Biofuel Cells Select for Microbial Consortia That Self-Mediate Electron Transfer

Microbial fuel cells hold great promise as a sustainable biotechnological solution to future energy needs. Current efforts to improve the efficiency of such fuel cells are limited by the lack of knowledge about the microbial ecology of these systems. The purposes of this study were (i) to elucidate whether a bacterial community, either suspended or attached to an electrode, can evolve in a microbial fuel cell to bring about higher power output, and (ii) to identify species responsible for the electricity generation. Enrichment by repeated transfer of a bacterial consortium harvested from the anode compartment of a biofuel cell in which glucose was used increased the output from an initial level of 0.6 W m⁻² of electrode surface to a maximal level of 4.31 W m⁻² (664 mV, 30.9 mA) when plain graphite electrodes were used. This result was obtained with an average loading rate of 1 g of glucose liter⁻¹ day⁻¹ and corresponded to 81% efficiency for electron transfer from glucose to electricity. Cyclic voltammetry indicated that the enhanced microbial consortium had either membrane-bound or excreted redox components that were not initially detected in the community. Dominant species of the enhanced culture were identified by denaturing gradient gel electrophoresis and culturing. The community consisted mainly of facultative anaerobic bacteria, such as Alcaligenes faecalis and Enterococcus gallinarum, which are capable of hydrogen production. Pseudomonas aeruginosa and other Pseudomonas species were also isolated. For several isolates, electrochemical activity was mainly due to excreted redox mediators, and one of these mediators, pyocyanin produced by P. aeruginosa, could be characterized. Overall, the enrichment procedure, irrespective of whether only attached or suspended bacteria were examined, selected for organisms capable of mediating the electron transfer either by direct bacterial transfer or by excretion of redox components.     

Amperometric estimation of BOD by using living immobilized yeasts

Summary A microbial electrode consisting of immobilized living whole cells of yeasts, porous membrane and an oxygen electrode was prepared for continuous estimation of biochemical oxygen demand (BOD). Immobilized Trichosporon cutaneum was employed for the microbial electrode sensor for BOD. When a sample solution containing the equivalent amount of glucose and glutamic acid was injected into the sensor system, the current of the electrode decreased markedly with time until steady state was reached. The response time was within 18 min. A linear relationship was observed between the current decrease and the concentration below 41 mg l ^– of glucose and 41 mg l ^– glutamic acid (5-day BOD 60 mg l ^–). The current decrease was reproducible within ± 6% of the relative error when a sample solution containing 27 mg l ^– of glucose and 27 mg l ^– of glutamic acid (5-day BOD 40 mg l ^–) was employed. The microbial electrode sensor was applied to untreated waste waters from a fermentation factory. Good comparative results were obtained between BOD estimated by the microbial electrode and that determined by the conventional 5-day method (regression coefficient was 1.2). Furthermore, the effect of various compounds on BOD estimation was also examined. The current output of the microbial electrode sensor was almost constant for 17 d and 400 tests.     

Removal of selenite from wastewater using microbial fuel cells

Simultaneous electricity generation and selenium removal was evaluated in single-chamber microbial fuel cells (MFCs) with acetate and glucose as carbon sources. Power output was not affected by selenite up to 125 mg l⁻¹ with glucose as substrate. Coulombic efficiencies of MFCs with glucose increased from 25% to 38% at 150 mg Se l⁻¹. About 99% of 50 and 200 mg Se l⁻¹ selenite was removed in 48 and 72 h for MFCs fed with acetate and glucose, respectively, demonstrating the potential of using MFC technology for Se remediation.     

