Production of 1,3-Propanediol by Clostridium butyricum VPI 3266 in continuous cultures with high yield and productivity

The effects of dilution rate and substrate feed concentration on continuous glycerol fermentation by Clostridium butyricum VPI 3266, a natural 1,3-propanediol producer, were evaluated in this work. A high and constant 1,3-propanediol yield (around 0.65 mol/mol), close to the theoretical value, was obtained irrespective of substrate feed concentration or dilution rate. Improvement of 1,3-propanediol volumetric productivity was achieved by increasing the dilution rate, at a fixed feed substrate concentration of 30, 60 or 70 g l⁻¹. Higher 1,3-propanediol final concentrations and volumetric productivities were also obtained when glycerol feed concentration was increased from 30 to 60 g l⁻¹, at D=0.05–0.3 h⁻¹, and from 60–70 g l⁻¹, at D=0.05 and 0.1 h⁻¹·30 g l⁻¹ of 1,3-propanediol and the highest reported value of productivity, 10.3 g l⁻¹ h⁻¹, was achieved at D=0.30 h⁻¹ and 60 g l⁻¹ of feed glycerol. A switch to an acetate/butyrate ratio higher than one was observed for 60 g l⁻¹ of feed glycerol and a dilution rate higher than 0.10 h⁻¹; moreover, at D=0.30 h⁻¹ 3-hydroxypropionaldehyde accumulation was observed for the first time in the fermentation broth of C. butyricum.     

Production of acetone and butanol by Clostridium acetobutylicum in continuous culture with cell recycle

Summary To increase the solvent productivity of the acetone-butanol fermentation, a continuous culture of Clostridium acetobytylicum with cell recycling was used. At a dry cell mass concentration of 8 g l^-1 and a dilution rate of D=0.64 h^-1, a solvent productivity of 5.4 g l^-1 h^-1 was attained. To prevent degeneration of the culture, which occurs with high concentrations of solvents (acetone, butanol and ethanol), different reactor cascades were used. A two-stage cascade with cell recycling and turbidostatic cell concentration control turned out to be the best solution, the first stage of which was kept at relatively low cell and product concentrations. A solvent productivity of 3 and 2.3 g l^-1 h^-1, respectively, was achieved at solvent concentrations of 12 and 15 g l^-1.Symbols D Dilution rate (h^-1) - r _p solvent productivity (g l^-1 h^-1) - s residual glucose concentration (g l^-1) - V _R reactor volume (l) - V _O overall volume (l) - x (dry) cell mass concentration (g l^-1) - Y _P/S solvent yield (g g^-1)     

Automated feeding strategies for high-cell-density fed-batch cultivation of Pseudomonas putida KT2440

Four automatic substrate feeding strategies were developed and investigated in this study to obtain rapid, repeatable, and reliable high cell densities of Pseudomonas putida KT2440 from glucose. Growth yield data of the key nutrients, Y _X/Glucose, Y _X/NH4, Y _X/PO4, Y _X/Mg, and Y _CO2/Glucose, were determined to be 0.41, 5.44, 13.70, 236, and 0.65 g g⁻¹, respectively. Although standard exponential feeding strategy worked well when the predetermined μ was set at 0.25 h⁻¹, an exponential glucose feeding strategy with online μ _max estimation resulted in a higher average biomass productivity (3.4 vs 2.8 g l⁻¹ h⁻¹). A CO₂ production rate based pulse glucose feeding strategy also resulted in good overall productivity (3.0 g l⁻¹ h⁻¹) and can be used as an alternative to pH-stat or DO-stat feeding. A cumulative CO₂ production based continuous feed with real-time cumulative glucose consumption estimation resulted in much higher biomass productivity (4.3 g l⁻¹ h⁻¹) and appears to be an excellent and reliable approach to fully automating high-cell-density fed-batch cultivation of P. putida.     

