Effect of polydimethylsiloxane oxygen carriers on the biological bleaching of hardwood kraft pulp by Trametes versicolor

Summary Improving the availability of oxygen by adding polydimethylsiloxanes (PDMS) oxygen carriers to Trametes versicolor cultures increased pulp brightening. The presence of the oxygen carriers in cultures of T. versicolor with hardwood kraft pulp increased the growth rate of the fungus, but not the ultimate biomass yield. The PDMS also stimulated brightening of hardwood kraft pulp by it T. versicolor immobilized in polyurethane foam. A threefold increase in the oxygen uptake rate in T. versicolor cultures with PDMS was observed. This increase can be explained by elevated oxygen transfer rate and attributed to the surfactant properties of PDMS. Offprint requests to: E. ZiomekIssued as NRCC 32760     

Physical and physiological impacts of different foam control strategies during a process involving hydrophobic substrate for the lipase production by Yarrowia lipolytica

The potentialities for the intensification of the process of lipase production by the yeast Yarrowia lipolytica on a renewable hydrophobic substrate (methyl oleate) have to be investigated. The key factor governing the lipase yield is the intensification of the oxygen transfer rate, considering the fact that Y. lipolytica is a strict aerobe. However, considering the nature of the substrate and the capacity for protein excretion and biosurfactant production of Y. lipolytica, intensification of oxygen transfer rate is accompanied by an excessive formation of foam. Two different foam control strategies have thus been implemented: a classical chemical foam control strategy and a mechanical foam control (MFM) based on the Stirring As Foam Disruption principle. The second strategy allows foam control without any modifications of the physico-chemical properties of the broth. However, the MFM system design induced the formation of a persistent foam layer in the bioreactor. This phenomenon has led to the segregation of microbial cells between the foam phase and the liquid phase in the case of the bioreactors operated with MFM control, and induced a reduction at the level of the lipase yield. More interestingly, flow cytometry experiments have shown that the residence time of microbial cells in the foam phase tends to induce a dimorphic transition which could potentially explain the reduction of lipase excretion.     

Oxygen effect on batch cultures of Leuconostoc mesenteroides: relationship between oxygen uptake,growth and end-products

Growth and lactose metabolism of a Leuconostoc mesenteroides strain were studied in batch cultures at pH 6.5 and 30° C in 101 modified MRS medium sparged with different gases: nitrogen, air and pure oxygen. In all cases, growth occurred, but in aerobiosis there was oxygen consumption, leading to an improvement of growth yield Y _x/s and specific growth rate compared to anaerobiosis. Whatever the extent of aerobic growth, oxygen uptake and biomass production increased with the oxygen transfer rate so that the oxygen growth yield, Y _x/o2, remained at a constant value of 11 g dry weight of biomass/mol oxygen consumed. Pure oxygen had a positive effect on Leuconostoc growth. Oxygen transfer was limiting under air, but pure oxygen provided bacteria with sufficient dissolved oxygen and leuconostocs were able to consume large amounts of oxygen. Acetate production increased progressively with oxygen consumption so that the total molar concentration of acetate plus ethanol remained constant. Maximal Y _x/s was obtained with a 120 l/h flow rate of pure oxygen: the switch from ethanol to acetate was almost complete. In this case, a 46.8 g/mol Y _x/s and a 0.69 h^–1 maximal growth rate could be reached.     

Batch kinetics and oxygen consumption ofCandida lipolytica 4-1 onn-alkanes

Detailed batch kinetics ofCandida lipolytica 4-1 onn-hexadecane for varying dispersed phase volume from 0.5 to 5% v/v is presented. All batch growth curves exhibited a linear growth region, indicating a substrate uptake limit. System productivities derived from the linear region were correlated to the dispersed phase volume. The correlation coefficient was identical with that obtained on gas oil. This implies that a correlation coefficient of interfacial area to the dispersed phase volume is identical for both systems. Dissolved oxygen profile and uptake of oxygen from gas phase were also measured. The oxygen uptake rate, volumetric oxygen transfer rate and oxygen demand (requirement) were calculated by means of the balance method. Under limiting dissolved oxygen concentration the maximal oxygen transfer of the fermenter was assessed.     

Mass cultivation of Medicago sativa growing on lactose: Kinetic aspects

Summary Batch cultures of Medicago sativa cells have been carried out in the dark under aerobic conditions using lactose as the sole carbon source. The stoichiometric analysis has been correlated with both the oxygen demand and the cell productivity in an oxygen-limited cultivation. The minimum oxygen transfer has been estimated to be 12.5 h^–1, i.e., 0.3 v.v.m; this initial aeration rate led to cell necrosis. Starting with a low oxygen transfer coefficient k_L·a and increasing the air flow rate during the course of fermentation gave an exponential growth phase. The maximum specific growth rate was 0.012 h^–1 and the growth yield was 0.43 g.d.w./g. of lactose. On the basis of the mass-balance relation the maintenance coefficient and the maximum growth yield have been calculated.     

