A new method for on-line measurement of the volumetric oxygen uptake rate in membrane aerated animal cell cultures

Oxygen is a key substrate in animal cell metabolism and its consumption is thus a parameter of great interest for bioprocess monitoring and control. A system for measuring it based on an oxygen balance on the liquid phase was developed. The use of a gas-permeable membrane offered the possibility to provide the required quantity of oxygen into the culture, while avoiding problems of foaming or shear stress generally linked to sparging. This aeration system allowed moreover to keep a known and constant k(L)a value through cultures up to 400 h. Oxygen uptake rate (OUR) was measured on-line with a very good accuracy of +/-5%, and the specific OUR for a CHO cell line was determined during batch (growth phase) and continuous culture as, respectively, equal to 2. 85x10(-13) and 2.54x10(-13) mol O(2) cell(-1) h(-1). It was also shown that OUR continuous monitoring gives actually more information about the metabolic state of the culture than the cell concentration itself, especially during transition phases like the end of the growth phase in a batch culture.     

The effect of dissolved oxygen tension and the utility of oxygen uptake rate in insect cell culture

Dissolved oxygen tension and oxygen uptake rate are critical parameters in animal cell culture. However, only scarce information of such variables is available for insect cell culture. In this work, the effect of dissolved oxygen tension (DOT) and the utility of on-line oxygen uptake rate (OUR) measurements in monitoring Spodoptera frugiperda (Sf9) cultures were determined. Sf9 cells were grown at constant dissolved oxygen tensions in the range of 0 to 30%. Sf9 metabolism was affected only at DOT below 10%, as no significant differences on specific growth rate, cell concentration, amino acid consumption/production nor carbohydrates consumption rates were found at DOT between 10 and 30%. The specific growth rate and specific oxygen uptake rate followed typical Monod kinetics with respect to DOT. The calculated _max and max were 0.033 h^-1 and 3.82×10^-10 mole cell^-1h^-1, respectively, and the corresponding saturation constants were 1.91 and 1.57%, respectively. In all aerated cultures, lactate was consumed only after glucose and fructose had been exhausted. The yield of lactate increased with decreasing DOT. It is proposed, that an apparent DOT in non-instrumented cultures can be inferred from the lactate yield of bioreactors as a function of DOT. Such a concept, can be a useful and important tool for determining the average dissolved oxygen tension in non-instrumented cultures. It was shown that the dynamic behavior of OUR can be correlated with monosaccharide (fructose and glucose) depletion and viable cell concentration. Accordingly, OUR can have two important applications in insect cell culture: for on-line estimation of viable cells, and as a possible feed-back control variable in automatic strategies of nutrient addition.Abbreviations DOT Dissolved oxygen tension - OUR Oxygen uptake rate - specific oxygen uptake rate - specific growth rate - X_v viable cell concentration - C_L, C^*, and oxygen concentrations in liquid phase, in equilibrium with gas phase, and medium molar concentration, respectively - H Henry's constant - K_La volumetric oxygen transfer coefficient - P_T total pressure - oxygen partial pressure - oxygen molar fraction - i discrete element     

Comparison of a kayaking ergometer protocol with an arm crank protocol for evaluating peak oxygen consumption

The purpose of this study was to compare a kayak ergometer protocol with an arm crank protocol for determining peak oxygen consumption (V(.-)O2). On separate days in random order, 10 men and 5 women (16-24 years old) with kayaking experience completed the kayak ergometer protocol and a standardized arm crank protocol. The kayak protocol began at 70 strokes per minute and increased by 10 strokes per minute every 2 minutes until volitional fatigue. The arm crank protocol consisted of a crank rate of 70 revolutions per minute, initial loading of 35 W and subsequent increases of 35 W every 2 minutes until volitional fatigue. The results showed a significant difference (p < 0.01) between the kayak ergometer and the arm crank protocols for relative peak V(.-)O2 (47.5 +/- 3.9 ml x kg(-1) x min(-1) vs. 44.2 +/- 6.2 ml x kg(-1) x min(-1)) and absolute peak V(.-)O2 (3.38 L x min(-1) +/- 0.53 vs. 3.14 +/- 0.64 L x min(-1)). The correlation between kayak and arm crank protocol was 0.79 and 0.90, for relative and absolute V(.-)O2 peak, respectively (both p < 0.01). The higher peak V(.-)O2 on the kayak ergometer may be due to the greater muscle mass involved compared to the arm crank ergometer. The kayak ergometer protocol may therefore be more specific to the sport of kayaking than an arm crank protocol.     