Accumulation of organic matter along a pollution gradient: Application of odum's theory of ecosystem energetics

Forest soil biology in Scots pine forests of the Empetrum-Vaccinium type was studied around the industrialized city of Oulu, northern Finland since 1987. The forest sites lie along a sulphur and nitrogen concentration gradient in the mor humus ranging from 1.6 to 3.9 mg S g^–1 organic matter (OM) and from 14 to 23 mg N g^–1 OM. A number of biological parameters have earlier been found to vary along this gradient, thus indicating that the ecosystems are subjected to a pollution stress. Total microbial biomass and various activity parameters were studied in 1991. The different methods are discussed and the results interpreted within the light of Odum's theory of the energetic stabilization of ecosystems. Microbial biomass C determined by the fumigation-extraction (FE) technique varied from 5 to 10 mg g^–OM, N from 0.5 to 1.0 mg g^–1OM, and basal respiration rate from 0.040 to 0.097 mg CO₂ h^–1 g^–1OM. All decreased along the pollution gradient. Substrate induced respiration values (SIR) varied from 0.025 to 0.085 mg CO₂-C h^–1 g^–1dw. SIR correlated well with the biomass values determined by the FE technique. The lag time of the microbial community after glucose addition (varying from 13 to 22 h) was shortened and the specific respiration increment of the microbial community in response to glucose addition increased along the gradient. The metabolic quotient (respiration/biomass) of the microflora strongly depended on the technique and equation used in calculating the microbial biomass. The results show a reduced biomass, but a more intensive regeneration and intensified activity per biomass unit of microorganisms in polluted forest soil. This in turn denotes an alteration in the microbial community in favor of a higher proportion of r-strategists under the disturbed conditions. In contrast, K-strategists may be more dominant under less polluted conditions. This interpretation is presented with some reservations concerning methodology. There is a need for the calibration of each method for determining microbial biomass in different types of soil.     

Summer abundance and activities of bacteria in the freshwater lakes of Schirmacher Oasis,Antarctica

Bacterial biomass and heterotrophic potential (using ¹⁴C-labeled glucose, glutamic acid and sodium acetate) of water, ice and sediment microbial populations were studied from different lakes of the Schirmacher Oasis, Antarctica. Epifluorescence counts of total bacteria in these lakes were observed to be lower by a factor when compared to some of the ultraoligotrophic Antarctic lakes. Biovolumes of bacteria from different samples did not show significant variations, suggesting that regulatory factors were oligotrophy and low temperatures rather than microzoan grazing. Microbial uptake rates of glutamic acid were generally the fastest, followed by glucose and/or sodium acetate in the lakewater samples. The mean values of V_max cell^–1 for glutamic acid, sodium acetate and glucose were 3.81, 0.91 and 0.71 pgCh^–1. Results of this study are potentially useful in recognizing the relative abundance and activity of limnetic microbial populations in the Schirmacher Oasis during summer — the active period of microbial growth — and for comparing their activities with other ecosystems elsewhere in continental Antarctica.     

Natural and constrainment-induced factors influencing the breakdown of dogwood and oak leaves

Breakdown rates and microbial colonization patterns of dogwood and oak leaves were measured between November and June of 1987–88 and 1988–89. Leaves were placed in artificial streams loose (unconstrained), in bags, or in packs. Discharge was maintained at approximately 0.25 s^–1, and no shredders were present in the streams. Average microbial biomass as ATP, for all species and treatments, increased from near 0 mg g^–1 AFDW in November to over 8 mg g^–1 AFDW in June. Microbial respiration increased from about 0.01 µg glucose respired hr-g^–1 AFDW in November to about 0.03 µg hr-g^–1 AFDW in June. Microbial biomass and activity were significantly greater on dogwood leaves than on oak leaves. Dogwood and oak leaf breakdown rates were fastest when unconstrained, –0.0034 and –0.0027 degree-day^–1 respectively. Breakdown rates of dogwood leaves were faster in bags (–0.0025 degree-day^–1) than in packs (–0.0015 degree-day^–1) while rates of oak leaves were not significantly different between bags and packs (–0.0014 and –0.0018 degree-day^–1 respectively). Breakdown rates of dogwood and oak leaves obtained in this study were much slower than those obtained by other investigators either in the presence or absence of shredders. A comparison of results from this study with results from other studies revealed that dogwood leaves may be affected more by turbulence, while oak leaves may be influenced more by shredder activity.     

