Maximum Oxygen Transfer Capacity Research Articles

Black glucose-releasing silicon elastomer rings for fed-batch operation allow measurement of the oxygen transfer rate from the top and optical signals from the bottom for each well of a microtiter plate

BackgroundIn industrial microbial biotechnology, fed-batch processes are frequently used to avoid undesirable biological phenomena, such as substrate inhibition or overflow metabolism. For targeted process development, fed-batch options for small scale and high throughput are needed. One commercially available fed-batch fermentation system is the FeedPlate®, a microtiter plate (MTP) with a polymer-based controlled release system. Despite standardisation and easy incorporation into existing MTP handling systems, FeedPlates® cannot be used with online monitoring systems that measure optically through the transparent bottom of the plate. One such system that is broadly used in biotechnological laboratories, is the commercial BioLector. To allow for BioLector measurements, while applying the polymer-based feeding technology, positioning of polymer rings instead of polymer disks at the bottom of the well has been proposed. This strategy has a drawback: measurement requires an adjustment of the software settings of the BioLector device. This adjustment modifies the measuring position relative to the wells, so that the light path is no longer blocked by the polymer ring, but, traverses through the inner hole of the ring. This study aimed at overcoming that obstacle and allowing for measurement of fed-batch cultivations using a commercial BioLector without adjustment of the relative measurement position within each well.ResultsDifferent polymer ring heights, colours and positions in the wells were investigated for their influence on maximum oxygen transfer capacity, mixing time and scattered light measurement. Several configurations of black polymer rings were identified that allow measurement in an unmodified, commercial BioLector, comparable to wells without rings. Fed-batch experiments with black polymer rings with two model organisms, E. coli and H. polymorpha, were conducted. The identified ring configurations allowed for successful cultivations, measuring the oxygen transfer rate and dissolved oxygen tension, pH, scattered light and fluorescence. Using the obtained online data, glucose release rates of 0.36 to 0.44 mg/h could be determined. They are comparable to formerly published data of the polymer matrix.ConclusionThe final ring configurations allow for measurements of microbial fed-batch cultivations using a commercial BioLector without requiring adjustments of the instrumental measurement setup. Different ring configurations achieve similar glucose release rates. Measurements from above and below the plate are possible and comparable to measurements of wells without polymer rings. This technology enables the generation of a comprehensive process understanding and target-oriented process development for industrial fed-batch processes.

Implementation of Perforated Concentric Ring Walls Considerably Improves Gas-Liquid Mass Transfer of Shaken Bioreactors.

Since their first use in the 1930s, shake flasks have been a widely used bioreactor type for screening and process development due to a number of advantages. However, the limited gas-liquid mass transfer capacities—resulting from practical operation limits regarding shaking frequency and filling volumes—are a major drawback. The common way to increase the gas-liquid mass transfer in shake flasks with the implementation of baffles is generally not recommended as it comes along with several severe disadvantages. Thus, a new design principle for shaken bioreactors that aims for improving the gas-liquid mass transfer without losing the positive characteristics of unbaffled shake flasks is introduced. The flasks consist of cylindrical glass vessels with implemented perforated concentric ring walls. The ring walls improve the gas-liquid mass transfer via the formation of additional liquid films on both of its sides, whereas the perforations allow for mixing between the compartments. Sulfite oxidation experiments revealed over 200% higher maximum oxygen transfer capacities (OTRmax) compared to conventional shake flasks. In batch cultivations of Escherichia coli BL21 in mineral media, unlimited growth until glucose depletion and oxygen transfer rates (OTR) of up to 138 mmol/L/h instead of an oxygen limitation at 57 mmol/L/h as in normal shake flasks under comparable conditions could be achieved. Even overflow metabolism could be prevented due to sufficient oxygen supply without the use of unconventional shaking conditions or oxygen enrichment. Therefore, we believe that the new perforated ring flask principle has a high potential to considerably improve biotechnological screening and process development steps.