A novel in situ probe for oxygen uptake rate measurement in mammalian cell cultures

The newly developed in situ oxygen uptake rate (in situ OUR) probe presented in this article is based on the in situ microscope technology platform. It is designed to measure the oxygen uptake rate (OUR) of mammalian cells, an important parameter for metabolic flux analysis, inside a reactor (in situ) and in real-time. The system isolates a known volume of cell culture from the bulk inside the bioreactor, monitors the oxygen consumption over time, and releases the sample again. The sample is mixed during the measurement with a new agitation system to keep the cells in suspension and prevent oxygen concentration gradients. The OUR measurement system also doubles as a standard dissolved oxygen (DO) probe for process monitoring when it is not performing OUR measurements. It can be equipped with two different types of optical sensors (i.e., DO, pH) simultaneously or a conventional polarographic DO-probe (Clark type). This new probe was successfully tested in baby hamster kidney perfusion cell cultures.     

On-line gas analysis in animal cell cultivation: II. Methods for oxygen uptake rate estimation and its application to controlled feeding of glutamine

Different methods for oxygen uptake rate (OUR) determinations in animal cell cultivation were investigated using a high quality mass spectrometer. Dynamic measurements have considerable disadvantages because of disturbances of the growing cells by the necessary variations of dissolved oxygen concentration. Only infrequent discrete measurements are possible using this method. Stationary liquid phase balance yielded better results with much higher frequency. Gas phase balancing has the advantage of not requiring dissolved oxygen measurement and knowledge of K(L)a, both of them are easily biased. It was found that simple gas phase balancing is either very inaccurate (error larger than expected signal) or very slow, with gas phase residence times of several hours. Therefore, a new method of aeration was designed. Oxygen and CO(2) transfer are mainly achieved via sparging. The gas released to the headspace is diluted with a roughly 100-fold stream of an inert gas (helium). Through this dilution, gas ratios are not changed for O(2), CO(2), Ar, and N(2). The measurement of lower concentrations (parts per million and below) is easy using mass spectrometry with a secondary electron multiplier. With this new method an excellent accuracy and sufficient speed of analysis were obtained. All these on-line methods for OUR measurement were tested during the cultivation of animal cells. The new method allowed better study of the kinetics of animal cell cultures as was shown with a hybridoma cell line (HFN 7.1, ATCC CRL 1606) producing monoclonal antibodies against human fibronectin. With the aid of these methods it was possible to find a correlation between a rapid decrease in oxygen uptake rate (OUR) and glutamine concentration. The sudden decrease in OUR can be attributed to glutamine depletion. This provided a basis for the controlled addition of glutamine to reduce the formation of ammonia produced by hydrolysis. This control method based on OUR measurement resulted in increased cell concentration and threefold higher product concentration. (c) 1995 John Wiley & Sons, Inc.     