Oxygen requirement in pullulan fermentation

Summary Oxygen was essential for the biosynthesis of pullulan by Aureobasidium pullulans. In a growth medium, pullulan yield and synthesis rate were proportional to the oxygen availability. However, under controlled oxygen environment in a non-growth medium, the synthesis rate and the yield of pullulan were inversely proportional to the oxygen tension. A relationship between melanin production and oxygen transfer conditions was also observed. The elapsed time prior to the appearance of the pigment was dependent upon the degree of oxygen availability.     

Characterization and improvement of oxygen transfer in pilot plant external air-lift bioreactor for mycelial biomass production

The oxygen transfer dynamics in a pilot plant external air-lift bioreactor (EALB) during the cultivation of mycelial biomass were characterized with respect to hydrodynamic parameters of gas holdup (), oxygen transfer coefficient (K_La) and superficial gas velocity (U _g), and dissolved oxygen (DO). An increased flow rate of air supply was required to meet the increased oxygen demand with mycelial biomass growth. Consequently, an increase in air flow rate led to an increase in , K_La and the DO level. The enhancement of oxygen transfer rate in the cultivated broth system, however, was limited with highly increased viscosity of the mycelial broth. An increase in air flow rate from 1.25 to 2.00 v/v/m resulted in a low increment of oxygen transfer. The newly designed pilot plant EALB with two air spargers significantly improved processing reliability, aeration rate and K_La. The pilot plant EALB process, operated under a top pressure from 0 to 1.0 bars, also demonstrated a significant improvement of oxygenation efficiency by more than 20% in DO and K_La. The performance of the two sparger EALB process under top pressure demonstrated an efficient and economical aerobic system with fast mycelial growth and high biomass productivity in mycelial biomass production and wastewater treatment.     

Substrate availability in phenanthrene biodegradation: Transfer mechanism and influence on metabolism

 The mechanism of phenanthrene transfer to the bacteria during biodegradation by a Pseudomonas strain was investigated using a sensitive respirometric technique (Sapromat equipment) allowing the quasi-continuous acquisition of data on oxygen consumption. Several systems of phenanthrene supply, crystalline solid and solutions in non-water-miscible solvents (silicone oil and 2,2,4,4,6,8,8-heptamethylnonane) were studied. In all cases, analysis of the kinetics of oxygen consumption demonstrated an initial phase of exponential growth with the same specific growth rate. In order to analyze the second phase of growth and phenanthrene degradation, a study of the kinetics of phenanthrene transfer to the aqueous phase was conducted by direct experimentation, with the crystal and silicone oil systems, in abiotic conditions. The data allowed the validation of a model based on phase-transfer laws, describing the variations, with substrate concentrations, of rates of phenanthrene transfer to the aqueous phase. Analysis of the biodegradation curves then showed that exponential growth ended in all cases when the rates of phenanthrene consumption reached the maximal transfer rates. Thereafter, the biodegradation rates closely obeyed, for all systems, the transfer rate values given by the model. These results unambiguously demonstrated that, in the present case, phenanthrene biodegradation required prior transfer to the aqueous phase. With the silicone oil system, which allowed high transfer and biodegradation rates, phenanthrene was directed towards higher metabolite production and lower mineralization, as shown by oxygen consumption and carbon balance determinations. Received: 30 November 1994/Accepted: 11 January 1995     

Citric acid production by Aspergillus niger immobilized on polyurethane foam

Summary Citric acid was produced using Aspergillus niger immobilized on polyurethane foam in a bubble column reactor. Most of the adsorbed cells remained on the support and, as a result, high oxygen tension was maintained during the reactor operation. However, uncontrolled growth of the pellets made continuous reactor operation difficult. The citric acid productivity obtained from 15 vol.% foam particles containing immobilized cells was 0.135 g/l per hour. This productivity of immobilized cells was almost the same as that of free cells. The oxygen level dropped to half saturation in 5 days in the immobilized cell culture in contrast to 2 days in the free cell culture.     

Effect of chemical defoaming agents on the mass exchange rate in the oxytetracycline biosynthesis process

The effect of chemical foam suppressors of various classes on the intensity of Streptomyces rimosus respiration, concentrations of dissolved oxygen and carbon dioxide and gas levels in the fermentation broth of oxytetracycline was studied. Its was shown that addition of the foam suppressors, as a rule, resulted in a decreased rate of the oxygen mass transfer due to the decreased surface of the phase contact gas-liquid. This decrease was not so significant as that in case of uninhibited intensity of the fermentation broth foaming. A different character of the effect of different foam suppressors on the biosynthetic process was noted. The effect of the foam suppressors was decreased by their regular addition in low amounts.     