Granular activated carbon single-chamber microbial fuel cells (GAC-SCMFCs): A design suitable for large-scale wastewater treatment processes

As an emerging biotechnology capable of removing contaminants and producing electricity, microbial fuel cells (MFCs) hold a promising future in wastewater treatment. However, several main problems, including the high internal resistance (R_in), low power output, expensive material, and complicated configuration have severely hindered the large-scale application of MFCs. The study targeted these challenges by developing a novel MFC system, granular activated carbon single-chamber MFC, termed as GAC-SCMFC. The batch tests showed that GAC was a good substitute for carbon cloth and GAC-SCMFCs generated high and stable power outputs compared with the traditional two-chamber MFCs (2CMFCs). Critical operational parameters (i.e. wastewater substrate concentrations, GAC amount, electrode distance) affecting the performance of GAC-SCMFCs were examined at different levels. The results showed that the R_in gradually decreased from 60 Ω to 45 Ω and the power output increased from 0.2 W/m³ to 1.2 W/m³ when the substrate concentrations increased from 100 mg/L to 850 mg/L. However, at high concentrations of 1000–1500 mg/L, the power output leveled off. The R_in of MFCs decreased 50% when the electrode distance was reduced from 7.5 cm to 1 cm. The highest power was achieved at the electrode distance of 2 cm. The power generation increased with more GAC being added in MFCs due to the higher amount of biomass attached. Finally, the multi-anode GAC-SCMFCs were developed to effectively collect the electrons generated in the GAC bed. The results showed that the current was split among the multiple anodes, and the cathode was the limiting factor in the power production of GAC-SCMFCs.     

The influence of cyclic glucose feeding on a continuous bakers' yeast culture

Conclusions Except for the pronounced adaptation-hysteresis effect, the pulse experiments exhibited the expected trend: deviation from the steady feed reference curve was greatest at lowest dilution rates. Under conditions of lowest glucose level the effect of pulsing would be expected to cause the largest fluctuations in glucose, causing a certain fraction of the cells to ferment. Generally over the entire dilution rate range the biomass production was decreased and the ethanol was increased by pulsing the feed stream. There is, however, some evidence that pulse feeding can trigger quite unexpected results. Point (6) at D=0.3 h^–1 in Fig. 1 exhibit a biomass productivity which was about 20% greater than the continuous feeding reference value (DX=3.6 kg m^–3 h^–1 as compared with 3.0 kg m^–3 h^–1). Such performance would be of significant commercial value, but the poor reproducibility due to adaptation, as seen here, certainly would preclude its application.The results obtained should also be applicable to fed batch operation at the corresponding glucose level. Further experiments including the variation of the glucose feeding period would be necessary to obtain a conclusive picture. The observed phenomena are likely to occur in other fermentations and could eventually explain some of the problems existing with scale up of fermentation processes.Symbols D dilution rate h^–1 - P product (ethanol) concentration kg m^–3 - Q_O2 specific oxygen uptake rate mol kg^–1 s^–1 - Q_CO2 specific CO₂ production rate mol kg^–1 s^–1 - S substrate (glucose) concentration kg m^–3 - X biomass concentration kg m^–3 - Y_P/S yield of ethanol from glucose kg kg^–1 - Y_X/S yield of biomass from glucose kg kg^–1     