Flexible Sheet-Type Sensor for Noninvasive Measurement of Cellular Oxygen Metabolism on a Culture Dish

A novel flexible sensor was developed for the noninvasive oxygen metabolism measurement of cultivated cells and tissues. This device is composed of a transparent double-layered polymer sheet of ethylene-vinyl alcohol (EVOH) and poly(dimethylsiloxane) (PDMS) having an array of microhole structures of 90 μm diameter and 50 μm depth on its surface. All the microhole structures were equipped with a 1-μm-thick optical chemical sensing layer of platinum porphyrin-fluoropolymer on their bottom. The three-dimensional microstructures of the sensor were fabricated by a newly developed simple and low-cost production method named self-aligned hot embossing. The device was designed to be attached slightly above the cells cultivated on a dish to form a temporarily closed microspace over the target cells during measurement. Since the change in oxygen concentration is relatively fast in the microcompartmentalized culture medium, a rapid evaluation of the oxygen consumption rate is possible by measuring the phosphorescence lifetime of the platinum porphyrin-fluoropolymer. The combined use of the device and an automated optical measurement system enabled the high-throughput sensing of cellular oxygen consumption (100 points/min). We monitored the oxygen metabolism of the human breast cancer cell line MCF7 on a Petri dish and evaluated the oxygen consumption rate to be 0.72 ± 0.12 fmol/min/cell. Furthermore, to demonstrate the utility of the developed sensing system, we demonstrated the mapping of the oxygen consumption rate of rat brain slices and succeeded in visualizing a clear difference among the layer structures of the hippocampus, i.e., the cornu ammonis (CA1 and CA3) and dentate gyrus (DG).     

On line estimation of oxygen uptake rate for insect cells growing in a modified spinner flask

The simple design of traditional spinner flasks makes the on-line estimation of cellular metabolism impossible. An on-line estimation system has been developed and used for the monitoring of oxygen uptake rate (OUR) for insect cells growing in a modified spinner flask. Neglect of oxygen desorption from culture media is a common source of error in OUR measurements for Sf21 cells. Therefore, an algorithm was developed to compensate for the affect of such desorption process on the determination of OUR. A modified spinner flask was successfully used as a low-volume bioreactor for insect cell cultivation and the OUR measurement developed here is both convenient and reliable.     

A sensitive measurement of oxygen uptake rate using a optochemical sensor

Summary The measurement of the specific oxygen uptake rate (OUR) of slow growing organisms using a small sample size is often hampered by the consumption of oxygen by the electrode used. Using a optochemical sensor we measured the OUR of carrot embryos with approximately 1000 cell clusters and of hybridoma with approximately 10⁶ cells. An OUR as low as 0.02 mol/h can be accurately measured.     

Hydrogel-perfluorocarbon composite scaffold promotes oxygen transport to immobilized cells

Cell encapsulation provides cells a three-dimensional structure to mimic physiological conditions and improve cell signaling, proliferation, and tissue organization as compared to monolayer culture. Encapsulation devices often encounter poor mass transport, especially for oxygen, where critical dissolved levels must be met to ensure both cell survival and functionality. To enhance oxygen transport, we utilized perfluorocarbon (PFC) oxygen vectors, specifically perfluorooctyl bromide (PFOB) immobilized in an alginate matrix. Metabolic activity of HepG2 liver cells encapsulated in 1% alginate/10% PFOB composite system was 47-104% higher than alginate systems lacking PFOB. A cubic model was developed to understand the oxygen transport mechanism in the alginate/PFOB composite system. The theoretical flux enhancement in alginate systems containing 10% PFOB was 18% higher than in alginate-only systems. Oxygen uptake rates (OURs) of HepG2 cells were enhanced with 10% PFOB addition under both 20% and 5% O2 boundary conditions, by 8% and 15%, respectively. Model predictions were qualitatively and quantitatively verified with direct experimental OUR measurements using both a perfusion reactor and oxygen sensing plate, demonstrating a greater OUR enhancement under physiological O2 boundary conditions (i.e., 5% O2). Inclusion of PFCs in an encapsulation matrix is a useful strategy for overcoming oxygen limitations and ensuring cell viability and functionality both for large devices (>1 mm) and over extended time periods. Although our results specifically indicate positive enhancements in metabolic activity using the model HepG2 liver system encapsulated in alginate, PFCs could be useful for improving/stabilizing oxygen supply in a wide range of cell types and hydrogels.     

