Properties of bacterial cellulose produced in a pilot-scale spherical type bubble column bioreactor

The saccharogenic liquid obtained by the enzymatic saccharification of food wastes was used as a medium for production of bacterial cellulose (BC). The enzymatic saccharification of food wastes (SFW) was carried out by the cultivation supernatant of Trichoderma inhamatum KSJ1. 5.6 g/L of BC was produced in a new modified 50 L bubble column bioreactor by Acetobacter xylinum KJ1. The productivity was similar to that of a modified 10 L bubble column bioreactor (5.8 g/L). When pure oxygen was supplied into the scaled-up culture conditions, 6.8 g/L (12% enhancement) of BC was produced, indicating a very useful method for BC mass production. The oxygen uptake rate (OUR) and q _O2 (specific oxygen uptake rate) were 0.214 mg-DO/L·min and 0.257 mg-DO/g-cell·min, respectively. The physical properties, such as morphology, molecular weight, crystallinity, and tensile strength of BCs produced in static culture (A), 10 L (B) and 50 L (C) modified bubble column cultures were investigated. All BCs showed fibrils with highly networking structure. The number average molecular weight of BCs in A, B and C was 2,314,000, 1,878,000, and 1,765,000, respectively. All of the BCs had a form of cellulose I representing pure cellulose. The relative degree of crystallinity showed the range of 79.6–86.0%. Tensile strengths of BC sheet in A, B and C were 1.75, 1.21, and 1.19 kg/mm², respectively. In conclusion, BC production by the modified bubble column culture mode of 50 L brought more favorable results in terms of the physical properties and its ease of scale-up.     

Determination of the stoichiometry and critical oxygen tension in the production culture of bacterial cellulose using saccharified food wastes

The stoichiometry of the entire reaction in a 50 L scaled-up production culture of bacterial cellulose (BC), using saccharified food wastes (SFW), was analyzed in this study. The stoichiometric analysis was carried out using the chemical formula, yield, degrees of reduction of the major components, and the respiratory quotient (RQ). Based on the stoichiometric analysis, the amounts of substrate, oxygen supply and BC production etc., were able to be predicted. In addition, the amount of energy generated in the culture was predicted based on the oxygen consumption via the stoichiometric analysis. The stoichiometry of BC production using SFW in a 50 L large scale reactor will be useful as a standard for mass production of the culture. The stoichiometric analysis can also help the designers of reactors decide on the boiler capacity and oxygen supply for a large scale bioreactor system. The OUR (oxygen uptake rate) of Acetobacter xylinum KJ1 in a 12 hour-age cultivation was 0.21 mg DO/L·min, from which the critical DO concentration was suggested to be maintained above 3.10 ppm to prevent oxygen limitation during the BC production culture. The results indicated that pure oxygen should be supplied during the exponential phase, where DO depletion was observed. An ascertainment experiment, with the addition of pure oxygen into the culture system, showed BC production of 7.37 g/L, which was considerable productivity.     

Optimization of citric acid production by <Emphasis Type="Italic">Aspergillus niger</Emphasis> NRRL 567 grown in a column bioreactor

Citric acid production using Aspergillus niger NRRL 567 grown on peat moss has been optimized in a column bioreactor using a statistically based method. A 23 full factorial design with eight fermentation conditions was applied to evaluate significance on citric acid production and their interactions between variables, where the three independent variables evaluated were aeration rate, bed depth and temperature. Aeration rate and fermentation temperature were identified to be significant variables. Citric acid production markedly increases with aeration rate and fermentation temperature; however, the bed depth of solid substrate showed an insignificant effect on citric acid production. The optimum fermentation condition for citric acid production in a column bioreactor consisted of aeration rate of 0.84 vvm, bed depth of 22 cm and fermentation temperature of 32 °C. Under a given condition, a maximum citric acid production of 120.6 g/l was predicted and matched well with the experimental value of 123.9 g/kg.     

