Baffles increase performance of solid-state fermentation in rotating drum bioreactors

Summary Rhizopus oligosporus grew better on wheat bran in a rotating drum when baffles were fitted. The maximum O₂ uptake rates for the baffled and unbaffled runs were 9.0 and 5.7 mmol/min.kg initial dry substrate respectively. The RQ remained at 1.0 throughout the baffled run but varied between 1.0 and 1.2 for the unbaffled run.     

The potential for establishment of axial temperature profiles during solid-state fermentation in rotating drum bioreactors

The mixing and heat transfer phenomena within rotating drum bioreactors (RDBs) used for solid-state fermentation processes are poorly studied. The potential for the establishment of axial temperature gradients within the substrate bed was explored using a heat transfer model. For growth of Aspergillus oryzae on wheat bran within a 24 L RDB with air at a superficial velocity of 0.0023 m s(-1) and 15% relative humidity, the model predicts an axial gradient between the air inlet and outlet of 2 degrees C during rapid growth, compared to experimental axial temperature gradients of between 1 and 4 degrees C. Undesirably high temperatures occur throughout the bed under these operating conditions, but the model predicts that good temperature control can be achieved using humid air (90% relative humidity) at superficial velocities of 1 m s(-1) for a 204 L RDB. For a 2200 L RDB, good temperature control is predicted with superficial velocities as low as 0.4 m s(-1) with the airflow being switched from 90% to 15% relative humidity whenever the temperature at the outlet end of the drum exceeds the optimal temperature for growth. This work suggests that significant axial temperature gradients can arise in those RDBs that lack provision for axial mixing. It is therefore advisable to use angled lifters within RDBs to promote axial mixing.     

Characterizing mixing in a rotating drum bioreactor for solid-state fermentation

A method, based on the use of wheat bran particles dyed with Rhodamine-WT as tracer particles, was developed to characterize mixing in a 200 l rotating drum bioreactor used for solid state fermentation. The extraction process contributes a 15% relative error in determining Rhodamine concentrations. Extraction efficiency is not affected by autoclaving of the bran and there is no inter-particle transfer of dye during the mixing of bran within the drum. For an unbaffled drum rotated at 5 rpm the axial dispersion coefficient is 9.15 cm² min^–1.     

Mass transfer correlations for rotating drum bioreactors

Evaporative cooling is extremely important for large-scale operation of rotating drum bioreactors (RDBs). Outlet water vapour concentrations were measured for a RDB containing wet wheat bran with the aim of determining the mass transfer coefficient for evaporation from the bran bed to the headspace. Mass transfer was expressed as the mass transfer coefficient times the area for transfer per unit volume of void space in the drum. Values of ka' were determined under combinations of aeration superficial velocities ranging from 0.006 to 0.017 ms(-1) and rotation rates ranging from 0 to 9 rpm. Mass transfer coefficients were evaluated using a variety of residence time distributions (RTDs) for flow in the gas phase including plug flow and well-mixed and a Central Jet RTD based on RTD studies. If plug flow is assumed, the degree of holdup at low effective Peclet (Pe(eff)) numbers gives an apparent under-estimate of ka' compared with empirical correlations. Values of ka' calculated using the Central Jet RTD agree well with values of ka' from literature correlations. There was a linear relationship between ka' and effective Peclet number: ka' = 2.32 x 10(-3)Pe(eff).     

O2 uptake during solid-state fermentation in a rotating drum bioreactor

The maximum O2 uptake by Rhizopus oligosporus grown in a 200 litre rotating drum bioreactor at 0.5 rpm ranged from 6.7 to 7.6 mmol per min per kg initial dry substrate (IDS), for runs done with 4 baffles each 17 cm wide, and 12 baffles each 5 cm wide. Without baffles, the maximum O2 uptake rate at 5 rpm was 6.9 mmol/(min.kg IDS), compared to 5.1 mmol/(min.kg IDS) obtained at 0.5 rpm. Therefore O2 supply is adequate in rotating drum bioreactors as long as slumping flow regimes of the substrate bed are avoided. © Rapid Science Ltd. 1998     