Development of a bioartificial pancreas: II. Effects of oxygen on long-term entrapped betaTC3 cell cultures

Tissue-engineered pancreatic constructs based on immunoisolated, insulin-secreting cells are promising in providing an effective, relatively inexpensive, long-term treatment for type I (insulin-dependent) diabetes. An in vitro characterization of construct function under conditions mimicking the in vivo environment is essential prior to any extensive animal experimentation. Encapsulated cells may experience hypoxic conditions postimplantation as a result of one or more of the following: the design of the construct; the environment at the implantation site; or the development of fibrosis around the construct. In this work, we studied the effects of 3- and 4-day-long hypoxic episodes on the metabolic and secretory activities and on the levels of intracellular metabolites detectable by phosphorus-31 nuclear magnetic resonance ((31)P NMR) of alginate/poly-L-lysine/alginate entrapped betaTC3 mouse insulinomas continuously perfused with culture medium. Results show that, upon decreasing the oxygen concentration in the surrounding medium, the encapsulated cell system reached a new, lower metabolic and secretory state. Hypoxia drove the cells to a more anaerobic glycolytic metabolism, increased the rates of glucose consumption (GCR) and lactate production (LPR), and reduced the rates of oxygen consumption (OCR) and insulin secretion (ISR). Furthermore, hypoxia reduced the levels of intracellular nucleotide triphosphates (NTP) and phosphorylcholine (PC) and caused a rapid transient increase in inorganic phosphate (P(i)). Upon restoration of the oxygen concentration in the perfusion medium, all parameters returned to their prehypoxic levels within 2 to 3 days following either gradual unidirectional changes (ISR, NTP, PC) or more complicated dynamic patterns (OCR, GCR, LPR). A further increase in oxygen concentration in the perfusion medium drove OCR, ISR, NTP, PC, and P(i) to new, higher levels. It is concluded that (31)P NMR spectroscopy can be used for the prolonged noninvasive monitoring of the bioenergetic changes of encapsulated betaTC3 cells occurring with changes in oxygen tension. The data also indicate that the oxygen-dependent states might be related to the total number of viable, metabolically active cells supported by the particular oxygen level to which the system is exposed. These findings have significant implications in developing and non-invasively monitoring a tissue-engineered bioartificial pancreas based on transformed beta cells, as well as in understanding the biochemical events pertaining to insulin secretion from betaTC3 insulinomas.     

Cell cycle- and growth phase-dependent variations in size distribution, antibody productivity, and oxygen demand in hybridoma cultures

Simultaneous determination of cell size and DNA content of hybridomas (HB-32) revealed a direct correlation between average cell volume and progression through the cell cycle. Pseudocontinuous experiments showed that G(1) cells, as estimated from cell size measurements, secreted monoclonal antibody at rates higher than those of cells in other stages of interphase and mitosis. Similarly, fed-batch and batch experiments suggested that specific oxygen uptake rate (qO(2)) is also a function of cell cycle, being minimum for cells in G(0) and G(1) phase. In batch cultures, HB-32 showed a rapid decrease in oxygen uptake rate (OUR) just prior to reaching maximum cell concentration. The OUR steadily increased from 0.01-0.05 to 0.5-0.7 mmol O(2)/L h as the cells went from the lag to the midexponential phase. The qO(2) increased from 0.3 x 10(-10)-0.9 x 10(-10) mmol O(2)/cell h at inoculation to 3.3 x 10(-10)-3.7 x 10(-10) mmol O(2)/cell h during the early exponential phase where it remained relatively constant. Several hours before maximum cell concentration was reached, OUR and qO(2) rapidly decreased to levels below those observed at inoculation. The time at which the shift in OUR and qO(2) occurred and the onset of decrease in the average cell size corresponded to the time of glutamine depletion. Based on monitoring OUR on-line in batch cultures, glutamine was supplemented, resulting in increased cell concentration, extension of culture viability, and increased MAb concentration.     