Detailed small-scale characterization and scale-up of active YFP inclusion body production with Escherichia coli induced by a tetrameric coiled coil domain

During heterologous protein production with Escherichia coli, the formation of inclusion bodies (IBs) is often a major drawback as these aggregated proteins are usually inactive. However, different strategies for the generation of IBs consisting of catalytically active proteins have recently been described. In this study, the archaeal tetrameric coiled-coil domain of the cell-surface protein tetrabrachion was fused to a target reporter protein to produce fluorescent IBs (FIBs). As the cultivation conditions severely influence IB formation, the entire cultivation process resulting in the production of FIBs were thoroughly studied. First, the cultivation process was scaled down based on the maximum oxygen transfer capacity, combining online monitoring technologies for shake flasks and microtiter plates with offline sampling. The evaluation of culture conditions in complex terrific broth autoinduction medium showed strong oxygen limitation and leaky expression. Furthermore, strong acetate formation and pH changes from 6.5 to 8.8 led to sub-optimal cultivation conditions. However, in minimal Wilms-MOPS autoinduction medium, defined culture conditions and a tightly controlled expression were achieved. The production of FIBs is strongly influenced by the induction strength. Increasing induction strengths result in lower total amounts of functional protein. However, the amount of functional FIBs increases. Furthermore, to prevent the formation of conventional inactive IBs, a temperature shift from 37°C to 15°C is crucial to generate FIBs. Finally, the gained insights were transferred to a stirred tank reactor batch fermentation. Hereby, 12g/L FIBs were produced, making up 43 % (w/w) of the total generated biomass.

Shake flask methodology for assessing the influence of the maximum oxygen transfer capacity on 2,3-butanediol production

BackgroundProduction of 2,3-butanediol from renewable resources is a promising measure to decrease the consumption of fossil resources in the chemical industry. One of the most influential parameters on biotechnological 2,3-butanediol production is the oxygen availability during the cultivation. As 2,3-butanediol is produced under microaerobic process conditions, a well-controlled oxygen supply is the key parameter to control biomass formation and 2,3-butanediol production. As biomass is on the one hand not the final product, but on the other hand the essential biocatalyst, the optimal compromise between biomass formation and 2,3-butanediol production has to be defined.ResultsA shake flask methodology is presented to evaluate the effects of oxygen availability on 2,3-butanediol production with Bacillus licheniformis DSM 8785 by variation of the filling volume. A defined two-stage cultivation strategy was developed to investigate the metabolic response to different defined maximum oxygen transfer capacities at equal initial growth conditions. The respiratory quotient was measured online to determine the point of glucose depletion, as 2,3-butanediol is consumed afterwards. Based on this strategy, comparable results to stirred tank reactors were achieved. The highest space–time yield (1.3 g/L/h) and a 2,3-butanediol concentration of 68 g/L combined with low acetoin concentrations and avoided glycerol formation were achieved at a maximum oxygen transfer capacity of 13 mmol/L/h. The highest overall 2,3-butanediol concentration of 78 g/L was observed at a maximum oxygen transfer capacity of 4 mmol/L/h.ConclusionsThe presented shake flask approach reduces the experimental effort and costs providing a fast and reliable methodology to investigate the effects of oxygen availability. This can be applied especially on product and by-product formation under microaerobic conditions. Utilization of the maximum oxygen transfer capacity as measure for the oxygen availability allows for an easy adaption to other bioreactor setups and scales.

Quasi-continuous parallel online scattered light, fluorescence and dissolved oxygen tension measurement combined with monitoring of the oxygen transfer rate in each well of a shaken microtiter plate.