Rapid immobilization of applied nitrogen in saline–alkaline soils

The dynamics of inorganic N are important in soil, and this applies particularly to the saline–alkaline soils of the former lake Texcoco in Mexico with high pH and salinity where a forestation program was started in the 1970s. In soils of lake Texcoco, in Mexico, more than 50% of applied N could not be accounted for one day after application of 200 mg kg^–1 soil along with glucose amendment. It was not clear whether this was due to abiotic or biotic processes, the form of inorganic N applied or the result of applying an easily decomposable substrate. We investigated this by adding glucose and 200 mg kg^–1 soil as (NH₄)₂SO₄-N or KNO₃-N to sterilized and unsterilized soil. The changes in inorganic and ninhydrin N, microbial biomass C and production of CO₂ were then monitored. Between the time of applying N and extraction with 0.5 M K₂SO₄, i.e., after ca 2 h, approximately 110 mg NH₄ ⁺-N kg^–1 dry soil could not be accounted for in the unsterilized and sterilized soil and that remained so for the entire incubation in the sterilized soil. After 1 day this increased to 140 mg NH₄ ⁺-N kg^–1 dry soil in the unsterilized control and 170 mg NH₄ ⁺-N kg^–1 dry soil in C amended soil. Volatilization of NH₃ accounted for 56 mg NH₄ ⁺-N kg^–1 so the rest appeared to be adsorbed on the soil matrix. The NH₃ volatilization and NH₄ ⁺ fixed in the soil matrix remained constant over time and no oxidation to NO₂ ^– or NO₃ ^– had occurred, so unaccounted N in unsterilized soil was probably incorporated into the microbial biomass in excess of what was required for metabolic activity. The unaccounted N was ca 70 mg NO₃ ^––N in nitrate amended soil after 3 days and 138 NO₃ ^––N when glucose was additionally added. Losses through abiotic processes were absent as inferred from changes in sterilized soil and the aerobic incubation inhibited possible losses through denitrification. It was inferred that NO₃ ^– that could not be accounted for was taken up by micro-organisms in excess of what was required for metabolic activity.     

Effect of the dilution rate on the biomass yield ofBacillus thuringiensis and determination of its rate coefficients under steady-state conditions

Depending on the biomass yield on glucose and the cell morphology ofBacillus thuringiensis, three different metabolic states were observed in continuous culture. At dilution rates between 0.18 h^–1 and 0.31 h^–1 vegetative cells, sporulating bacteria and spores coexisted, while glucose and amino acids were consumed. Only vegetative cells were observed at dilution rates between 0.42 h^–1 and 0.47 h^–1 and glucose was used as the main carbon and energy source. AtD = 0.50 h^–1 the biomass yield on glucose decreases sharply. To define better the specific growth rate range in which the microorganism uses mainly glucose, a dilution rate of 0.25–0.45 h^–1 was studied. The experimental data could be adjusted to a Monod model and the following rate coefficients and growth yields were determined: maximum specific growth rate 0.54 h^–1, saturation constant 0.56 mg glucose ml^–1, biomass growth yields 0.43 g cells (g glucose)^–1, and 0.76 g cells (g oxygen)^–1, and maintenance coefficients 0.065 g glucose (g cells)^–1 h^–1 and 0.039 g oxygen (g cells)^–1 h^–1.     

Heterotrophic glucose uptake and respiration in Lake Kinneret

The turnover times of glucose, averaged for 0–10 m in the upper waters of Lake Kinneret and measured by the addition of single or multiple concentrations of substrate, ranged from 23 to 188 hours and 1 to 87 hours respectively. Potential uptake rates (estimated as V_max) ranged from 0.095 to 1.94 µg glucose l^–1h^–1, while measured uptake rates varied from 0.09 to 1.1 µg glucose l^–1h^–1. Concentrations of dissolved carbohydrates and glucose averaged 0.71 mg glucose equivalents l^–1 and 39 µg glucose l^–1 respectively. No evident relationships between glucose cycling and any fractions of dissolved organic matter, phytoplankton biomass or primary productivity were found. Turnover times were generally most rapid immediately after the decline of the spring Peridinium bloom. The respiration percentage of incorporated glucose ranged from 25% to 61% with highest values during the summer months. Respiration may be influenced by the nature of the indigenous bacterial population as well as by temperature. Daily heterotrophic glucose carbon uptake was about 9% of the photosynthetic incorporation and could provide a bacterial yield of about 7 × 10⁴ ml^–1d^–1.     