Scaling up the Bioconversion of Cheese Whey Permeate into Fungal Oil by Mucor circinelloides

The bioconversion of hydrolyzed whey permeate into an oil-rich biomass by Mucor circinelloides was scaled up from 250 mL to 4 L with the use of an aerated stirred tank bioreactor. Biomass production and oil accumulation were strongly influenced by agitation speed (99–451 rpm) and aeration rate (0.29–1.70 vvm). Higher agitation and aeration rates (e.g., >400 rpm, >1 vvm), resulted in significantly higher biomass yield due to increased oxygen transfer capabilities and better mixing. Additionally, oil accumulation in the fungal biomass was related to high agitation (>400 rpm), while aeration (0.5–1.5 vvm) had no significant effect within the range evaluated. The predictive model was validated at the optimal conditions of 450 rpm and 1 vvm. Maximum biomass yield of 10.7 g L⁻¹ and lipid content of 32% dry biomass were achieved during 120 hours of fermentation. Simultaneous optimization of agitation and aeration in a bioreactor was found to not only improve fungal growth but also lipid content (24% vs. 32%), lipid yield (2.2 vs. 3.1 g L⁻¹) and γ-linoleic production (73–464 mg L⁻¹) compared to that of shake-flask. This study resulted in a scaled-up and optimized fermentation process that increased production of M. circinelloides biomass for subsequent use as raw material for food, feed, and fuel applications. This signifies a starting point for further studies aimed at assessing the development of a fully functioning fungi-to-food/fuel system on an industrial scale for several agricultural streams.     

Assessment of Relevant Factors Influencing Lipolytic Enzyme Production by Thermus thermophilus HB27 in Laboratory‐Scale Bioreactors

Nowadays, there is not much information on the large‐scale production of thermostable lipolytic enzymes by thermophilic organisms. Therefore, in this study the lipolytic enzyme production by Thermus thermophilus HB27 in laboratory‐scale bioreactors was undertaken. In order to determine the most suitable bioreactor, two configurations were investigated: stirred‐tank and airlift bioreactor. It was demonstrated that the stirred‐tank configuration led to the highest lipolytic extracellular activities, about 2.3‐fold higher than those found in the corresponding cultivation in the airlift bioreactor. On the other hand, the influence of several factors such as culture nutrients concentration, aeration, and agitation rate on the production of thermophilic lipolytic enzymes in a 5‐L stirred‐tank bioreactor was assayed. At reduced nutrients concentration (50 % with respect to the basal medium), a higher product/biomass yield was attained, without any operational problems. From the relationship between mass transfer coefficient (K_La), aeration, and agitation rates it was concluded that there is a lesser dependence on the aeration than the agitation rate. In addition, the mechanical stirring of the bioreactor could tear the membranes that contain the rotund bodies often found in cultures of thermophilic microorganisms, thus increasing the extracellular enzyme production.     

Determination of agitation and aeration conditions for scale-up of cellulolytic enzymes production by <Emphasis Type="Italic">Trichoderma inhamatum</Emphasis> KSJ1

10 to 35 L jar fermentation scale-up cultures were performed to determine the optimum agitation and aeration rates in the cellulolytic enzymes production culture by Trichoderma inhamatum KSJ1. The optimum agitation rate in the 35 L jar fermenter was provisionally determined to be 150 rpm by using a geometrically resembled scale up method from the 10 L jar fermenter. The optimum aeration rate was determined to be 0.5 vvm by applying the mean values of superficial velocity and vvm. The DO (Dissolved Oxygen) concentration of the culture liquid was maintained below the critical DO concentration (2.336 mg/L) at 150 rpm in the 35 L jar fermenter. To increase the DO above the critical DO concentration, the agitation rate was increased from 150 to 200 rpm, with the aeration rate maintained at 0.5 vvm. As a result, the DO was maintained above critical DO concentration. The OUR (Oxygen Uptake Rate) and k _L a values were 0.91 mg-DO/L·min and 11.1 hr⁻¹, respectively. The amylase and FPase (filter paper activity) activities were 4.48 and 0.74 U/mL, respectively, in the 35 L jar fermenter, which was comparable to that in the 10 L fermenter (4.2 and 0.5 U/mL, respectively). Therefore, the scale-up conditions, 0.5 vvm and 200 rpm, were concluded to be the optimum aeration and agitation rates in the 35 L jar fermenter.     