Simulation of oxygen carrier mediated oxygen transport to C3A hepatoma cells housed within a hollow fiber bioreactor

A priori knowledge of the dissolved oxygen (O2) concentration profile within a hepatic hollow fiber (HF) bioreactor is important in developing an effective bioartificial liver assist device (BLAD). O2 provision is limiting within HF bioreactors and we hypothesize that supplementing a hepatic HF bioreactor's circulating media with bovine red blood cells (bRBCs), which function as an O2 carrier, will improve oxygenation. The dissolved O2 concentration profile within a single HF (lumen, membrane, and representative extra capillary space (ECS)) was modeled with the finite element method, and compared to experimentally measured data obtained on an actual HF bioreactor with the same dimensions housing C3A hepatoma cells. Our results (experimental and modeling) indicate bRBC supplementation of the circulating media leads to an increase in O2 consumed by C3A cells. Under certain experimental conditions (pO2,IN) = 95 mmHg, Q = 8.30 mL/min), the addition of bRBCs at 5% of the average in vivo human red blood cell concentration (% hRBC) results in approximately 50% increase in the O2 consumption rate (OCR). By simply adjusting the operating conditions (pO2,IN) = 25 mmHg, Q = 1.77 mL/min) and increasing bRBC concentration to 25% hRBC the OCR increase is approximately 10-fold. However, the improved O2 concentration profile experienced by the C3A cells could not duplicate the full range of in vivo O2 tensions (25-70 mmHg) typically experienced within the liver sinusoid with this particular HF bioreactor. Nonetheless, we demonstrate that the O2 transport model accurately predicts O2 consumption within a HF bioreactor, thus setting up the modeling framework for improving the design of future hepatic HF bioreactors.     

Oxygen consumption rate in rotifers

The oxygen consumption rate (OCR) is a cumulative index of metabolic losses during aerobic metabolism. The generalized relationship of oxygen consumption rate (R, n1 O₂ ind^–1 h^–1) and dry body mass (M, µg) for rotifers is described by the equation: R = 9.15M^0.716. The level of rotifer metabolism is slightly lower than that of multicellular poikilothermic animals. Differences of OCR values in ontogenesis are substantial. Embryos and senile individuals are characterized by minimal OCR values. The OCR of oviparous females in the beginning of reproduction exceeds 2–3 times OCR values of juveniles. Differences in oxygen consumption intensity (OCI) are not so essential. OCR depends on food concentration. An increase of food concentration from 1.4 to 7.0 µg dry mass m1^–1 resulted in Brachionus calyciflorus in an OCR escalation of 2.5 times at 30°C, and 0.5 times at 25°C. Maximal OCR values occur at food concentration close to the saturation concentration for population growth rate. An exponential equation is adequate to describe R-t dependence for animals, long-term adapted to different constant temperatures (2 < Q₁₀ < 3). Acclimation effects observed during sharp temperature changes are determined by peculiarities of compensation reactions in species and separate populations. The formation of a zone of relative temperature independence of OCR (Q¹⁰ 1) at fluctuating temperature is observed. It is necessary to study enzymatic activities parallel to OCR and OCI measurements.     

Lipopolysaccharide-induced alterations in oxygen consumption and radical generation in endothelial cells

Oxygen consumption rate (OCR) and generation of superoxide and nitric oxide (NO) in mouse aortic endothelial cells (MAECs) treated with lipopolysaccharide (LPS) were studied. The OCR was determined in cell suspensions at 37 °C by electron paramagnetic resonance (EPR) spectroscopy. LPS significantly altered the OCR in a dose and time-dependent fashion. The OCR was significantly elevated immediately following the treatment of MAECs with LPS (5 and 10 μg/ml) and NADPH (100 μM) whereas the same was depressed 1 h after exposure to similar conditions of incubation. Under similar experimental conditions, superoxide generation was also determined by EPR spectroscopy and cytochrome c reduction assays. A marginal increase in the superoxide production was observed when the cells were treated with LPS and NADPH alone whereas the same was further enhanced significantly when the cells were treated with LPS and NADPH together. The increase in oxygen consumption and superoxide production caused by LPS was inhibited by diphenyleneiodonium (DPI), suggesting the involvement of NAD(P)H oxidase. A significant increase in the NO production by MAECs was noticed 1 h after treatment with LPS and was inhibited by L-NAME, further suggesting the involvement of nitric oxide synthase (NOS). Thus, on a temporal scale, LPS-induced alterations in oxygen consumption by MAECs may be under the control of dual regulation by NAD(P)H oxidase and NOS. (Mol Cell Biochem 278: 119–127, 2005)     