Production of surfactin and fengycin by Bacillus subtilis in a bubbleless membrane bioreactor

Surfactin and fengycin are lipopeptide biosurfactants produced by Bacillus subtilis. This work describes for the first time the use of bubbleless bioreactors for the production of these lipopeptides by B. subtilis ATCC 21332 with aeration by a hollow fiber membrane air–liquid contactor to prevent foam formation. Three different configurations were tested: external aeration module made from either polyethersulfone (reactor BB1) or polypropylene (reactor BB2) and a submerged module in polypropylene (reactor BB3). Bacterial growth, glucose consumption, lipopeptide production, and oxygen uptake rate were monitored during the culture in the bioreactors. For all the tested membranes, the bioreactors were of satisfactory bacterial growth and lipopeptide production. In the three configurations, surfactin production related to the culture volume was in the same range: 242, 230, and 188 mg l⁻¹ for BB1, BB2, and BB3, respectively. Interestingly, high differences were observed for fengycin production: 47 mg l⁻¹ for BB1, 207 mg l⁻¹ for BB2, and 393 mg l⁻¹ for BB3. A significant proportion of surfactin was adsorbed on the membranes and reduced the volumetric oxygen mass transfer coefficient. The degree of adsorption depended on both the material and the structure of the membrane and was higher with the submerged polypropylene membrane.     

Foaming and cell flotation in suspended plant cell cultures and the effect of chemical antifoams

Foam development and stability in Atropa belladonna suspensions were investigated as a function of culture conditions. Foaming was due mainly to properties of the cell-free broth and was correlated with protein content; effects due to presence of cells increased towards the end of batch culture. Highest foam levels were measured 11 days after inoculation. Air flow rate was of major importance in determining foam volume; foam volume and stability were also strongly dependent on pH. Foam flotation of plant cells was very effective. After 30 min foaming, ca. 55% of cells were found in the foam; this increased to ca. 75% after 90 min. Polypropylene glycol 1025 and 2025, Pluronic PE 6100, and Antifoam-C emulsion were tested as chemical antifoams. Polypropylene glycol 1025 and Antifoam C at concentrations up to 600 ppm had no adverse effect on growth in shake flasks; Pluronic PE 6100 has an inhibitory effect at all levels tested. Concentrations of polypropylene glycol 2025 and Pluronic PE 6100 as low as 20 ppm reduced foam volumes by a factor of ca. 10. Addition of antifoam reduced k(L)a values in bubble-column and stirred-tank bioreactors. After operation of a stirred reactor for 2 days using Antifoam C for foam control, cell production was limited by oxygen due to the effect of antifoam on mass transfer. Theoretical analysis showed that maximum cell concentrations and biomass levels decline with increasing reactors working volume due to greater consumption of antifoam to prevent foam overflow. The results indicate that when chemical foam control is used in plant cell cultures, head-space volume and tolerable foam levels must be considered to optimize biomass production. (c) 1994 John Wiley & Sons, Inc.     

Oxygen transfer rate,respiration and yields in batch and chemostat cultures ofKlebsiella aerogenes

The effect was studied of oxygen supply on the changes in total and specific rate of oxygen consumption by the cells, oxygen transfer rate, saturation concentrations of dissolved oxygen and the yields of batch and continuous cultivations. Experiments were done on the microorganismKlebsiella aerogenes CCM 2318 growing on synthetic glucose medium. Continuous cultivations were carried out at dilution rates of 0.96 and 0.178 h⁻¹. The rate of oxygen transfer was determined by the sulphite method and the coefficient K_La was assessed using the dynamic method with a correction for changes in the saturations of dissolved oxygen. A lowered oxygen supply in batch cultivations caused deformations in the course of cell respiration. Comparison of results of batch and continuous cultivations showed that the highest yields Y_x/s and Y_x/o are attained at low dilution rates without oxygen limitation. Batch cultivations, on the other hand, exhibit the lowest yields and the highest cell respiration levels. In both types of cultivations, a respiration peak was ascertained under the conditions of growth limitation by oxygen.     

Ligninolytic enzyme production by Phanerochaete chrysosporium immobilized on different carriers

In this study, several different carriers were employed in a Phanerochaete chrysosporium BVH-F-1767 cell immobilization study. Polystyrene foam was shown to be the optimum carrier material from organism biomass measurements and maximum MnP production (915.62 U L(-1)). The maximum MnP activity of polystyrene foam system was achieved 2-5 days sooner than with the other carrier systems studied. It was thus clear that the polystyrene foam approach shortened the culture cycle. Analysis of the carrier mechanisms employed in this study revealed that polystyrene foam had larger internal spaces and a greater surface area, and thus the potential to enhance the transfer efficiency of oxygen and nutrients to the fungus and accelerate its growth. The mycelia of the fungus were able to associate closely with the unique internal pore structure of the polystyrene foam, providing a more quiescent microenvironment and helping to maintain the stability of the cultivation system.     