Production of antifungal lipopeptide from <Emphasis Type="Italic">Bacillus subtilis</Emphasis> in submerged fermentation using shake flask and fermentor

Optimization of the production of antifungal lipopeptide, iturin A, by B. subtilis was carried out in submerged cultural fermentation. In a shake-flask experiment, response surface methodology (RSM) was employed to optimize the cultivation conditions of Bacillus subtilis S3 for the enhancement of iturin A production. The optimal conditions for iturin A production obtained from RSM were pH 6.0, 0.93% maltodextrin, 1.11% glucose, 0.72% corn steep powder (C.S.P), 1.5 mM MgSO₄, 0.75 mM KH₂PO₄, rotation speed 180 rpm and area of aeration 4.35 cm². 57% increase of iturin A productivity (from 47.19 mg/L to 74.22 mg/L) was observed using the one-factor-at -a-time technique; however, 180% increase of iturin A productivity (from 47.19 mg/L to 132.23 mg/L) was observed with RSM. The iturin A production was further studied in a 5-L fermentor with a variation of agitation speed, aeration and baffles. In the 5-L fermentor, it was found that increased agitation speed improved the growth of B. subtilis and the production of iturin A. Aeration at 2 vvm gave excellent production of iturin A (175.52 mg/L) at 6 d of fermentation. The addition of baffles in the fermentor has significantly influenced the oxygen mass transfer coefficient (KLa) and iturin A production.     

Batch cultivation of kluyveromyces fragilis in cheese whey

Kluyveromyces fragilis was cultivated batchwise in an open pond rectangular bioreactor at 30°C, 2vvm of aeration, under non-sterile conditions and uncontrolled pH. The culture medium contained 7% cheese whey powder, 0.25% KH₂PO₄, 0.5% (NH₄)₂SO₄ and was adjusted to an initial pH of 4.0 with phosphoric acid. The lactose was almost completely consumed after 16 h and COD reduction attained 80% after 64 h. The maximum suspended solids concentration obtained was 11.7 g/L. The cheese whey which had initially low protein and high lactose contents was converted by this system into a high protein and low lactose carbohydrate product.     

Influence of medium composition on oxygen transfer rate in animal cell culture

Experiments were conducted in a 0.25 m diameter bubble column to investigate the effect of medium composition on oxygen transfer rate. Aqueous solutions, the composition of which mimics a mammalian cell culture medium, are used. The effect on oxygen transfer rate of additives used to protect cells against local hydrodynamic stresses induced by bubble coalescence and bursting is addressed, in the range of operating conditions (aeration rates) met in animal cell cultures. The mass transfer coefficient, the liquid viscosity and surface tension, and the bubble size distribution are measured as a function of liquid composition and of gas superficial velocity, allowing to decouple the effects of the different additive on k_L and on a.     

Comparison of photobioreactors and optimal light regime for rapid vegetative propagation of transgenic Undaria pinnatifida gametophytes

BACKGROUND: Previously, tachyplesin gene (tac) has been successfully transferred into Undaria pinnatifida gametophytes using the method of microprojectile bombardment transformation. The objectives of this study were to compare and evaluate the performance of bubble‐column and airlift bioreactors to determine a preferred configuration of bioreactor for vegetative propagation of transgenic U. pinnatifida gametophytes, and to then investigate the influence of light on vegetative propagation of these gametophytes, including incident light intensity, photoperiod and light quality to resolve the problems of rapid vegetative propagation within the selected bioreactor. RESULTS: Experimental results showed that final dry cell density in the airlift bioreactor was 12.7% higher than that in the bubble‐column bioreactor under the optimal aeration rate of 1.2 L air min^?1 L^?1 culture. And a maximum final dry cell density of 2830 mg L^?1 was obtained within the airlift bioreactor using blue light at 40 µmol m^?2 s^?1 with a light/dark cycle of 14/10 (h). Polymerase chain reaction (PCR) analysis indicated that genes (bar and tac) were not lost during rapid vegetative propagation within the airlift bioreactor. CONCLUSION: The airlift bioreactor was shown to be much more suitable for rapid vegetative propagation of transgenic U. pinnatifida gametophytes than the bubble‐column bioreactor in the laboratory. The use of blue light allows improvement of vegetative propagation of transgenic U. pinnatifida gametophytes. Copyright © 2009 Society of Chemical Industry     