Biodegradation of tannery wastewater using sequencing batch reactor--respirometric assessment

This investigation proved that respirometry combined with sequencing batch reactor (SBR) could be an effective way for the removal of COD in tannery wastewater. Measurement of oxygen uptake rates (OUR) and corresponding COD uptake rates showed that a 12-h operating cycle was optimum for tannery wastewater. The removal of COD by degradation was stoichiometric with oxygen usage. A plot of OUR values provided a good indication of the biological activity in the reactor. A high OUR value corresponded to the feed period; at the end of the cycle, when the substrate was depleted, the OUR value was low. At a 12-h SBR cycle with a loading rate of 1.9-2.1 kgm(-3) d(-1), removal of 80-82% COD, 78-80% TKN and 83-99% NH(3)-N were achieved. These removal efficiencies were much higher than the conventional aerobic systems. A simple method of COD fractionation was performed from the OUR and COD uptake rate data of the SBR cycle. About 66-70% of the influent COD was found to be readily biodegradable, 10-14% was slowly degradable and 17-21% was non-biodegradable. The oxygen mass transfer coefficient, K(L)a (19 +/- 1.7 h(-1)) was derived from respirometry. It was observed that with the exception of high organic load at the initial feed the oxygen transfer capacity was in excess of the OUR, and aerobic condition was generally maintained. Simultaneous nitrification-denitrification was observed in the SBR during the feed period as proved by mass balance.     

Comparison of the three optical platforms for measurement of cellular respiration

We compared three optical platforms for measurement of cellular respiration: absolute oxygen consumption rates (OCRs) in hermetically sealed microcuvettes, relative OCRs measured in a 96-well plate with oil seal, and steady-state oxygenation of cells in an open 96-well plate. Using mouse embryonic fibroblasts cell line, the phosphorescent intracellular O₂ probe MitoXpress-Intra, and time-resolved fluorescence reader, we determined algorithms for conversion of relative OCRs and cell oxygenation into absolute OCRs, thereby allowing simple high-throughput measurement of absolute OCR values.     

Planktonic microbial community oxygen consumption rate in Cananéia waters (25°S 48°W), Brazil

Water samples were taken monthly in 1976 at three sites along the Cananéia estuarine-lagoon system (25°S 48°W), on the southern coast of São Paulo State, Brazil. After screening through a 100 μm mesh silk screen, samples from two sites were size fractionated in classes between 100–50 μm, 50–10 μm and 10–0.45 μm and, concentrated. Oxygen consumption rates (OCR) of the fractionated and unfractionated surface samples were estimated after incubating in dark bottles at ‘in situ’ temperature. OCR varied from 0.26±0.95 to 41.61±2.56 μl O₂ l^?1 h^?1. Highest OCR was observed in summer, at the site under fresh-water influence. At this site, for most of the year, the smallest size category contributed mostly to total OCR. Bacteria attached to particulate matter appear to be responsible for most of the oxygen consumption measured. At the more saline waters, OCR was found to be associated with organisms in the 10–100 μm size class. At this site, size analysis of chlorophylla showed that the 0.45–10 μm size category comprised the major portion of total chlorophyll. The lack of association between the OCR dominant size class and the prevailing phytoplankton and bacteria size categories, suggests the oxygen consumption to be due to other organisms, probably nano- and microzooplankton. OCR measured in the unfractionated samples, at site 2, showed the same order of magnitude to those of the fractionated ones. At this site the importance of the bacterial metabolism in relation to the total OCR measured is discussed. It is concluded that in the surface waters of the Cananéia estuarine lagoon system the contribution of bacterial metabolism to total oxygen consumption differs along the system, being significant at the upper reaches of the estuary, where the inputs of organic matter are larger. In saline waters, in spite of high bacterial densities, oxygen consumption seems to reflect the metabolism of larger organisms.     

