【正文】 the shortest time lag, the fastest growth ocurred at the lowest stirrer speed. (Fig. 4A).Neither the ammonium uptake, nor the phosphate uptake was higher at high stirrer speeds, than atlower ones. The sugar uptake was considerably lower at 750 rpm, than at 500 or 300 rpm. (Fig. 4B). The xylanase activity was the highest at 300 rpm,and with increasing stirrer speed, it decreased (Fig4C). The low xylanase activity at high stirrer speed can be explained by the less intimate contact between the fungal mycelium and the wheat bran... Culti6ations in the airlift tower loop reactor，The runs in the airlift tower loop reactor were performed under following conditions: The preculture was incubated at 150 rpm, and 30176。C for 10 h in plex medium. The culture medium in the reactor was inoculated with 5 104 spores ml ? 1, and cultivated at 25176。C, pH and vvm. The speci?c power input and the shear stress on the cells were low in this reactor, in parison with those in the stirred tank reactor. The?lamentous mycelium formed was partly freely suspended in the medium, partly attached to the wheat bran particles. With increasing cell mass concentration, the viscosity of the cultivation medium increased considerably. In spite of that,the supply of the cells with oxygen was satisfactory, as can be seen in Fig. 5A, which shows the courses of the relative dissolved oxygen concentrations pO2 (with respect to the saturation) in the riser and downer during the batch growth rate, the reducing sugars and glucose uptake rates and xylanase production rate were in this reactor considerably higher than in the stirredtank reactor (Fig. 5B, C). After the glucose was used up, the xylanase activity quickly increased and attained a constant and very high value already at 100 h. The phosphate and ammonium concentrations decreased during the ?rst 40 h,then between 40 and 60 h they increased anddecreased and at 70 h (phosphate) and 80 h(ammonium), respectively, then they increasedagain (Fig. 5D, E). The maxima of the phosphateand ammonium concentrations between 40 and 80h could not be observed in the stirred tank reactor. The investigations were performed in the airlifttower loop reactor with ground wheat bran, under the same cultivation conditions as used withnon ground wheat bran. Filamentous myceliumwas formed, but in contrary to the non groundwheat bran run, the wheat bran fragments werepletely covered by the mycelium. They partlyformed clumps and pellets. This ?lamentous network increased the medium viscosity considerably, which caused a reduction of the mediumcirculation and the dissolved oxygen concentration (Because of the cell growth on the membrane of the oxygen electrode, the exact measurement ofpO2 was not possible). A parison of the runs with non ground andground wheat brans indicated, that with the latterthe growth rate and CPR were much lower (Fig.6A), but the release of reducing sugars and glucose and their consumption rate differed onlyslightly (Fig. 6B, C). However, the xylanase activity was much higher with nonground wheat branthan with ground one (Fig. 6D).4. Discussion. Fungal morphology and product formation inpresence of solid substratesSeveral papers were published on the investigation of the fungal morphology (Paul et al., 1994。Packer et al., 1992a,b), however, no informationwas presented on, how the solid substrates in?uence the fungal morphology and the productivity.The present investigations reveal, that the in?uence of the solid substrate on the fungal morphology depends on various factors, as speci?c powerinput, size of the particles, phosphate concentration. At low speci?c power input, pellets wereformed in stirred tank reactors, and ?lamentousmycelium in airlift tower loop reactors. With increasing speci?c power input in the stirred tankreactor the pellet size decreased, and as long asthe intimate contact between the fungus and thesolid substrate was not impaired, the productivityimproved. However, at too high speci?c powerinput the shear forces separated the fungus fromthe solid substrate, which caused a decrease of theproductivity. If the solid substrate is large enough(. 5–6 mm), it can protect the fungus fromshear stress, when the fungus can grow inside ofthe solid substrate. By grinding, the solid substrate can bee too small, to protect the fungus. This accelerates the growth, but has anadverse effect on the productivity. In reactorswith low speci?c power input, small solid substrate particles can cause the formation of a?lamentous network with high viscosity and wallgrowth, which has a detrimental effect on thegrowth and product formation. At high phos phate concentrations in the airlift tower loop reactor large hollow pellets were formed in syntheticmedium (Siedenberg et al., 1997b). This effect isexpected in plex medium as well. Large hollow pellets have a negative effect on the attachment of the fungus on the solid substrate and thuson the enzyme production.D. Siedenberg et al. / Journal of Biotechnology 56 (1997) 205–216Fig. 5. Batch cultivation in airlift tower loop reactor. (A) Dissolved oxygen concentrations。 (B) CPR, reducing sugar concentrationsand xylanase activities。 (C) CPR, glucose concentrations and xylanase activities。 (D) Orthophosphate concentrations。(E) Ammonium concentrations. D. Siedenberg et al. / Journal of Biotechnology 56 (1997) 205–216 Airlift tower loop reactors can only be operatedat low speci?c power input at which the oxygentransfer has a high ef?ciency with respect to thepower input. However, their optimal operationtank reactors. Diluted cultivation medium has tobe used, to reduce the cel