Biodiesel production from Stichococcus strains at laboratory scale

BACKGROUND: This paper reports the results of an experimental campaign of autotrophic cultures of Stichococcus strains aiming at selecting the most promising strain for biofuel production. The strain selected—S. bacillaris 158/11—was cultivated in 1 L lab‐scale bubble column photobioreactors under fed‐batch and semi‐continuous conditions. A Bold basal medium supplemented with NaNO₃ as nitrogen source was adopted. Tests were carried out at 23 °C, 140 µE m^?2 s^?1, and air flow rate ranging between 0.4 and 4 vvm. Cultures were characterized in terms of pH, concentration of total nitrogen, total organic carbon, total inorganic carbon, biomass, lipid fraction and methyl‐ester distribution of transesterified lipids. RESULTS: S. bacillaris 158/11 proved to be the best strain to produce biodiesel. Methyl‐ester distribution was characterized by a large fraction of methyl palmitate, methyl linolenate, methyl linoleate, and methyl oleate along with phytol. The process photosynthetic efficiency—fraction of available light stored as chemical energy ‐ was about 1.5%. Specific biomass productivity was ～60 mg_DM L^?1 day^?1 under the semi‐continuous conditions tested. Total lipid productivity was 14 mg L^?1 day^?1 at a dilution rate of 0.050 L day^?1. CONCLUSION: S. bacillaris 158/11 is a potential strain for massive microalgae cultures for biofuel production. Higher biomass/total‐lipid productivity could be obtained in sunlight. Copyright © 2011 Society of Chemical Industry     

A study of mycelial growth and exopolysaccharide production from a submerged culture of <Emphasis Type="Italic">Mycoleptodonoides aitchisonii</Emphasis> in an air-lift bioreactor

We determined the optimal culture and medium conditions for effective production of mycelial mass and exopolysaccharide from a liquid culture of Mycoleptodonoides aitchisonii in an air-lift bioreactor. The mycelial growth and exopolysaccharide production were found to be optimal at a temperature of 25 °C and pH of 6.5. When 60 g/L of lactose was used as a carbon source, the maximum mycelial growth and exopolysaccharide production were obtained. The polypeptone and yeast extract were the most appropriate nitrogen sources for mycelial growth and exopolysaccharide production. In addition, when a mixture of 20 g/L of polypeptone and 5 g/L of yeast extract was used, the exopolysaccharide production increased 50% compared to that of the sole nitrogen source. CaCl₂·2H₂O (1.0 g/L) was the most effective mineral source. Using the optimal culture and medium conditions, batch cultures with basal and designed medium on mycelial growth and exopolysaccharide production in a 5 L air-lift bioreactor were carried out for 16 days. The mycelial growth and exopolysaccharide production increased with an increase of culture time at 14 days, and the maximum mycelial growth and exopolysaccharide production were 20.3 and 6.2 g/L, respectively, after 14 days of culture. The developed model in an air-lift bioreactor showed good agreement with experimental data. These results indicate that exopolysaccharide production is associated with the mycelial growth of M. aitchisonii in an air-lift bioreactor.     

Effect of viscosity-inducing factors on oxygen transfer in production culture of bacterial cellulose

Bacterial cellulose (BC) production culture requires high oxygen transfer rate (representatively k _L a) at a low shear force. Considering that oxygen exhaustion is observed at the latter half of the exponential growth phase where BC production actually begins, it is highly probable that the drastic reduction of k _L a (oxygen volumetric transfer coefficient) is caused by the drastic increase of the soluble and insoluble viscous materials. Therefore, we examined the apparent viscosity-inducing materials generated during BC culture and investigated their effects on k _L a. Using the saccharified liquid from food waste as the culture medium, we discussed the relationship between the concentration of the generated solid matters, especially BC and the viscosity, the relationship between the BC concentration and k _L a, and the correlation between the viscosity and k _L a. The relationship between the solid matter (BC), which is the insoluble viscosity-inducing material, and k _L a showed that the BC concentration and k _L a were in exponentially reciprocal proportion with the linear regression equation. In case of using agar as the soluble viscosity-inducing material, the correlation between the viscosity and k _L a showed that the viscosity depending on the agar concentration was in exponentially reciprocal proportion with k _L a in both tap water and the saccharified liquid medium. The results indicated that the effect of the BC concentration on k _L a was not great in the saccharified liquid medium. As the agar concentration increased in tap water and the saccharified liquid medium, the viscosity was increased and k _L a was decreased gradually, showing a linear relation between the logarithm of the viscosity as agar and k _L a. In conclusion, the effect of the soluble viscosity on k _L a was greater than that of solid matter (BC). Also, it was suggested that the soluble viscosity-inducing matters like agar were rather more effective than k _L a in BC production.     