Microbial biomass and nitrogen cycling responses to fertilization and litter removal in young northern hardwood forests

The influence of site fertility on soil microbial biomass and activity is not well understood but is likely to be complex because of interactions with plant responses to nutrient availability. We examined the effects of long-term (8 yr) fertilization and litter removal on forest floor microbial biomass and N and C transformations to test the hypothesis that higher soil resource availability stimulates microbial activity. Microbial biomass and respiration decreased by 20–30 % in response to fertilization. Microbial C averaged 3.8 mg C/g soil in fertilized, 5.8 mg C/g in control, and 5.5 mg C/g in litter removal plots. Microbial respiration was 200 µg CO₂-C g^–1 d^–1 in fertilized plots, compared to 270 µg CO₂-C g^–1 d^–1 in controls. Gross N mineralization and N immobilization did not differ among treatments, despite higher litter nutrient concentrations in fertilized plots and the removal of substantial quantities of C and N in litter removal plots. Net N mineralization was significantly reduced by fertilization. Gross nitrification and NO₃ ^– immobilization both were increased by fertilization. Nitrate thus became a more important part of microbial N cycling in fertilized plots even though NH₄ ⁺ availability was not stimulated by fertilization.Soil microorganisms did not mineralize more C or N in response to fertilization and higher litter quality; instead, results suggest a difference in the physiological status of microbial biomass in fertilized plots that influenced N transformations. Respiration quotients (qCO₂, respiration per unit biomass) were higher in fertilized plots (56 µg CO₂-C mg C^–1 d^–1) than control (48 µg CO₂-C mg C^–1 d ^–1) or litter removal (45 µg CO₂-C mg C^–1 d^–1), corresponding to higher microbial growth efficiency, higher proportions of gross mineralization immobilized, and lower net N mineralization in fertilized plots. While microbial biomass is an important labile nutrient pool, patterns of microbial growth and turnover were distinct from this pool and were more important to microbial function in nitrogen cycling.     

Microbial diversity of Minnesota peatlands

Microbial diversity, numbers, and metabolic activities in Minnesota peatlands were investigated using a variety of microbial enrichment and enumeration procedures together with radioisotopic measurements of microbial degradative processes. Minnesota peatlands were shown to contain large microbial populations of wide metabolic diversity. Direct counts of bacteria using epifluorescence microscopy indicated bacterial populations of about 10⁸ ml^–1 of peatland water, irrespective of depth. Radioisotopic most-probable-number (MPN) counts of heterotrophs able to mineralize¹⁴C-labeled substrates to¹⁴CO₂ showed significant populations of glucose degraders (10⁴–10⁶ ml^–1) as well as degraders of benzoate (10²–10³ ml^–1), 2,4-dichlorophenoxyacetate (10²–10⁵ ml^–1), and sphagnum (10³–10⁷ ml^–1) in the various peatlands examined. The MPNs of NO₃ ^– reducers varied from 10³–10⁶ ml^–1, SO₄ ^– reducers from 10²–10³ ml^–1, methanogenic bacteria from 10³–10⁶ ml^–1, and methane oxidizers from 10³–10⁴ ml^–1, depending on sampling site and depth. Eighty pure cultures of aerobic bacteria and fungi were isolated from Minnesota peats. Most of those cultures tested were able to grow on at least 20 organic compounds (carbohydrates, aromatic molecules, hydrocarbons, etc.) as sole sources of carbon and energy. One isolate, aBacillus, was able to fix atmospheric N₂. Several of the isolates were able to mineralize¹⁴C-labeled lignin.     