Influence of dissolved oxygen concentration on the oxygen kinetics of Gluconobacter oxydans

Summary As shown in earlier studies in production scale bioreactors oxygen limited zones occur. Microorganisms in these reactors are therefore subjected to concentrations of oxygen varying with time. To simulate these conditions, the effect of low oxygen concentrations upon product formation and kinetics of oxygen of Gluconobacter oxydans are studied at laboratory scale.Under these oxygen limited conditions comparable kinetic parameters for oxygen are observed as under normally aerated conditions.So, a saturation constant for oxygen K _{O 2}=6.9 mol/l is observed, which is equivalent to a DOT value of about 3% of air saturation.For optimization purposes of production scale conditions, gassing with oxygen enriched air or with pure oxygen is one of the possibilities.To study the effect of high oxygen concentrations upon kinetics and product formation, the organisms are also cultivated under these extreme conditions. Although at oxygen concentrations larger then 60% saturation with pure oxygen, still growth was observed, the growth rate and also the product formation rate were strongly diminished.From these experiments it can be concluded that gassing with pure oxygen to achieve higher oxygen transfer rates at production scale will be restricted.     

The oxygen transfer rate as key parameter for the characterization of<Emphasis Type="Italic"> Hansenula polymorpha</Emphasis> screening cultures

Screening cultures are usually non-monitored and non-controlled due to a lack of appropriate measuring techniques. A new device for online measurement of oxygen transfer rate (OTR) in shaking-flask cultures was used for monitoring the screening of Hansenula polymorpha. A shaking frequency of 300 rpm and a filling volume of 20 ml in 250-ml flasks ensured a sufficient oxygen transfer capacity of 0.032 mol (l h)^–1 and thus a respiration not limited by oxygen. Medium buffered with 0.01 mol phosphate l^–1 (pH 6.0) resulted in pH-inhibited respiration, whereas buffering with 0.12 mol phosphate l^–1 (pH 4.1) resulted in respiration that was not inhibited by pH. The ammonium demand was balanced by establishing fixed relations between oxygen, ammonium, and glycerol consumption with 0.245±0.015 mol ammonium per mol glycerol. Plate precultures with complex glucose medium reduced the specific growth rate coefficient to 0.18 h^–1 in subsequent cultures with minimal glycerol medium. The specific growth rate coefficient increased to 0.26 h^–1 when exponentially growing precultures with minimal glycerol medium were used for inoculation. Changes in biomass, glycerol, ammonium, and pH over time were simulated on the basis of oxygen consumption.     

Influence of oxygen transfer rate on vinegar production by Acetobacter aceti in submerged fermentation

Summary A study was carried out on the influence of oxygen transfer rate on bacterial growth and acid production in submerged vinegar fermentation by an industrial culture of Acetobacter aceti. The production rate grew as did growth rate even if no increment in cellular mass was noticed when oxygen transfer rate was increased. The relationship between growth and production followed a mixed growth associated model.     

Microbial growth rates and local external mass transfer coefficients in a porous bed biofilm system measured by 19F magnetic resonance imaging of structure,oxygen concentration,and flow velocity

¹⁹F nuclear magnetic resonance (NMR) oximetry and ¹H NMR velocimetry were used to noninvasively map oxygen concentrations and hydrodynamics in space and time in a model packed bed biofilm system in the presence and absence of flow. The development of a local oxygen sink associated with a single gel bead inoculated with respiring Escherichia coli was analyzed with a phenomenological model to determine the specific growth rate of the bacteria in situ, returning a value (0.66 hr⁻¹) that was close to that measured independently in planktonic culture (0.62 hr⁻¹). The decay of oxygen concentration in and around the microbiologically active bead was delayed and slower in experiments conducted under continuous flow in comparison to no-flow experiments. Concentration boundary layer thicknesses were determined and Sherwood numbers calculated to quantify external mass transfer resistance. Boundary layers were thicker in no-flow experiments compared to experiments with flow. Whereas the oxygen concentration profile across a reactive biofilm particle was symmetric in no-flow experiments, it was asymmetric with respect to flow direction in flow experiments with Sherwood numbers on the leading edge (Sh = 7) being larger than the trailing edge (Sh = 3.5). The magnitude of the experimental Sh was comparable to values predicted by a variety of correlations. These spatially resolved measurements of oxygen distribution in a geometrically complex model reveal in innovative detail the local coupling between microbial growth, oxygen consumption, and external mass transfer.