Conversion of cellulose into fungal cell mass

Summary Chaetomium cellulolyticum (ATCC 32319) was cultivated on glucose, Avicel and/or Sigmacell in a 20-1 stirred tank batch reactor. The substrate (cellulose) concentration, the cell mass concentration (through protein and/or nitrogen content), reducing sugar concentration, the enzyme activity, the alkali consumption rate, the dissolved O₂ and CO₂ concentrations in the outlet gas were measured. The specific growth rate, the substrate yield coefficient, cell productivity, the oxygen consumption rate, the CO₂ production rate and the volumetric mass transfer coefficient were determined. At the beginning of the growth phase the oxygen utilization rate exhibits a sharp maximum. This maximum could be used to start process control. Because of the long lag phase periodic batch operation is recommended.Symbols CP cell protein concentration (g l^–1) - FPA FP enzyme activity (IU l^–1) - GP dissolved protein concentration (g l^–1) - IU international unit of enzyme activity - k_La volumetric mass tranfer coefficient (h^–1) - LG alkali (1 n NaOH) consumption (ml) - LGX specific alkali consumption rate per cell mass (ml g^–1 h^–1) - P cell mass productivity (g l^–1 h^–1) - specific oxygen consumption rate per cell mass (g g^–1 h^–1) - Q aeration rate (volumetric gas flow rate per volume of medium, vvm) (min^–1) - N impeller speed (revolution per minute, rpm) (min^–1) - S substrate concentration (g l^–1) - S⁰ S at t_F=0 (g l^–1) - S₀ S in feed (g l^–1) - SR acid consumption (ml) - TDW total dry weight (g l^–1) - T temperature (° C) - t_F cultivation time (h) - U substrate conversion - X cell mass concentration (g l^–1) - Y_X/S vield coefficient - specific growth rate (h^–1) - _m maximum specific growth rate (h^–1)     

Low salt medium for lactate and ethanol production by recombinant Escherichia coli B

Individual nutrient salts were experimentally varied to determine the minimum requirements for efficient l(+)-lactate production by recombinant strains of Escherichia coli B. Based on these results, AM1 medium was formulated with low levels of alkali metals (4.5 mM and total salts (4.2 g l⁻¹). This medium was equally effective for ethanol production from xylose and lactate production from glucose with average productivities of 18–19 mmol l⁻¹ h⁻¹ for both (initial 48 h of fermentation).     

Production of single cell protein from rice polishings using Candida utilis

Microbial protein was produced from defatted rice polishings using Candida utilis in shake-flasks and a 14-l fermentor to optimize fermentation conditions before producing biomass in a 50-l fermentor. The organism supported maximum values of 0.224 h⁻¹, 0.94, 1.35, 1.75, 2.12 g l⁻¹ h⁻¹, 0.62 g cells g⁻¹ substrate utilized and 0.38 g g⁻¹ for specific growth rate, true protein productivity, crude protein productivity, cell mass productivity, substrate consumption rate, cell yield, crude protein yield, respectively in 50-l fermentor studies using optimized cultural conditions. Maximum values compared favourably or were superior to published data in literature. The biomass protein in the 50-l fermentor contained 22.3, 27.8, 19.2, 9.5, 38.12, 8.5 and 0.27% true protein, crude protein, crude fibre, ash, carbon, cellulose and RNA content, respectively. The dried biomass showed a gross metabolizable energy value of 2678 kcal kg⁻¹ and contained all essential and non-essential amino acids. Yeast biomass as animal feed may replace expensive feed ingredients currently being used in poultry feed and may improve the economics of feed produced in countries like Pakistan. This revised version was published online in August 2006 with corrections to the Cover Date.     

Lactic acid production from cellulosic waste by immobilized cells of Lactobacillus delbrueckii

Industrial waste corn cob residue (from xylose manufacturing) without pretreatment was hydrolyzed by cellulase and cellobiase. The cellulosic hydrolysate contained 52.4 g l⁻¹ of glucose and was used as carbon source for lactic acid fermentation by cells of Lactobacillus delbrueckii ZU-S2 immobilized in calcium alginate gel beads. The final concentration of lactic acid and the yield of lactic acid from glucose were 48.7 g l⁻¹ and 95.2%, respectively, which were comparative to the results of pure glucose fermentation. The immobilized cells were quite stable and reusable, and the average yield of lactic acid from glucose in the hydrolysate was 95.0% in 12 repeated batches of fermentation. The suitable dilution rate of continuous fermentation process was 0.13 h⁻¹, and the yield of lactic acid from glucose and the productivity were 92.4% and 5.746 g l⁻¹ h⁻¹, respectively. The production of lactic acid by simultaneous saccharification and fermentation (SSF) process was carried out in a coupling bioreactor, the final concentration of lactic acid was 55.6 g l⁻¹, the conversion efficiency of lactic acid from cellulose was 91.3% and the productivity was 0.927 g l⁻¹ h⁻¹. By using fed-batch technique in the SSF process, the final concentration of lactic acid and the productivity increased to 107.6 g l⁻¹ and 1.345 g l⁻¹ h⁻¹, respectively, while the dosage of cellulase per gram substrate decreased greatly. This research work should advance the bioconversion of renewable cellulosic resources and reduce environmental pollution.     