Production of xanthan by in-flow cultures of Xanthomonas campestris

The kinetics of xanthan formation in Xanthomonas campestris continuous and fed-batch fermentations was studied along with metabolic changes due to growth rate variation. A maximum growth rate within the range 0.11–0.12 h^–1 was obtained from the continuous culture data in defined medium, producing xanthan at rates up to 0.36 g l^–1 h^–1 corresponding to a maximum 67% glucose conversion at a dilution rate (D) of 0.05 h^–1. Comparatively, fed-batch cultivation was more efficient, producing maximum xanthan at 0.75 g l^–1 h^–1 and 63% glucose conversion at 0.1 h^–1. When reaching D=0.062 h^–1 in continuous cultures, a change was observed and the values of the specific rate of substrate consumption shifted, initiating an uncoupled growth region expressing a lack of balance of the catabolic and anabolic reactions. The deviation was not accompanied by a change in specific xanthan production indicating that xanthan metabolism was not affected by D. For fed-batch-grown X. campestris cells within the range D=0.03–0.1 h^–1, both metabolic parameters changed linearly with the growth rate showing a wide region coupled to growth. Outside that range, glucose accumulated and the specific xanthan production dropped, suggesting substrate inhibition. Correspondence to: J. C. Roseiro     

Heterotrophic microbial activity in shallow aquifer sediments of Long Island,New York

Bacterial numbers and activities (as estimated by glucose uptake and total thymidine incorporation) were investigated at two sites in Long Island, New York aquifer sediments. In general, bacterial activities were higher in shallow (1.5–4.5 m below the water table or BWT), oxic sediments than in deep (10–18 m BWT), anoxic sediments. The average total glucose uptake rates were 0.18 ± 0.10 ng gdw^–1 h^–1 in shallow sediments and 0.09 ± 0.11 ng gdw^–1 h^–1 in deep sediments; total thymidine incorporation rates were 0.10 ± 0.13 pmol gdw^–1 h^–1 and 0.03 ± 0.03 pmol gdw^–1 h^–1 in shallow and deep sediments, respectively. Incorporation of glucose was highly efficient, as only about 10% of added label was recovered as CO₂. Bacterial abundance (estimated from acridine orange direct counts) was 2.5 ± 2.0 × 10⁷ cells gdw^–1 and 2.0 ± 1.3 × 10⁷ cells gdw^–1 in shallow and deep sediments, respectively. These bacterial activity and abundance estimates are similar to values found in other aquifer environments, but are 10- to 1000-fold lower than values in soil or surface sediment of marine and estuarine systems. In general, cell specific microbial activities were lower in sites from Connetquot Park, a relatively pristine site, when compared to activities found in sites from Jamesport, which has had a history of aldicarb (a pesticide) contamination. To our knowledge, this is the first report of bacterial activity measurements in the shallow, sandy aquifers of Long Island, New York.Correspondence to: D.G. Capone     

Selective production of bikaverin in a fluidized bioreactor with immobilized Gibberella fujikuroi

The best culture medium composition for the production of bikaverin by Gibberella fujikuroi in shake-flasks, i.e. 100 g glucose l^–1; 1 g NH₄Cl l^–1; 2 g rice flour l^–1; 5 g KH₂PO₄ l^–1 and 2.5 g MgSO₄ l^–1, was obtained through a fractional factorial design and then scaled-up to a fluidized bioreactor. The effects of carbon and nitrogen concentrations, inoculum size, aeration, flow rate and bead sizes on batch bikaverin production using immobilized G. fujikuroi in a fluidized bioreactor were determined by an orthogonal experimental design. Concentrations of up to 6.83 g bikaverin l^–1 were obtained when the medium contained 100 g glucose l^–1 and 1 g NH₄Cl l^–1 with an inoculum ratio of 10% v/v, an aeration rate of 3 volumes of air per volume of medium min^–1, and a bead size of 3 mm. Based on dry weight, the bikaverin production was 30–100 times larger than found in submerged culture and approximately three times larger than reported for solid substrate fermentation.     

