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A multimeter was used to determinevoltage and current under stirring conditions with andwithout ventilation at room temperature. According to theUnited States patent application (20110116342), which isentitled ‘‘method for scaling mixing operations’’, formulaEq. (1) was used to calculate the mixing power:Po ¼ 1:732IV ð1ÞThe power number is calculated as:Np ¼ PoqN2D5ð2ÞThe units of these parameters are as follows: Po (W), q(kg/m3), N (r/s), D (m). Oxygen transfer coefficient KLaThe stability of the agitation speed and aeration rate wasmaintained in the determination of oxygen transfer. TheDO concentration was recorded using an appropriateelectrode. The response time of electrode (sr) ranges gen-erally around 5 s. Briefly, the N2 was first injected into thebioreactor and then a certain rate of air was injected toreplace the N2 when DO had dropped to below 5 %. Thechange of DO was recorded every 10 s until the value wasstable. KLa can be calculated using the changes in thedissolved oxygen concentration curve according to thefollowing formula.lnC L CLC L C0¼ KLaðt t0Þð3ÞSeed culturePleurotus ostreatus 9506 was purchased from the ChineseAcademy of Agricultural Sciences. Pleurotus ostreatus9506 was initially grown on PDA medium in a tube for7 days at 28 C, and then transferred to the seed culturemedium with a sterilized inoculating loop. The seed culturewas grown in a 250-ml flask containing 50 ml of fermen-tation medium (g L-1) 15.2 glucose, 9.0 yeast extract, 0.03CuSO4 5H2O, 0.3 MgSO4, 0.3 (NH4)2SO4, 0.06 CaCO3,and 10 ml trace elements (0.1 % FeSO4, 0.09 % ZnSO4,0.02 % MnSO4)at28 C on a rotary shaker incubator at140 rpm for 7 days.Stirred-tank bioreactor and fermentation processThe fermentation medium was inoculated with 2 % (v/v) ofthe seed culture and then cultivated at 28 Cina5Lstirred-tank reactor (Biotech, Shanghai, China) equippedwith pH and oxygen electrodes (sr, 5 s).
Unless otherwisespecified, according to our previous study, fermentationswere performed under the following conditions: tempera-ture, 28 C; aeration rate, 4 vvm; agitation speed, 300 rpm;pH natural; inoculum size, 2.5 %; working volume, 3 L.The seed culture was transferred to the fermentationmedium and was cultivated for 8 days. All experimentswere performed at least in duplicate. In batch experiments, 100-ml samples were taken at 24-hintervals and the fermentation parameters (pH, DO,agitation speed) were also recorded. Mycelium dryweight was measured after repeated washing of the cellpellets with distilled water and drying overnight at90 C. The filtrate from membrane filtration was ana-lyzed quantitatively for residual glucose concentrationusing D-Glucose Assay Kit (Equl-Megazyme, Ireland)according to the manufacturer’s instructions. The laccaseactivity of Pleurotus ostreatus was monitored at 25 Cusing ABTS as substrate. Laccase activity in media wasdetermined quantitatively by measuring the maximumoxidation of ABTS at 420 nm after 1 min. The reactionmixture (3 ml) contained 0.1 ml of enzyme solution,0.4 ml of 1 mmol/L ABTS, and 2.5 ml sodium acetatebuffer solution (pH 4.5). One unit of enzyme activitywas defined as the amount of enzyme catalyzing theoxidation of 1 lmolofsubstrateperminute.Thefiltratekinematic viscosity was determined using the Ubbelohdeviscometer, and l was calculated according to the fol-lowing formula:l ¼ c q ð4ÞThe units of these parameters are as follows: l(10-6Pa s), q (kg/m3), c (mm2/s).The value for the shear stress can be interpreted ascmax / pDN or (cave / N). Here ut has been calculatedusing p, N and D, and the formulaut;1 ¼ pND ð5Þand is close to cmax.The blade tip speed of the new stirrer can be calculatedusing the formula:ut;2 ¼ pND sin45 ð6ÞThe flow pattern changes in the mixing vessel can becharacterized by the Re, which is calculated from N, D, qand l using the formulaRe ¼ ND2qlð7ÞThe flow patterns in a bioreactor with baffles can beclassified into two types depending on the value of Re, oneof which is laminar flow (Rei10) and the other turbulentmixing (Rei104). The mixing power is related to the flowpattern and the structure of the reactor and it increases withshear stress. Therefore, the total power consumption can becalculated as the following formula by determining themotor current [10].W ¼ZIVcosU dt ð8Þ Results and discussionMixing power characteristicsThe power input of the two types of agitator was measuredunder non-aerated conditions. As shown in Fig. 2, thestirring power increased with stirring speed but the powerconsumption of the straight diagonal-pitched blade stirreris lower than that of the Rushton turbine at a given speed.The power number decreased with increase of Re. Theresults showed that the power number of this new impelleris lower than that of the Rushton turbine at low Re,