade stirrer (closed triangle) small at the beginning of fermentation. In the fermentationprocess with the new impeller, the fermentation broth pHgradually increased and reached a stable value (Fig. 7),which was different from pH profile in the fermentationprocess using the Rushton turbine. When using the Rushtonturbine, at the latter stages of fermentation process, the pHdeclined and the mycelia began to autolyse. The mainreason for different pH profiles is the fact that myceliummorphologies are very different in two fermentation runs.Mycelium morphologies are mainly affected by the shearstress of stirrer. Compared to the Rushton turbine, the newstirrer can reduce the shear stress on biological cells, whichis beneficial to the intracellular accumulation of metabo-lites. At the same time, the lower shear stress cannotfacilitate the extracellular secretion of metabolites in shortperiod and change the fermentation broth pH greatly andrapidly. This is also the reason that the fermentation brothpH gradually increased and reached a stable value in thefermentation process with the new impeller.ConclusionsIn this work, a straight diagonal-pitched blade stirrer wasdesigned, built and characterized in a 5-L fermenter. At thesame time, the new stirrer was installed in a 5-L bioreactorand evaluated in submerged fermentation of the ediblefungus Pleurotus ostreatus. The results showed that,comparing with submerged cultivation using the Rushtonturbine, the maximum dry weight of mycelium is increasedby 47 % and reached 7.47 g/L, and the maximum laccaseactivity is increased by 15 % up to 2,277 U/L. Glucoseconsumption was also found to be relatively faster. Finally, the power consumption of this new stirrer is 2.8 % lowerthan that of the Rushton turbine. In submerged fermenta-tion of higher fungi, the straight diagonal-pitched bladestirrer should improve the production of mycelia andmetabolites and reduce production costs.
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