BackgroundMicrotiter plates (MTP) are often applied as culture vessels in high-throughput screening programs. If online measuring techniques are available, MTPs can also be applied in the first steps of process development. For such small-scale bioreactors dipping probes are usually too large; therefore, optical measurements are often used. For example, the BioLector technology allows for the online monitoring of scattered light and fluorescence in each well of a continuously orbitally shaken MTP. Although this system provides valuable data, these measurements are mainly of a semi-quantitative nature. Therefore, signal calibration is required to obtain absolute values. With the µRAMOS technology it became possible for the first time to quantify the oxygen transfer rate (OTR) separately in each well of an MTP. In this work, a device is presented that combines both techniques, to provide a hitherto unparalleled high amount of information from each single well.ResultsBecause both systems (BioLector and µRAMOS) are based on optical measurements, the measurements need to be synchronized to avoid interferences with the optical signals. The new experimental setup was applied for online monitoring in cultures of Escherichia coli and Hansenula polymorpha. It has been demonstrated that the well-to-well reproducibility is very high, and that the monitored signals provide reliable and valuable information about the process. With varying filling volumes, different maximum oxygen transfer capacities (OTRmax) were adjusted in oxygen-limited cultures. The different degrees of stress during the culture due to oxygen limitation affected microbial growth and also impacted reproducibility from culture to culture. Furthermore, it was demonstrated that this new device significantly simplifies the experimental efforts: instead of parallel cultures in a shake flask and MTP, just one single experiment in MTP needs to be conducted to measure the OTR, dissolved oxygen tension (DOT), scattered light and fluorescence.ConclusionsThe new device is a very suitable system for the online monitoring of cultures in continuously orbitally shaken MTPs. Due to the high number of parameters that can simultaneously be measured with this small-scale device, deeper insight into the investigated microbial system can be achieved. Furthermore, the experimental efforts to obtain OTR, DOT, scattered light and fluorescence signals during a culture are decreased. Ultimately, this new technology and the resulting high amount of collected data will eliminate the currently existing separation between screening and process development.Graphical abstractPicture of the combined μRAMOS and BioLector setup which allows for measurements of the oxygen transfer rate (OTR), dissolved oxygen tension (DOT), scattered light and fluorescence in each single well of an orbitally shaken microtiter plate. Electronic supplementary materialThe online version of this article (doi:10.1186/s12934-016-0608-2) contains supplementary material, which is available to authorized users.

Correlation for the maximum oxygen transfer capacity in shake flasks for a wide range of operating conditions and for different culture media

Characterization of shake flasks is essential to ensure a sufficient oxygen supply for aerobic microorganism. To determine the volumetric mass transfer coefficient kLa or the maximum oxygen transfer capacity OTRmax the sulfite system is commonly applied. It has been used for non-baffled shake flasks based on the assumption of a proportionality between the kLa of the sulfite system and of a biological culture. This work proves this to be valid only for shaking frequencies above 300rpm. Below the proportionality factor depends on the shaking frequency. More than 350 experiments with different concentrations of the sulfite system, 16 different media compositions and 10 different microorganisms were evaluated varying the shaking diameter (1.25–10cm), flask diameter (51–131mm), filling volume (2–160mL) and shaking frequency (100–450rpm). For the first time the properties of the culture media are coherently represented by a term based on the easily accessible osmolality. An universal consistent equation to calculate the OTRmax for non-baffled shake flasks for an exhaustive range of shaking conditions was developed and validated by experimental data. The deviations are within a remarkable narrow range of ±5mmolL−1h−1.

Parallel use of shake flask and microtiter plate online measuring devices (RAMOS and BioLector) reduces the number of experiments in laboratory-scale stirred tank bioreactors.

BackgroundConventional experiments in small scale are often performed in a ‘Black Box’ fashion, analyzing only the product concentration in the final sample. Online monitoring of relevant process characteristics and parameters such as substrate limitation, product inhibition and oxygen supply is lacking. Therefore, fully equipped laboratory-scale stirred tank bioreactors are hitherto required for detailed studies of new microbial systems. However, they are too spacious, laborious and expensive to be operated in larger number in parallel. Thus, the aim of this study is to present a new experimental approach to obtain dense quantitative process information by parallel use of two small-scale culture systems with online monitoring capabilities: Respiration Activity MOnitoring System (RAMOS) and the BioLector device.ResultsThe same ‘mastermix’ (medium plus microorganisms) was distributed to the different small-scale culture systems: 1) RAMOS device; 2) 48-well microtiter plate for BioLector device; and 3) separate shake flasks or microtiter plates for offline sampling. By adjusting the same maximum oxygen transfer capacity (OTRmax), the results from the RAMOS and BioLector online monitoring systems supplemented each other very well for all studied microbial systems (E. coli, G. oxydans, K. lactis) and culture conditions (oxygen limitation, diauxic growth, auto-induction, buffer effects).ConclusionsThe parallel use of RAMOS and BioLector devices is a suitable and fast approach to gain comprehensive quantitative data about growth and production behavior of the evaluated microorganisms. These acquired data largely reduce the necessary number of experiments in laboratory-scale stirred tank bioreactors for basic process development. Thus, much more quantitative information is obtained in parallel in shorter time.Electronic supplementary materialThe online version of this article (doi:10.1186/s13036-015-0005-0) contains supplementary material, which is available to authorized users.