Limitations in substrate utilization efficiency by Zymomonas mobilis

Summary Optimal growth conditions for Zymomonas mobilis have been established using continuous cultivation methods. Optimal substrate utilization efficiency occurs with 2.5 g l^–1 yeast extract, 2.0 g l^–1 ammonium sulfate and 6.0 g l^–1 magnesium sulfate in the media. Catabolic activity is at its maximum with glucose uptake rates of 16–18 g l^–1 h^–1 and ethanol production rates of 8–9 g l^–1 h^–1, Q_g values of 22–26 and Q_p values between 11 and 13, which results in 40 g l^–1 h^–1 ethanol yields using a 100 g l^–1 substrate feed. Any increase in these parameters goes on cost of substrate utilization efficiency. Calcium pantothenate can not substitute yeast extract.Abbreviations G Glucose (%) - Pant Calcium pantothenate (mg l^–1) - D Dilution rate (h^–1) - NH₄ Ammonium sulfate (%) - M_g Magnesium sulfate (%) - S₁ Residual glucose in the fermenter (g l^–1) - S₀ Glucose feed (g l^–1) - Eth Ethanol concentration (g l^–1) - GUR Glucose uptake rate (g l^–1 h^–1) - Q_g Specific glucose uptake rate (g g^–1 h^–1) - Q_p Specific ethanol production rate (g g^–1 h^–1) - EPR Ethanol production rate (g l^–1 h^–1) - Y_g Yield coefficient for glucose (g g^–1) - Y_p Conversion efficiency (%) - C Biomass concentration (g l^–1) Present address: (Until June 1982) Institut für Mikrobiologie, TH Darmstadt, 6100 Darmstdt, Federal Republic of Germany     

Metabolic,physiological and kinetic aspects of the alcoholic fermentation of whey permeate by Kluyveromyces fragilis NRRL 665 and Kluyveromyces lactis NCYC 571

Optimum growth conditions for the fermentation of non-concentrated whey permeate by Kluyveromyces fragilis NRRL 665 have been defined. Use of 3.75 g yeast extract l^?1, a growth temperature of 38°C and a pH of 4.0 allowed a maximum productivity of 5.23 g ethanol l^?1 h^?1 in continuous culture with a yield 91% of theoretical. Complete batch fermentation of permeate with 100 g lactose l^?1 was possible with a maximum specific growth rate of 0.276 h^?1 without any change in ethanol yield. Fermentation of concentrated permeate resulted, however, in a general decrease of specific substrate consumption rate, demonstrated by the inability to completely convert an initial 90 or 150 g lactose l^?1 in continuous culture, even at dilution rates as low as 0.05 and 0.08 h^?1, respectively. The decrease could be related to substrate inhibition, to an increase in osmotic pressure caused by lactose and salts, and to ethanol inhibition of both alcohol and biomass yield. The decrease in specific productivity could be counterbalanced by use of high cell density cultures, obtained by cell recycle of K. fragilis. Fermentation of a non-concentrated permeáte at a dilution rate of 1 h^?1 resulted in a productivity of 22 g l^?1 h^?1 at 22 g ethanol l^?1. Cell recycle using flocculating Kluyveromyces lactis NCYC 571 was also tested. With this strain a productivity of 9.3 g l^?1 h^?1 at 45 g product l^?1 was attained at a dilution rate of 0.2 h^?1, with an initial lactose concentration of 95 g l^?1.     

Continuous fermentation of sweet whey permeate for propionic acid production in a CSTR with UF recycle

Summary Propionic acid was produced byPropionibacterium acidi-propionici from sweet-whey permeate in a stirred tank reactor (CSTR) with cell recycle by ultrafiltration. The highest volumetric productivity achieved was 14.3 g.l^–1. h^–1, with a biomass of 100 g.l^–1 (dry weight). More concentrated product can be obtained by electrodialysis of the cell free fermentation medium.     