Three-dimensional simulation of grain mixing in three different rotating drum designs for solid-state fermentation

A previously published two-dimensional discrete particle simulation model for radial mixing behavior of various slowly rotating drums for solid-state fermentation (SSF) has been extended to a three-dimensional model that also predicts axial mixing. Radial and axial mixing characteristics were predicted for three different drum designs: (1) without baffles; (2) with straight baffles; and (3) with curved baffles. The axial mixing behavior was studied experimentally with video- and image-analysis techniques. In the drum without baffles and with curved baffles the predicted mixing behavior matched the observed behavior adequately. The predicted axial mixing behavior in the drum with straight baffles was predicted less accurately, and it appeared to be strongly dependent on particle rotation, which was in contrast to the other drum designs. In the drum with curved baffles complete mixing in the radial and axial direction was achieved much faster than in the other designs; that is, it was already achieved after three to four rotations. This drum design may therefore be very well suited to SSF. It is concluded that discrete particle simulations provide valuable detailed knowledge about particle transport processes, and this may help to understand and optimize related heat and mass transfer processes in SSF.     

Performance of an intermittent agitation rotating drum type bioreactor for solid-state fermentation of wheat straw

A laboratory bioreactor, designed for solid-state fermentation of thermophilic microorganisms, was operated for production of cellulases and hemicellulases by the thermophilic fungus Thermoascus aurantiacus. The suitability of the apparatus for the effective control of important operating variables affecting growth of microbes in solid-state cultivation was determined. Application of the optimum conditions found for the moisture content of the medium, growth temperature and airflow rate produced enzyme yields of 1709 U endoglucanase, 4 U cellobiohydrolase, 79 U beta-glucosidase, 5.5 U FPA, 4490 U xylanase and 45 U beta-xylosidase per g of dry wheat straw. The correlation between microorganism growth and production of enzymes was efficiently described by the Le Duy kinetic model.     

Solid-state fermentation in rotating drum bioreactors: operating variables affect performance through their effects on transport phenomena

Aspergillus oryzae ACM 4996 was grown on an artificial gel-based substrate and on steamed wheat bran during solid-state fermentations in 18.7 L rotating drum bioreactors. For gel fermentations fungal growth decreased as rotational speed increased, presumably due to increased shear. For wheat bran fermentations fungal growth improved under agitated compared to static culture conditions, due to superior heat and mass transfer. We conclude that the effects of operational variables on the performance of SSF bioreactors are mediated by their effects on transport phenomena such as mixing, shear, heat transfer, and mass transfer within the substrate bed. In addition, the substrate characteristics affect the need for and the rates of these transport processes. Different transport phenomena may be rate limiting with different substrates. This work improves understanding of the effects of bioreactor operation on SSF performance.     

Residence time distributions of gas flowing through rotating drum bioreactors

Residence time distribution studies of gas through a rotating drum bioreactor for solid-state fermentation were performed using carbon monoxide as a tracer gas. The exit concentration as a function of time differed considerably from profiles expected for plug flow, plug flow with axial dispersion, and continuous stirred tank reactor (CSTR) models. The data were then fitted by least-squares analysis to mathematical models describing a central plug flow region surrounded by either one dead region (a three-parameter model) or two dead regions (a five-parameter model). Model parameters were the dispersion coefficient in the central plug flow region, the volumes of the dead regions, and the exchange rates between the different regions. The superficial velocity of the gas through the reactor has a large effect on parameter values. Increased superficial velocity tends to decrease dead region volumes, interregion transfer rates, and axial dispersion. The significant deviation from CSTR, plug flow, and plug flow with axial dispersion of the residence time distribution of gas within small-scale reactors can lead to underestimation of the calculation of mass and heat transfer coefficients and hence has implications for reactor design and scale-up.     