Biotechnology and Bioengineering: Volume 111, Number 10, October 2014

Cover Legend Liquid films on shake flask walls explains increasing maximum oxygen transfer capacities with elevating viscosity. See related article by Giese et. al., Biotechnology and Bioengineering, Volume 111, Number 2 (February 2014), pages 295–308. (Image courtesy of Heiner Giese et. al.) image

Cross-section perimeter is a suitable parameter to describe the effects of different baffle geometries in shaken microtiter plates

BackgroundBiotechnological screening processes are performed since more than 8 decades in small scale shaken bioreactors like shake flasks or microtiter plates. One of the major issues of such reactors is the sufficient oxygen supply of suspended microorganisms. Oxygen transfer into the bulk liquid can in general be increased by introducing suitable baffles at the reactor wall. However, a comprehensive and systematic characterization of baffled shaken bioreactors has never been carried out so far. Baffles often differ in number, size and shape. The exact geometry of baffles in glass lab ware like shake flasks is very difficult to reproduce from piece to piece due to the hard to control flow behavior of molten glass during manufacturing. Thus, reproducibility of the maximum oxygen transfer capacity in such baffled shake flasks is hardly given.ResultsAs a first step to systematically elucidate the general effect of different baffle geometries on shaken bioreactor performance, the maximum oxygen transfer capacity (OTRmax) in baffled 48-well microtiter plates as shaken model reactor was characterized. This type of bioreactor made of plastic material was chosen, as the exact geometry of the baffles can be fabricated by highly reproducible laser cutting. As a result, thirty different geometries were investigated regarding their maximum oxygen transfer capacity (OTRmax) and liquid distribution during shaking. The relative perimeter of the cross-section area as new fundamental geometric key parameter is introduced. An empirical correlation for the OTRmax as function of the relative perimeter, shaking frequency and filling volume is derived. For the first time, this correlation allows a systematic description of the maximum oxygen transfer capacity in baffled microtiter plates.ConclusionsCalculated and experimentally determined OTRmax values agree within ± 30% accuracy. Furthermore, undesired out-of-phase operating conditions can be identified by using the relative perimeter as key parameter. Finally, an optimum well geometry characterized by an increased perimeter of 10% compared to the unbaffled round geometry is identified. This study may also assist to comprehensively describe and optimize the baffles of shake flasks in future.

Improvement and scale-down of a Trichoderma reesei shake flask protocol to microtiter plates enables high-throughput screening

Nowadays, high-throughput screening is essential for determining the best microbial strains and fermentation conditions. Although microtiter plates allow higher throughput in screening than shake flasks, they do not guarantee sufficient oxygen supply if operated at unsuitable conditions. This is especially the case in viscous fermentations, potentially leading to poor liquid movement and surface growth. Therefore, in this study, two aims were pursued. First, an industrial Trichoderma reesei shake flask protocol is improved with respect to oxygen supply and production. Second, this improved shake flask protocol is scaled down into microtiter plate under consideration of similar oxygen supply. For this purpose, the respiration activity monitoring system (RAMOS) was applied. An approach based on a sulfite system was introduced to ensure equal maximum oxygen transfer capacities (OTRmax) in microtiter plates and shake flasks. OTRmax-values of 250 mL shake flasks and 24-well microtiter plates were determined in a wide range of operating conditions. These sulfite datasets were used to identify operating conditions leading to the same oxygen supply for T. reesei in shake flasks and 24-well microtiter plates. For 24-well microtiter plates, the shake flask OTRmax of 20 mmol/L/h of an industrial protocol was obtained under the following optimal operating conditions: 1 mL filling volume per well, 200 rpm shaking frequency and 50 mm shaking diameter. With these conditions almost identical oxygen transfer rates and product concentrations were measured in both scales. The proposed approach is a fast and accurate means to scale-down established screening procedures into microtiter plates to achieve high-throughput.