Increased erythritol production in fed-batch cultures of Torula sp. by controlling glucose concentration

The effect of glucose concentration on erythritol production by Torula sp. was investigated. The maximum volumetric productivity of erythritol was obtained at an initial glucose concentration of 300 g l⁻¹ in batch culture. The volumetric productivity was maximal at a controlled glucose concentration of 225 g l⁻¹, reducing the lag time of the erythritol production. A fed-batch culture was established with an initial glucose concentration of 300 g l⁻¹ and with a controlled glucose concentration of 225 g l⁻¹ in medium containing phytic acid as a phosphate source. In this fed-batch culture, a final erythritol production of 192 g l⁻¹ was obtained from 400 g l⁻¹ glucose in 88 h. This corresponded to a volumetric productivity of 2.26 g l⁻¹ h⁻¹ and a 48% yield. Journal of Industrial Microbiology & Biotechnology (2001) 26, 248–252. Received 26 September 2000/ Accepted in revised form 16 January 2001     

Effect of oxygen supply and dilution rate on the production of 2,3-butanediol in continuous bioreactor by Klebsiella pneumoniae

Kinetics of 2,3-butanediol production by Klebsiella pneumoniae (NRRL B199) from glucose have been studied in a continuous bioreactor. The effect of oxygen supply rate and dilution rate on the product output rate and yield of 2,3-butanediol were investigated. For a feed glucose concentration of 100 g l⁻¹, the optimum oxygen transfer rate is between 25.0–35.0 mmol l⁻¹ h⁻¹. Under these conditions, maximum product concentration obtained was 35 g l⁻¹ at a dilution rate of 0.1 h⁻¹ and the maximum product output rate obtained was 4.25 g l⁻¹ h⁻¹. The product yield based on the substrate utilized approached the theoretical value (50%) at low values of oxygen transfer rate but decreased with increasing oxygen transfer rate.     

Improvement in lactic acid production from starch using α-amylase-secreting <Emphasis Type="Italic">Lactococcus lactis</Emphasis> cells adapted to maltose or starch

To achieve direct and efficient lactic acid production from starch, a genetically modified Lactococcus lactis IL 1403 secreting α-amylase, which was obtained from Streptococcus bovis 148, was constructed. Using this strain, the fermentation of soluble starch was achieved, although its rate was far from efficient (0.09 g l⁻¹ h⁻¹ lactate). High-performance liquid chromatography revealed that maltose accumulated during fermentation, and this was thought to lead to inefficient fermentation. To accelerate maltose consumption, starch fermentation was examined using L. lactis cells adapted to maltose instead of glucose. This led to a decrease in the amount of maltose accumulation in the culture, and, as a result, a more rapid fermentation was accomplished (1.31 g l⁻¹ h⁻¹ lactate). Maximum volumetric lactate productivity was further increased (1.57 g l⁻¹ h⁻¹ lactate) using cells adapted to starch, and a high yield of lactate (0.89 g of lactate per gram of consumed sugar) of high optical purity (99.2% of l-lactate) was achieved. In this study, we propose a new approach to lactate production by α-amylase-secreting L. lactis that allows efficient fermentation from starch using cells adapted to maltose or starch before fermentation.     

High cell density cultivation of Brevibacterium linens and formation of proteinases and lipase

Brevibacterium linens forms hydrolytic enzymes which can be used to accelerate the ripening of cheese without causing bitterness. B. linens ATCC 9172 was grown to a high cell density (50 g dry wt l^–1 after 60 h) in a mineral medium containing lactic acid, soy-peptone and ammonium sulphate by applying a continuous feed of nutrients. The maximal activities of l-leucine aminopeptidase and cell-associated proteinase were 286 U l^–1 and 202 U l^–1, respectively. The cell-associated lipolytic activity exhibited a strong and sudden increase at 46 h, resulting in a maximum of 9.5 U g^–1 dry wt; thus the volumetric productivity of proteolytic and lipolytic activity was 4220 U l^–1 h^–1 and 7.3 U l^–1 h^–1, respectively.     

Batch and repeated batch production of l(+)-lactic acid by Enterococcus faecalis RKY1 using wood hydrolyzate and corn steep liquor

Lactic acid production was investigated for batch and repeated batch cultures of Enterococcus faecalis RKY1, using wood hydrolyzate and corn steep liquor. When wood hydrolyzate (equivalent to 50 g l⁻¹ glucose) supplemented with 15–60 g l⁻¹ corn steep liquor was used as a raw material for fermentation, up to 48.6 g l⁻¹ of lactic acid was produced with, volumetric productivities ranging between 0.8 and 1.4 g l⁻¹ h⁻¹. When a medium containing wood hydrolyzate and 15 g l⁻¹ corn steep liquor was supplemented with 1.5 g l⁻¹ yeast extract, we observed 1.9-fold and 1.6-fold increases in lactic acid productivity and cell growth, respectively. In this case, the nitrogen source cost for producing 1 kg lactic acid can be reduced to 23% of that for fermentation from wood hydrolyzate using 15 g l⁻¹ yeast extract as a single nitrogen source. In addition, lactic acid productivity could be maximized by conducting a cell-recycle repeated batch culture of E. faecalis RKY1. The maximum productivity for this process was determined to be 4.0 g l⁻¹ h⁻¹.     