New developments in solid-state fermentation: II. Rational approaches to the design, operation and scale-up of bioreactors

Over the last decade there has been a significant improvement in understanding how to design, operate and scale-up solid-state fermentation bioreactors. The key to these advances has been the application of mathematical modeling techniques to describe the biological and transport phenomena within the system. This review focuses on the advances in understanding that have come from this modeling work, and the insights it has given us into bioreactor design, operation and scale-up. It also highlights two promising bioreactor designs that have emerged over the last decade or so. For processes in which the substrate bed must remain static throughout the fermentation, the most promising design is the Zymotis design of ORSTOM at Montpellier, France, which involves closely spaced internal heat transfer plates within a packed-bed bioreactor. For those processes in which mixing can be tolerated, the stirred bioreactor developed at INRA, in Dijon, France, has been successfully demonstrated at scales of 1–25 t of substrate. Theoretical work suggests that mathematical models will be useful tools in the scale-up process, however, there are no reports that they have been used in the development of any current large-scale process. Rather, the models have been validated against data obtained from laboratory-scale bioreactors. There is an urgent need to test the accuracy and robustness of the models by applying them within real process development.     

Validation of a model describing two-dimensional heat transfer during solid-state fermentation in packed bed bioreactors

A two-dimensional heat transfer model was validated against two experimental studies from the literature which describe the growth of Aspergillus niger during solid-state fermentation in packed bed bioreactors. With the same set of model parameters, the two-dimensional model was able to describe both radial temperature gradients, which dominated in one of the studies, and axial temperature gradients, which dominated in the other study. The sensitivity of the model predictions to the characteristics of the substrate and the microbe were explored. The temperatures reached in the column are most sensitive to parameters which affect the peak heat load, including the substrate packing density, the maximum specific growth rate, and the maximum biomass concentration. Even though the bed is assumed to be aerated with saturated air, the increase in temperature with bed height increases the water-carrying capacity of the air and therefore enables evaporation to contribute significantly to cooling. The model suggests that evaporation can remove as much as 78% of the heat from the bed during times of peak heat generation. Our model provides a tool which can guide the design and operation of packed bed bioreactors. However, further improvements are necessary to do this effectively, the most important of which is the incorporation of a water balance.     

Uniform design: A new approach of designing fermentation media

Summary Uniform design was introduced as a new approach of designing fermentation medium. The production of ethanol by Saccharomyces cerevisiae was used as a model system to investigate its applicability. Six factors, each at 10 levels experiments based on uniform design were carried out. As a result, the empirical mathematical models could successfully predict the experimental results with very good accuracy. In contrast with orthogonal design, uniform design has advantages of less experiments and higher working effeciency. Furthermore, factor influences on operation performance can be easily analyzed.     

Mathematical modeling as a tool to investigate the design and operation of the zymotis packed-bed bioreactor for solid-state fermentation

Zymotis bioreactors for solid-state fermentation (SSF) are packed-bed bioreactors with internal cooling plates. This design has potential to overcome the problem of heat removal, which is one of the main challenges in SSF. In ordinary packed-bed bioreactors, which lack internal plates, large axial temperature gradients arise, leading to poor microbial growth in the end of the bed near the air outlet. The Zymotis design is suitable for SSF processes in which the substrate bed must be maintained static, but little is known about how to design and operate Zymotis bioreactors. We use a two-dimensional heat transfer model, describing the growth of Aspergillus niger on a starchy substrate, to provide guidelines for the optimum design and operation of Zymotis bioreactors. As for ordinary packed-beds, the superficial velocity of the process air is a key variable. However, the Zymotis design introduces other important variables, namely, the spacing between the internal cooling plates and the temperature of the cooling water. High productivities can be achieved at large scale, but only if small spacings between the cooling plates are used, and if the cooling water temperature is varied during the fermentation in response to bed temperatures.     