Liquid films on shake flask walls explain increasing maximum oxygen transfer capacities with elevating viscosity

In biotechnological screening and production, oxygen supply is a crucial parameter. Even though oxygen transfer is well documented for viscous cultivations in stirred tanks, little is known about the gas/liquid oxygen transfer in shake flask cultures that become increasingly viscous during cultivation. Especially the oxygen transfer into the liquid film, adhering on the shake flask wall, has not yet been described for such cultivations. In this study, the oxygen transfer of chemical and microbial model experiments was measured and the suitability of the widely applied film theory of Higbie was studied. With numerical simulations of Fick's law of diffusion, it was demonstrated that Higbie's film theory does not apply for cultivations which occur at viscosities up to 10 mPa s. For the first time, it was experimentally shown that the maximum oxygen transfer capacity OTRmax increases in shake flasks when viscosity is increased from 1 to 10 mPa s, leading to an improved oxygen supply for microorganisms. Additionally, the OTRmax does not significantly undermatch the OTRmax at waterlike viscosities, even at elevated viscosities of up to 80 mPa s. In this range, a shake flask is a somehow self-regulating system with respect to oxygen supply. This is in contrary to stirred tanks, where the oxygen supply is steadily reduced to only 5% at 80 mPa s. Since, the liquid film formation at shake flask walls inherently promotes the oxygen supply at moderate and at elevated viscosities, these results have significant implications for scale-up.

Potential errors in conventional DOT measurement techniques in shake flasks and verification using a rotating flexitube optical sensor

BackgroundDissolved oxygen tension (DOT) is an important parameter for evaluating a bioprocess. Conventional means to measure DOT in shake flasks using fixed Clark-type electrodes immersed in the bulk liquid are problematic, because they inherently alter the hydrodynamics of the systems. Other approaches to measure DOT that apply fluorescing sensor spots fixed at the inside wall of a shake flask are also suboptimal. At low filling volumes for cultivating microorganisms with a high oxygen demand, the measured DOT signal may be erroneous. Here, the sensor spot is sometimes exposed to gas in the head space of the flask. Merely repositioning the sensor spot elsewhere in the flask does not address this problem, since there is no location in the shake flask that is always covered by the rotating bulk liquid. Thus, the aim of this prospective study is first, to verify the systemic error of Clark-type electrodes for measuring DOT in shake flasks. The second principle aim is to use the newly built "flexitube optical sensor" to verify potential errors in conventional optical DOT measurements based on fixed sensor spots.ResultsWith the Clark-type electrode, the maximum oxygen transfer capacity in shake flasks rose compared to that of an analogous system without an electrode. This proves changed hydrodynamics in the system with the Clark-type electrode. Furthermore, regarding the sensor spot experiments under oxygen-limited conditions where the DOT value ought to approach zero, the acquired signals were clearly above zero. This implies that the sensor spot is influenced by oxygen present in the headspace and not only by oxygen in the bulk liquid.ConclusionsThe Clark-type electrode is unsuitable for measuring DOT. Moreover, the newly built rotating flexitube optical sensor is useful to verify potential errors of conventional optical DOT measurement techniques applying fixed sensor spots.

The baffled microtiter plate: Increased oxygen transfer and improved online monitoring in small scale fermentations

Most experiments in screening and process development are performed in shaken bioreactors. Today, microtiter plates are the preferred vessels for small-scale microbial cultivations in high throughput, even though they have never been optimized for this purpose. To interpret the experimental results correctly and to obtain a base for a meaningful scale-up, sufficient oxygen supply to the culture liquid is crucial. For shaken bioreactors this problem can generally be addressed by the introduction of baffles. Therefore, the focus of this study is to investigate how baffling and the well geometry affect the maximum oxygen transfer capacity (OTR(max)) in microtiter plates. On a 48-well plate scale, 30 different cross-section geometries of a well were studied. It could be shown that the introduction of baffles into the common circular cylinder of a microtiter plate well doubles the maximum oxygen transfer capacity, resulting in values above 100 mmol/L/h (k(L)a > 600 1/h). To also guarantee a high volume for microbial cultivation, it is important to maximize the filling volume, applicable during orbital shaking. Additionally, the liquid height at the well bottom was examined, which is a decisive parameter for online-monitoring systems such as the BioLector. This technology performs fiber-optical measurements through the well bottom, therefore requires a constant liquid height at all shaking frequencies. Ultimately, a six-petal flower-shaped well geometry was shown to be the optimal solution taking into account all aforementioned criteria. With its favorable culture conditions and the possibility for unrestricted online monitoring, this novel microtiter plate is an efficient tool to gain meaningful results for interpreting and scaling-up experiments in clone screening and bioprocess development.