Optimization of lactic acid production by immobilized <Emphasis Type="Italic">Lactococcus lactis</Emphasis> IO-1

Production of lactic acid from glucose by immobilized cells of Lactococcus lactis IO-1 was investigated using cells that had been immobilized by either entrapment in beads of alginate or encapsulation in microcapsules of alginate membrane. The fermentation process was optimized in shake flasks using the Taguchi method and then further assessed in a production bioreactor. The bioreactor consisted of a packed bed of immobilized cells and its operation involved recycling of the broth through the bed. Both batch and continuous modes of operation of the reactor were investigated. Microencapsulation proved to be the better method of immobilization. For microencapsulated cells at immobilized cell concentration of 5.3 g l⁻¹, the optimal production medium had the following initial concentrations of nutrients (g l⁻¹): glucose 45, yeast extract 10, beef extract 10, peptone 7.5 and calcium chloride 10 at an initial pH of 6.85. Under these conditions, at 37 °C, the volumetric productivity of lactic acid in shake flasks was 1.8 g l⁻¹ h⁻¹. Use of a packed bed of encapsulated cells with recycle of the broth through the bed, increased the volumetric productivity to 4.5 g l⁻¹ h⁻¹. The packed bed could be used in repeated batch runs to produce lactic acid.     

Performance,kinetics, and substrate utilization in a continuous yeast fermentation with cell recycle by ultrafiltration membranes

Summary A continuous single stage yeast fermentation with cell recycle by ultrafiltration membranes was operated at various recycle ratios. Cell concentration was increased 10.6 times, and ethanol concentration and fermentor productivity both 5.3 times with 97% recycle as compared to no recycle. Both specific growth rate and specific ethanol productivity followed the exponential ethanol inhibition form (specific productivity was constant up to 37.5 g/l of ethanol before decreasing), similar to that obtained without recycle, but with greater inhibition constants most likely due to toxins retained in the system at hight recycle ratios.By analyzing steady state data, the fractions of substrate used for cell growth, ethanol formation, and what which were wasted were accounted for. Yeast metabolism varied from mostly aerobic at low recycle ratios to mostly anaerobic at high recycle ratios at a constant dissolved oxygen concentration of 0.8 mg/kg. By increasing the cell recycle ratio, wasted substrate was reduced. When applied to ethanol fermentation, the familiar terminology of substrate used for Maintenance must be used with caution: it is not the same as the wasted substrate reported here.A general method for determining the best recycle ratio is presented; a balance among fermentor productivity, specific productivity, and wasted substrate needs to be made in recycle systems to approach an optimal design.Nomenclature B Bleed flow rate, l/h - C _T Concentration of toxins, arbitrary units - D Dilution rate, h^-1 - F Filtrate or permeate flow rate, removed from system, l/h - F _o Total feed flow rate to system, l/h - K _s Monod form constant, g/l - P Product (ethanol) concentration, g/l - P _o Ethanol concentration in feed, g/l - PP} Adjusted product concentration, g/l - PD Fermentor productivity, g/l-h - R Recycle ratio, F/F _o - S Substrate concentration in fermentor, g/l - S _o Substrate concentration in feed, g/l - V Working volume of fermentor, l - V _MB Viability based on methylene blue test - X Cell concentration, g dry cell/l - X _o Cell concentration in feed, g/l - Y _ATP Cellular yield from ATP, g cells/mol ATP - Y _ATPS Yield of ATP from substrate, mole ATP/mole glucose - Y _G True growth yield or maximum yield of cells from substrate, g cell/g glucose - Y _P Maximum theoretical yield of ethanol from glucose, 0.511 g ethanol/g glucose - Y _P/S Experimental yield of product from substrate, g ethanol/g glucose - Y _x/s Experimental yield of cells from substrate, g cell/g glucose - S _NP/X Non-product associated substrate utilization, g glucose/g cell - k ₁, k₂, k₃, k₄ Constants - k ₁ ^APP , k ₂ ^APP Apparent k ₁, k₃ - k ₁ ^TRUE True k ₁ - m Maintenance coefficient, g glucose/g cell-h - m ^* Coefficient of substrate not used for growth nor for ethanol formation, g glucose/g cell-h - Specific growth rate, g cells/g cells-h, reported as h^-1 - _m Maximum specific growth rate, h^-1 - v Specific productivity, g ethanol/g cell-h, reported as h^-1 - v _m Maximum specific productivity, h^-1     