合成气厌氧发酵生物反应器的研究进展

对合成气厌氧发酵生物反应器的研究进展进行综述,包括生物反应器操作原理、类型、构造、应用和对发酵过程的影响等,并对其未来的发展作出展望。     

Response surface methodology for optimizing the fermentation medium of alpha-galactosidase in solid-state fermentation

AIMS: Alpha-galactosidase is applied in food and feed industries for hydrolysing raffinose series oligosaccharides (RO) that are the factors primarily responsible for flatulence upon ingestion of soybean-derived products. The objective of the current work was to develop an optimal culture medium for the production of alpha-galactosidase in solid-state fermentation (SSF) by a mutant strain Aspergillus foetidus. METHODS AND RESULTS: Response surface methodology (RSM) was applied to evaluate the effects of variables, namely the concentrations of wheat bran, soybean meal, KH(2)PO(4), MnSO(4).H(2)O and CuSO(4).5H(2)O on alpha-galactosidase production in the solid substrate. A fractional factorial design (FFD) was firstly used to isolate the main factors that affected the production of alpha-galactosidase and the central composite experimental design (CCD) was then adopted to derive a statistical model for optimizing the composition of the fermentation medium. The experimental results showed that the optimum fermentation medium for alpha-galactosidase production by Aspergillus foetidus ZU-G1 was composed of 8.2137 g wheat bran, 1.7843 g soybean meal, 0.001 g MnSO(4).H(2)O and 0.001 g CuSO(4).5H(2)O in 10 g dry matter fermentation medium. CONCLUSIONS: After incubating 96 h in the optimum fermentation medium, alpha-galactosidase activity was predicted to be 2210.76 U g(-1) dry matter in 250 ml shake flask. In the present study, alpha-galactosidase activity reached 2207.19 U g(-1) dry matter. SIGNIFICANCE AND IMPACT OF THE STUDY: Optimization of the solid substrate was a very important measure to increase enzyme activity and realize industrial production of alpha-galactosidase. The process of alpha-galactosidase production in laboratory scale may have the potential to scale-up.     

The design of controllers for batch bioreactors

The implementation of control algorithms to batch bioreactors is often complicated by variations in process dynamics that occur during the course of fermentation. Such a wide operating range often renders the performance of fixed gain proportional-integral-differential (PID) controllers unsatisfactory. In this work, detailed studies on the control of batch fermentations are per formed. Two simple controller designs are presented with the intent to compensate for changing process dynamics. One design incorporates the concepts of static feedforward-feedback control. While this technique produces tighter control than feedback alone, it is not as successful as a controller based on gain scheduling. The gain-scheduling controller, a subclass of adaptive controllers, uses the oxygen uptake rate as an auxiliary variable to fine-tune the PID controller parameters. The control of oxygen tension in the bioreactor is used as a vehicle to convey the proposed ideas, analyses, and results. Simulation experiments indicate significant improvement in controller performance can be achieved by both of the proposed approaches even in the presence of measurement noise.     

The influence of process parameters in production of lipopeptide iturin A using aerated packed bed bioreactors in solid-state fermentation

The strain Bacillus iso 1 co-produces the lipopeptide iturin A and biopolymer poly-γ-glutamic acid (γ-PGA) in solid-state fermentation of substrate consisting of soybean meal, wheat bran with rice husks as an inert support. The effects of pressure drop, oxygen consumption, medium permeability and temperature profile were studied in an aerated packed bed bioreactor to produce iturin A, diameter of which was 50 mm and bed height 300 mm. The highest concentrations of iturin A and γ-PGA were 5.58 and 3.58 g/kg-dry substrate, respectively, at 0.4 L/min after 96 h of fermentation. The low oxygen uptake rates, being 23.34 and 22.56 mg O₂/kg-dry solid substrate for each air flow rate tested generated 5.75 W/kg-dry substrate that increased the fermentation temperature at 3.7 °C. The highest pressure drop was 561 Pa/m at 0.8 L/min in 24 h. This is the highest concentration of iturin A produced to date in an aerated packed bed bioreactor in solid-state fermentation. The results can be useful to design strategies to scale-up process of iturin A in aerated packed bed bioreactors. Low concentration of γ-PGA affected seriously pressure drop, decreasing the viability of the process due to generation of huge pressure gradients with volumetric air flow rates. Also, the low oxygenation favored the iturin A production due to the reduction of free void by γ-PGA production, and finally, the low oxygen consumption generated low metabolic heat. The results show that it must control the pressure gradients to scale-up the process of iturin A production.     