Fluorescence-based cell viability screening assays using water-soluble oxygen probes

A simple luminescence-based assay for screening the viability of mammalian cells is described, based on the monitoring of cell respiration by means of a phosphorescent water-soluble oxygen probe that responds to changes in the concentration of dissolved oxygen by changing its emission intensity and lifetime. The probe was added at low concentrations (0.3 microM to 0.5 nM) to each sample containing a culture of cells in the wells of a standard 96-well plate. Analysis of oxygen consumption was initiated by applying a layer of mineral oil on top of each sample followed by monitoring of the phosphorescent signal on a prompt or time-resolved fluorescence plate reader. Rates of oxygen uptake could be determined on the basis of kinetic changes of the phosphorescence (initial slopes) and correlated with cell numbers (10(5) to 10(7) cells/mL for FL5.12 lymphoblastic cell line), cell viability, or drug/effector action using appropriate control samples. The assay is cell noninvasive, more simple, robust, and cost-effective than existing microplate-based cell viability assays; is compatible with existing instrumentation; and allows for high-throughput analysis of cell viability.     

Dissolved oxygen as principal parameter for conidia production of biocontrol fungi Trichoderma viride in non-Newtonian wastewater

Dissolved oxygen (DO) concentration was selected as a principal parameter for translating results of shake flask fermentation of Trichoderma viride (biocontrol fungi) to a fermenter scale. All fermentations were carried out in a 7.5 l automated fermenter with a working volume of 4 l. Fermentation performance parameters such as volumetric oxygen transfer coefficient (k _L a), oxygen uptake rate (OUR), rheology, conidia concentration, glucose consumption, soluble chemical oxygen demand, entomotoxicity and inhibition index were measured. The conidia concentration, entomotoxicity and inhibition index were either stable or improved at lower DO concentration (30%). Variation of OUR aided in assessing the oxygen supply capacity of the fermenter and biomass growth. Meanwhile, rheological profiles demonstrated the variability of wastewater during fermentation due to mycelial growth and conidiation. In order to estimate power consumption, the agitation and the aeration requirements were quantified in terms of area under the curves, agitation vs. time (rpm h), and aeration vs. time (lpm h). This simple and novel strategy of fermenter operation proved to be highly successful which can be adopted to other biocontrol fungi.     

On-line gas analysis in animal cell cultivation: I. Control of dissolved oxygen and pH

To monitor gas reaction rates in animal cell culture at constant dissolved oxygen concentration (DO) and constant pH it was necessary to develop improved control methods. Decoupling of both controllrs was obtained by manipulation of molar fractions of oxygen and carbon dioxide in the gas phase. Two pairs of DO and pH controllers were designed and tested both in simulation and exprimental runs. The first controller pair was developed for headspace aeration only, whereas the second controller pair was designed for bubble aeration using a microsparger and flushing the headspace with helium. pH was controlled by a conventional discrete PID controller in its velocity form. For DO control two linear state space feedback controllers with parameter adaptation were established. In these controllers the oxygen uptake rate (OUR) was considered as a disturbance and was not included in the mathematical model. The feedback gain adaptation was based on the difference between the actual molar fraction of oxygen at time step n and the initial molar fraction. This difference is related to OUR and was used to increase or decrease the state feedback controller gain (k and k(1), respectively) in a slow manner. With these controllers it was possible to get an excellent online estimate of OUR. In the case of bubble aeration a simple gas phase mass balance was sufficient, whereas during the headspace aeration a liquid phase balance was required. It has been shown that determination of OUR using gas balance requires a significantly better controller performance compared to just keeping DO and pH within reasonable limits. (c) 1995 John Wiley & Sons, Inc.