Bioreactor control using a probing feeding strategy and mid-ranging control

The paper presents a fed-batch fermentation technique for bioreactors operating close to their maximum oxygen transfer capacity. The method combines the advantages of the probing feeding strategy and the temperature limited fed-batch technique. When the maximum oxygen transfer capacity of the reactor is reached, the temperature is decreased to lower the oxygen demand. To achieve a good control of the dissolved oxygen a mid-ranging controller manipulating the stirrer speed and the temperature is used. The feeding strategy is analysed and it is also illustrated by simulations and an experiment.

A cultivation technique for E. coli fed-batch cultivations operating close to the maximum oxygen transfer capacity of the reactor

A cultivation strategy combining the advantages of temperature-limited fed-batch and probing feeding control is presented. The technique was evaluated in fed-batch cultivations with E. coli BL21(DE3) producing xylanase in a 3 liter bioreactor. A 20% increase in cell mass was achieved and the usual decrease in specific enzyme activity normally observed during the late production phase was diminished with the new technique. The method was further tested by growing E. coli W3110 in a larger bioreactor (50 l). It is a suitable cultivation technique when the O2 transfer capacity of the reactor is reached and it is desired to continue to produce the recombinant protein.

Oxygen transfer phenomena in 48‐well microtiter plates: Determination by optical monitoring of sulfite oxidation and verification by real‐time measurement during microbial growth

Oxygen limitation is one of the most frequent problems associated with the application of shaking bioreactors. The gas-liquid oxygen transfer properties of shaken 48-well microtiter plates (MTPs) were analyzed at different filling volumes, shaking diameters, and shaking frequencies. On the one hand, an optical method based on sulfite oxidation was used as a chemical model system to determine the maximum oxygen transfer capacity (OTR(max)). On the other hand, the Respiration Activity Monitoring System (RAMOS) was applied for online measurement of the oxygen transfer rate (OTR) during growth of the methylotropic yeast Hansenula polymorpha. A proportionality constant between the OTR(max) of the biological system and the OTR(max) of the chemical system were indicated from these data, offering the possibility to transform the whole set of chemical data to biologically relevant conditions. The results exposed "out of phase" shaking conditions at a shaking diameter of 1 mm, which were confirmed by theoretical consideration with the phase number (Ph). At larger shaking diameters (2-50 mm) the oxygen transfer rate in MTPs shaken at high frequencies reached values of up to 0.28 mol/L/h, corresponding to a volumetric mass transfer coefficient (k(L)a) of 1,600 1/h. The specific mass transfer area (a) increases exponentially with the shaking frequency up to values of 2,400 1/m. On the contrary, the mass transfer coefficient (k(L)) is constant at a level of about 0.15 m/h over a wide range of shaking frequencies and shaking diameters. However, at high shaking frequencies, when the complete liquid volume forms a thin film on the cylindric wall of the well, the mass transfer coefficient (k(L)) increases linearly to values of up to 0.76 m/h. Essentially, the present investigation demonstrates that the 48-well plate outperforms the 96-well MTP and shake flasks at widely used operating conditions with respect to oxygen supply. The 48-well plates emerge, therefore, as an excellent alternative for microbial cultivation and expression studies combining the advantages of both the high-throughput 96-well MTP and the classical shaken Erlenmeyer flask.

Adaptive dissolved oxygen control through the glycerol feeding in a recombinant Pichia pastoris cultivation in conditions of oxygen transfer limitation

In high cell density cultivation processes the productivity is frequently constrained by the bioreactor maximum oxygen transfer capacity. The productivity can often be increased by operating the process at low dissolved oxygen concentrations close to the limitation level. This may be accomplished with a closed-loop controller that regulates the dissolved oxygen concentration by manipulating the dominant carbon source feeding rate. In this work we study this control problem in a pilot 50 l bioreactor with a high cell density recombinant P. pastoris cultivation in complex media. The study focuses on the design of accurate stable adaptive controllers, with guaranteed exponential convergence and its relation with the calibration of controller parameters. Two adaptive control strategies were tested in the pilot bioreactor: a model reference adaptive controller with a linear reference model and an integral feedback controller with adaptive gain. The latter alternative proved to be more robust to errors in the measurements of the off-gas composition. Concerning the instrumentation, algorithms were derived assuming that both the dissolved oxygen tension and off-gas composition are measured on-line, but also the case of only dissolved oxygen being measured is addressed. It was verified that the measurement of off-gas composition might not improve the controller performance due to measurement and process time delays.