On the dynamics of a spherical scaffold in rotating bioreactors

We analyze the dynamics of a spherical scaffold in rotating bioreactors (or clinostats). The idealized clinostat environment consists of a purely rotational flow that is perpendicular to a gravitational field. We confirm through a detailed analytical study that lift effects considerably alter the position of the equilibrium point reached by the scaffolds in the (vertical) direction collinear to the gravitational field. This result holds for small particle and shear Reynolds numbers. Our analysis shows that the inertial lift effect is negligible in the horizontal direction. We show that for all rotations of practical interest, and for the range of particle Reynolds number smaller than unity, the vertical coordinate of the equilibrium point is strongly affected by consideration of lift effects. For light (heavy) particles, inclusion of lift in the formation forces the equilibrium position to be below (above) the horizontal plane that contains the axis of rotation. The equilibrium point for light particles is stable and therefore is observable experimentally. The equilibrium point for heavy particles is unstable. We also estimate the stress level applied to the scaffold and derive an algebraic expression that indicates that the stress level acting on the scaffold decreases with increasing shear Reynolds number.     

Characterization of the solid-state fermentation of cassava

The solid-state fermentation of cassava (Mannlhot esculenta Crantz) was characterized by determining pH and biological oxygen demand (BOD). The results showed a strong association (=0.73) between pH of the fermenting slurry and that of the waste llquor. BOD of the liquor decreased as fermentation progressed. After 96 h fermentation, BOD was about 6×10² mg/l. Progress of cassava fermentation can probably be determined indirectly by following the changes in the pH and BOD profiles of the liquor.

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Résumé La fermentation en milleu solide du manioc (Mannihot esculenta Crantz) a été caractérisée en déterminant le pH et la demande biolgique en oxygène (DBO). Les résultats montrent une forte corrélation (=0.73) entre le pH de la pâte en fermentation et celui de la liquer résiduaire. La DBO de la liqueur décroît au fur et à mesure que la fermentation progresse. Après 96hh de fermentation, la DBO était de 6×10² mg/l. L'avancement de la fermentation du manioc peut probablement être déterminée indirectement ectement en sulvant l'évolution des profiles en pH et en DBO de la liqueur.

     

A pulsing device for packed-bed bioreactors: II. Application to alcoholic fermentation

When the immobilized cells are employed in packed-bed bioreactors several problems appear. To overcome these drawbacks, a new bioreactor based on the use of pulsed systems was developed [1]. In this work, we study the glucose fermentation by immobilized Saccharomyces cerevisiae in a packed-bed bioreactor. A comparative study was then carried out for continuous fermentation in two packed-bed bioreactors, one of them with pulsed flow. The determination of the axial dispersion coefficients indicates that by introducing the pulsation, the hydraulic behaviour is closer to the plug flow model. In both cases, the residence time tested varied from 0.8 to 2.6 h. A higher ethanol concentration and productivity (increases up to 16%) were achieved with the pulsated reactors. The volumes occupied by the CO₂ were 5.22% and 9.45% for fermentation with/without pulsation respectively. An activity test of the particles from the different sections revealed that the concentration and viability of bioparticles from the two bioreactors are similar. From the results we conclude that the improvements of the process are attributable to a mechanical effect rather than to physiological changes of microorganisms.List of Symbols D m²/s dispersion coefficient - K _is l/g inhibition substrate constant - K _ip l/g inhibition ethanol constant - K _s g/l Apparent affinity constant - P g/l ethanol concentration - q _p g/(gh) specific ethanol productivity - Q _p g/(lh) overall ethanol productivity - q _s g/(gh) specific glucose consumption rate - Q _s g/(lh) glucose consumption rate - S g/l residual glucose concentration - S(in0) g/l initial glucose concentration - V _max g/(lh) maximum rate - Y _p/s g/g yield in product