A novel UASB–MFC–BAF integrated system for high strength molasses wastewater treatment and bioelectricity generation

An up-flow anaerobic sludge blanket reactor–microbial fuel cell–biological aerated filter (UASB–MFC–BAF) system was developed for simultaneous bioelectricity generation and molasses wastewater treatment in this study. The maximum power density of 1410.2 mW/m² was obtained with a current density of 4947.9 mA/m² when the high strength molasses wastewater with chemical oxygen demand (COD) of 127,500 mg/l was employed as the influent. The total COD, sulfate and color removal efficiencies of the proposed system were achieved of 53.2%, 52.7% and 41.1%, respectively. Each unit of this system had respective function and performed well when integrated together. The UASB reactor unit was mainly responsible for COD removal and sulfate reduction, while the MFC unit was used for the oxidation of generated sulfide with electricity generation. The BAF unit dominated color removal and phenol derivatives degradation. This study is a beneficial attempt to combine MFC technology with conventional anaerobic–aerobic processes for actual wastewater treatment.     

Long-term performance of a plant microbial fuel cell with Spartina anglica

The plant microbial fuel cell is a sustainable and renewable way of electricity production. The plant is integrated in the anode of the microbial fuel cell which consists of a bed of graphite granules. In the anode, organic compounds deposited by plant roots are oxidized by electrochemically active bacteria. In this research, salt marsh species Spartina anglica generated current for up to 119 days in a plant microbial fuel cell. Maximum power production was 100 mW m⁻² geometric anode area, highest reported power output for a plant microbial fuel cell. Cathode overpotential was the main potential loss in the period of oxygen reduction due to slow oxygen reduction kinetics at the cathode. Ferricyanide reduction improved the kinetics at the cathode and increased current generation with a maximum of 254%. In the period of ferricyanide reduction, the main potential loss was transport loss. This research shows potential application of microbial fuel cell technology in salt marshes for bio-energy production with the plant microbial fuel cell.     

An automated flow injection analysis system for on-line monitoring of glucose and L-lactate during lactic acid fermentation in a recycle bioreactor

The study concerns on-line sequential analysis of glucose and L-lactate during lactic acid fermentation using a flow injection analysis (FIA) system. Enzyme electrodes containing immobilized glucose oxidase and L-lactate oxidase were used with an amperometric detection system. A 12-bit data acquisition card with 16 analog input channels and 8 digital output channels was used. The software for data acquisition was developed using Visual C++, and was devised for sampling every hour for sequential analyses of lactate and glucose. The detection range was found to be 2–100 g l^–1 for glucose and 1–60 g l^–1 for L-lactate using the biosensors. This FIA system was used for monitoring glucose utilization and L-lactate production by immobilized cells of Lactobacillus casei subsp. rhamnosus during a lactic acid fermentation process in a recycle batch reactor. After 13 h of fermentation, complete sugar utilization and maximal L-lactate production was observed. A good agreement was observed between analysis data obtained using the biosensors and data from standard analyses of reducing sugar and L-lactate. The biosensors exhibited excellent stability during continuous operation for at least 45 days.     

An adaptive on-line control feed rate system in a laboratory scale bakers yeast process

Summary A new control policy for the on-line optimization of the nutrient supply in bakers yeast process is proposed. A feed rate corresponding to minimal substrate uptake time was shown to be optimal for cell yield and specific growth rate. Cultivation results of baker's yeast are presented.Nomenclature c glucose concentration in wort (mol.l^–1) - C total glucose used (mol) - c_e ethanol concentration in wort (mg.l^–1) - c_p glucose concentration in fresh medium (mol.l^–1) - dt/dc glucose consumption time (sec.mol^–1) - F substrate feed rate (litre.hr^–1) - q_c glucose uptake rate (mol.hr^–1) - Q_c specific glucose uptake rate (moll.g^–1.hr^–1) - qO₂ oxygen uptake rate (mol.hr^–1) - QO₂ specific oxygen uptake rate (mol.g^–1.hr^–1) - r_x productivity (g.l^–1.hr^–1) - t time (hr) - x biomass concentration (g.l^–1) - X total biomass (g) - Y_x/c cell yield (g.g^–1): (g.mol^–1) - Y_o/c consumed oxygen to glucose ratio (mol.mol^–1)