Design of a stable adaptive controller for driving aerobic fermentation processes near maximum oxygen transfer capacity

In many industrial fermentation processes oxygen availability is the main limiting factor for product production. Typically the dissolved oxygen (DO) concentration decreases continuously at the beginning of the batch until it reaches a critical level where the oxygen transfer rate is very close to the vessel's maximum transfer capacity. The process may be further driven close to this sensitive operating point with a controller that manipulates the carbon source feed rate. This operating strategy is linked with important productivity issues and is still frequently realised in open-loop at production scale. The main purpose of the present study is to derive an effective closed-loop control solution and to demonstrate its economical advantage in relation to the open-loop form of operation. A stable model reference adaptive controller (MRAC) was designed based on a phenomenological model of the process. The implementation requires two on-line measurements: the DO tension and oxygen transfer rate (OTR) between gas–liquid phases, which are nowadays standard and easily available in production facilities. The controller performance is accessed with a simulation case study. The main results show that the adaptive controller is precise, stable and robust to disturbances and to inaccuracies like variability in raw materials typical in fermentations run in complex media. The controller is simple, easy to implement, and could possibly improve productivity in processes for which oxygen transfer capacity is limiting growth and product production.

Effect of oxygen supply on passaging, stabilising and screening of recombinant Hansenula polymorpha production strains in test tube cultures.

Twenty-four Hansenula polymorpha transformants were passaged and stabilised in glucose medium and screened in glycerol medium for recombinant phytase in shaken test tubes. The cultivations were performed under either limited or non-limited oxygen supply. Maximum oxygen transfer capacities of test tubes were assessed by sulfite oxidation. Oxygen-limited glucose cultures resulted in a partially anaerobic metabolism and formation of 4.1 g ethanol l(-1), which was subsequently aerobically metabolised. Non-limited oxygen supply led to overflow metabolism and to accumulation of 2.1 g acetic acid l(-1), reducing the biomass yield. The use of glycerol in the screening main cultures prevented by-product formation irrespective of oxygen supply. Preculturing in glucose medium under non-limited oxygen supply resulted in a 20-h lag phase of the screening main culture. This lag phase was not observed when preculturing was performed under oxygen limitation. Phytase activity was on average 25% higher in cultures passaged, stabilised and screened under limited oxygen supply than in cultures under non-limited oxygen supply.

Characterization of gas-liquid mass transfer phenomena in microtiter plates.

Gas-liquid mass transfer properties of shaken 96-well microtiter plates were characterized using a recently described method. The maximum oxygen transfer capacity (OTR(max)), the specific mass transfer area (a), and the mass transfer coefficient (k(L)) in a single well were determined at different shaking intensities (different shaking frequencies and shaking diameters at constant filling volume) and different filling volumes by means of sulfite oxidation as a chemical model system. The shape (round and square cross-sections) and the size (up to 2 mL maximum filling volume) of a microtiter plate well were also considered as influencing parameters. To get an indication of the hydrodynamic behavior of the liquid phase in a well, images were taken during shaking and the liquid height derived as a characteristic parameter. The investigations revealed that the OTR(max) is predominantly dependent on the specific mass transfer area (a) for the considered conditions in round-shaped wells. The mass transfer coefficient (k(L)) in round-shaped wells remains at a nearly constant value of about 0.2 m/h for all shaking intensities, thus within the range reported in the literature for surface-aerated bioreactors. The OTR(max) in round-shaped wells is strongly influenced by the interfacial tension, determined by the surface tension of the medium used and the surface properties of the well material. Up to a specific shaking intensity the liquid surface in the wells remains horizontal and no liquid movement can be observed. This critical shaking intensity must be exceeded to overcome the surface tension and, thus, to increase the liquid height and enlarge the specific mass transfer area. This behavior is solely specific to microtiter plates and has not yet been observed for larger shaking bioreactors such as shaking flasks. In square-shaped microtiter plate wells the corners act as baffles and cause a significant increase of OTR(max), a, and k(L). An OTR(max) of up to 0.15 mol/L/h can be reached in square-shaped wells.