Anaerobic fermentation of Salmonella typhimurium with and without pyruvate carboxylase

A plasmid that expressed pyruvate carboxylase (PYC) from Rhizobium etli was introduced into Salmonella typhimurium LT2. Anaerobic fermentations of S. typhimurium with and without PYC were compared with glucose as a carbon source. The presence of PYC increased the succinate yield from glucose from 0.044 g g^–1 to 0.22 g g^–1, while the lactate yield decreased from 0.31 g g^–1 to 0.16 g g^–1. Metabolic flux calculations during the early growth phase indicate that under these growth conditions in the presence of PYC more carbon flows to oxaloacetate via pyruvate carboxylase than via phosphoenolpyruvate carboxylase. Also, under these growth and induction conditions, the presence of PYC diminished the cell growth rate from 0.34 h^–1 to 0.28 h^–1, the specific rate of ATP formation from 45 mmol l^–1 h^–1 to 27 mmol l^–1 h^–1, and the specific rate of glucose consumption from 17 mmol l^–1 h^–1 to 10 mmol l^–1 h^–1.     

Production of 1,3-propanediol by Klebsiella pneumoniae from glycerol broth

Broth containing 152 g glycerol l⁻¹ from Candida krusei culture was converted to 1,3-propanediol by Klebsiella pneumoniae. Residual glucose in the broth promoted growth of K. pneumoniae while acetate was inhibitory. After desalination treatment of glycerol broth by electrodialysis, the acetate in the broth was removed. A fed-batch culture with electrodialytically pretreated broth as␣substrate was developed giving 53 g 1,3- propanediol l⁻¹ with a yield of 0.41 g g⁻¹ glycerol and a productivity of 0.94 g l⁻¹ h⁻¹.     

Continuous acetone-butanol production with direct product removal

Summary To eliminate the product inhibition and increase the productivity of butanol formation, a continuously operated membrane bioreactor was connected to a four-stage mixer-settler cascade. Clostridium acetobutylicum was cultivated in this reactor. Butanol was selectively extracted with butyric acid saturated n-decanol from the cell-free cultivation medium, and the butanol-free medium was refed into the reactor. Due to the high boiling point of decanol, the recovery of butanol from the decanol solution is easy. The partition coefficient and selectivity of butanol in the cultivation medium-decanol-system is sufficiently high for removing it from the medium. Direct contact of the cells with the decanol phase causes cell damage. However, decanol is practically insoluble in the fermentation medium, thus the contact of the cell-free medium with the solvent phase does not influence of cell growth and product formation. At a dilution rate of D _z=0.1 h^-1, the butanol productivity was increased by removing butanol from the medium by a factor of four. A further increase was prevented by a contaminant of the technical decanol, which was identified by GC-MS-analysis as 1-,3-hexandiol.Symbols D dilution rate, h^-1 - D _eff effective dilution rate (Eq. 3), h^-1 - D _Ex extraction dilution rate (Eq. 3), h^-1 - D _g dilution rate of cell suspension in reactor-filter-system, h^-1 - E degree of extraction (Eq. 3), l - P product concentration in medium after extraction, g l^-1 - P _O product concentration in reactor, g l^-1 - R _P productivity and product formation rate, g l^-1 h^-1 - q _p S specific product formation coefficient with regard to the cell growth rate, l - V _F volume of cell suspension in filter module, l - V _g volume of the cell suspension in reactor and in filter module V _g =V _R +V _F , l - V _R volume of cell suspension in ractor, l - v _O cell free feed rate, l h^-1 - v ₁ flow rate of cell suspension leaves the reactor, l h^-1 - v _E flow rate of decanol through the extractor, l h^-1 - v _w flow rate of the cell free medium through the filter modul, l h^-1 - X cell mass concentration, g l^-1 - specific growth rate of the cells, h^-1 Dedicated to Professor Dr. H. J. Rehm on the occasion of his 60th birthday