Comparison of lipase production by Geotrichum candidum in stirring and airlift fermenters

The production of lipase by Geotrichum candidum in both, stirred tank and airlift bioreactors were compared. G candidum an imperfect filamentous fungus, grows well in liquid medium, and produces a lipase with specific affinity for long‐chain fatty acids with cis‐9 double bonds but, lipase production is generally not efficient because the optimum medium composition and fermentation conditions are not known. Response surface methodology was used to optimize the agitation speed (100–500 rpm) and aeration (0.2–1.8 vvm) for production of lipase by G candidum in a bench‐scale stirred fermenter. A Central Composite Rotatable Design (CCRD) was used to optimize lipase activity and productivity. Lipase production in an airlift fermenter was also studied with aeration ranging from 1 to 3 vvm. A previously optimized culture medium containing 3.58% of peptone, 0.64% of soy oil and an initial pH of 7.0, was used in the experiments, incubating at 30°C. In the stirred reactor the optimum conditions of agitation and aeration for lipase production and productivity were 300 rpm and 1 vvm, leading to an activity of 20 U cm^?3 in 54 h of fermentation and 0.3900 (U cm^?3 h^?1) of productivity. The best aeration condition in the airlift fermenter was 2.5 vvm, which yielded similar lipase activity after 30 h of fermentation, resulting in a productivity of 0.6423 (U cm^?3 h^?1). In the absence of mechanical agitation similar lipase yields were achieved but in less time, resulting in productivity, about 60% greater than in a stirred fermenter; the lower energy demand for the same lipase yield offers economic advantages. Copyright © 2004 Society of Chemical Industry     

A comprehensive study of lipase production by Yarrowia lipolytica CECT 1240 (ATCC 18942): from shake flask to continuous bioreactor

BACKGROUND: Lipases are commercially important enzymes, and the development and optimization of their production processes are of great interest. The diversity of behaviours between strains stresses the need for research on this topic, especially when bioreactor culture is considered. The study of a continuous operating mode is especially attractive, since very scarce information is available on its application to microbial lipases production. RESULTS: Lipase production in submerged cultures of Yarrowia lipolytica CECT 1240 (ATCC 18 942) has been investigated. Significant lipolytic activity (over 700 U dm^?3), mostly extracellular and membrane‐bound, was obtained in shake flasks using medium supplemented with olive oil. The culture was carried out in air‐lift and stirred tank bench‐scale bioreactors and the latter was selected. The influence of aeration and agitation rates was assessed in batch cultures, and agitation from 400–700 rpm and low aeration rates (i.e. 0.2 vvm) are recommended. Batch, fed‐batch and continuous operation were investigated, and regular enzyme production (up to 600 U dm^?3) was achieved with the latter. CONCLUSION: Lipase production by the selected strain was successfully carried out in shake flasks and bench‐scale bioreactors. After studying batch, fed‐batch and continuous processes, continuous culture in a stirred tank bioreactor was found best in terms of regular enzyme production, exceptionally good operational stability and good fitting of the results to mathematical models. Copyright © 2009 Society of Chemical Industry     

Improvement of Staphylococcus xylosus lipase production through optimization of the culture conditions

The Staphylococcus xylosus strain produces without induction an original lipase named S. xylosus lipase (SXL). Since considerable interest has been given to microbial lipases for biotechnology applications like detergents, food, drugs and pharmaceutical products, improvement of their production is of great importance to reduce the final cost. This goal could be reached through the optimization of several physicochemical culture conditions. Indeed, an appropriate medium was formulated for SXL production. It was composed of 17 g/L pancreatic digest of casein, 2.5 g/L glucose, 6 g/L yeast extract, 0.75 g/L ammonium sulfate corresponding to a C/N ratio of 6, 1 g/L K₂HPO₄ and 1 g/L KH₂PO₄. In such a medium, SXL production reached 42 U/mL. Moreover, the usefulness of such a medium for large‐scale production of SXL was also evidenced in an automated fully controlled 2.6‐L fermenter. It was shown that aeration of the medium, which strongly affected the growth, regulated the lipase synthesis by the produced cells. It was found that when using a dissolved oxygen saturation of the medium of 50%, the SXL production reached 62 U/mL.     

Immobilization of trophic anaerobic acetogen for semi-continuous syngas fermentation

The massive consumption of fossil energy forces people to find new sources of energy. Syngas fermentation has become a hot research field as its high potential in renewable energy production and sustainable development. In this study, trophic anaerobic acetogen Morella thermoacetica was successfully immobilized by calcium alginate embedding method. The ability of the immobilized cells on production of acetic acid through syngas fermentation was compared in both airlift and bubble column bioreactors. The bubble column bioreactor was selected as the better type of bioreactor. The production of acetic acid reached 32.3 g·L^-1 in bubble column bioreactor with a space-time yield of 2.13 g·L^-1·d^-1. The immobilized acetogen could be efficiently reused without significant lag period, even if exposed to air for a short time. A semi-continuous syngas fermentation was performed using immobilized cells, with an average space-time acetic acid yield of 3.20 g·L^-1·d^-1. After 30 days of fermentation, no significant decrease of the acetic acid production rate was observed.     

Effect of oxygen transfer rates on alcohols production by Candida guilliermondii cultivated on soybean hull hydrolysate

BACKGROUND: In this research the use of soybean hull hydrolysate (SHH) as substrate for xylitol and ethanol production using an osmotolerant strain of Candida guilliermondii was studied. The production of alcohols was investigated in batch cultivations in which the variable parameter was the volumetric oxygen mass transfer coefficient (k_La) obtained from three different conditions of air supply: anaerobic (150 rpm, no aeration); microaerobic (300 rpm, 1 vvm), and aerobic (600 rpm, 2 vvm), corresponding to k_La values of 0; 8; and 46 h^?1, respectively. RESULTS: SHH, although presenting a very high osmotic pressure (1413 mOsm kg^?1), was completely metabolized under aerobic conditions with high biomass productivities of 0.49 g cells (L h)^?1, with little formation of ethanol. Xylitol was produced under microaeration, with product yield of 0.22 g g^?1 xylose, with the formation of glycerol as a by‐product. No xylose was metabolized under anaerobic conditions, but ethanol was produced from hexoses with high product yields of 0.5 g g^?1. CONCLUSION: These results suggest that the hydrolysis of soybean hull and its conversion to ethanol and other alcohols could be an important use of this agro‐industrial waste, which could be used for biofuel, xylitol or biomass production, depending on the aeration conditions of the cultures. Copyright © 2008 Society of Chemical Industry     

Bubble shape,gas flow and gas–liquid mass transfer in pulp fibre suspensions

Gas–liquid mass transfer in pulp fibre suspensions in a batch‐operated bubble column is explained by observations of bubble size and shape made in a 2D column. Two pulp fibre suspensions (hardwood and softwood kraft) were studied over a range of suspension mass concentrations and gas flow rates. For a given gas flow rate, bubble size was found to increase as suspension concentration increased, moving from smaller spherical/elliptical bubbles to larger spherical‐capped/dimpled‐elliptical bubbles. At relatively low mass concentrations (C_m = 2–3% for the softwood and C_m ? 7% for the hardwood pulp) distinct bubbles were no longer observed in the suspension. Instead, a network of channels formed through which gas flowed. In the bubble column, the volumetric gas–liquid mass transfer rate, k_La, decreased with increasing suspension concentration. From the 2D studies, this occurred as bubble size and rise velocity increased, which would decrease overall bubble surface area and gas holdup in the column. A minimum in k_La occurred between C_m = 2% and 4% which depended on pulp type and was reached near the mass concentration where the flow channels first formed.     

Effect of the aeration rate on pullulan production and fermentation broth rheological properties in an airlift reactor

The effect of aeration rate on pullulan production and the rheological properties of the fermentation broth in an airlift reactor was investigated. An airlift fermenter was shown to be an appropriate fermentation system for the production of pullulan. A maximum pullulan concentration (30 kg m^?3), biomass concentration (6.0 kg m^?3), pullulan yield (60%, w/w) and sugar utilization (100%, w/w) was obtained at an aeration rate of 2 vvm. The mycelium and the yeast‐like cells were the morphological forms responsible for pullulan production. The highest polysaccharide concentration was obtained when the mycelial forms and the yeast‐like cells were 60% and 40% (w/w) of the total biomass, respectively. The apparent viscosity of the broth was increased with the increase of the aeration rate from 1 to 2 vvm and then decreased at higher vvms. On the other hand, the dissolved oxygen concentration and the volumetric mass transfer coefficient continually increased with the increase of the aeration rate. The mycelial forms and the production of extracellular polysaccharide were responsible for the non‐Newtonian flow behaviour of the fermentation broth. The rheological behaviour can be characterized by a power law type of equation. The relationship between shear rate/shear stress and shear rate/apparent viscosity showed a non‐Newtonian behaviour of the fermentation broth. © 2001 Society of Chemical Industry