Degradation of 2-methylnaphthalene by free and immobilized cells of Pseudomonas sp. strain NGK1

A Pseudomonas sp. strain NGK1 (NCIM 5120) capable of utilizing 2-methylnaphthalene (2-MN) was immobilized in various matrices namely, polyurethane foam (PUF), alginate, agar and polyvinyl alcohol (PVA) (1.5 × 10¹² c.f.u. g^–1 beads). The degradation rates of 25 and 50 mM 2-MN by freely suspended cells (2 × 10¹¹ c.f.u. ml^–1) and immobilized cells in batches, semi-continuous with shaken culture and continuous degradation in a packed-bed reactor were compared. The PUF-immobilized cells achieved higher degradation of 25 and 50 mM of 2-MN than freely suspended cells and the cells immobilized in alginate, agar or PVA. The PVA- and PUF-immobilized cells could be reused for more than 30 and 20 cycles respectively, without losing any degradation capacity. The effect of dilution rates on the rate of degradation of 25 and 50 mM 2-MN with freely suspended and immobilized cells were compared in the continuous system. Increase in dilution rate increased the degradation rate only up to 1 h^–1 in free cells with 25 mM 2-MN and no significant increase was observed with 50 mM 2-MN. With immobilized cells, the degradation rate increased with increase in dilution rate up to 1.5 h^–1 for 25 mM and 1 h^–1 for 50 mM 2-MN. These results revealed that the immobilized cell systems are more efficient than freely suspended cells for biodegradation of 2-MN.     

Continuous production ofl-lactic acid from whey permeate by immobilizedLactobacillus casei subsp.casei

Summary The production of l-lactic acid from whey permeate, a waste product of the dairy industry, by fermentation with the lactic acid bacterium Lactobacillus casei subsp. casei was investigated. A fermentation medium consisting of permeate and supplements, which enables exponential growth of the organisms, was developed. A fast method for determination of free and immobilized biomass in solid-rich media, based on measurement of cellular ATP, was evolved. Continuous fermentations in a stirred tank reactor (STR) and in a fluidized bed reactor (FBR) with immobilized biomass were compared. In the STR a volumetric productivity of 5.5 g/l per hour at 100% substrate conversion [dilution rate (D) = 0.22 h^–1] was determined. In the FBR porous sintered glass beads were used for immobilization and a maximum biomass concentration of 105 g/kg support was measured. A productivity of 10 g/l per hour was obtained at D = 0.4 h^–1 (substrate conversion 93%) and of 13.5 g/l per hour at D = 1.0 h^–1 (substrate conversion 50%). Offprint requests to: W. Krischke     

Improvement of a mammalian cell culture process by adaptive,model-based dialysis fed-batch cultivation and suppression of apoptosis

Both conventional and genetic engineering techniques can significantly improve the performance of animal cell cultures for the large-scale production of pharmaceutical products. In this paper, the effect of such techniques on cell yield and antibody production of two NS0 cell lines is presented. On the one hand, the effect of fed-batch cultivation using dialysis is compared to cultivation without dialysis. Maximum cell density could be increased by a factor of ~5–7 by dialysis fed-batch cultivation. On the other hand, suppression of apoptosis in the NS0 cell line 6A1 bcl-2 resulted in a prolonged growth phase and a higher viability and maximum cell density in fed-batch cultivation in contrast to the control cell line 6A1 (100)3. These factors resulted in more product formation (by a factor ~2). Finally, the adaptive model-based OLFO controller, developed as a general tool for cell culture fed-batch processes, was able to control the fed-batch and dialysis fed-batch cultivations of both cell lines.Abbreviations A membrane area (dm²) - c _Glc,F glucose concentration in nutrient feed (mmol L^–1) - c _Glc,FD glucose concentration in dialysis feed (mmol L^–1) - c _Glc,i glucose concentration in inner reactor chamber (mmol L^–1) - c _Glc,o glucose concentration in outer reactor chamber (dialysis chamber) (mmol L^–1) - c _Lac,FD lactate concentration in dialysis feed (mmol L^–1) - c _Lac,i lactate concentration in inner reactor chamber (mmol L^–1) - c _Lac,o lactate concentration in outer reactor chamber (dialysis chamber) (mmol L^–1) - c _LS,FD limiting substrate concentration in dialysis feed (mmol L^–1) - c _LS,i limiting substrate concentration in inner reactor chamber (mmol L^–1) - c _LS,o limiting substrate concentration in outer reactor chamber (dialysis chamber) (mmol L^–1) - c _Mab monoclonal antibody concentration (mg L^–1) - F _D feed rate of dialysis feed (L h^–1) - F _Glc feed rate of nutrient concentrate feed (L h^–1) - K _d maximum death constant (h^–1) - k _d,LS death rate constant for limiting substrate (mmol L^–1) - k _Glc monod kinetic constant for glucose uptake (mmol L^–1) - k _Lac monod kinetic constant for lactate uptake (mmol L^–1) - k _LS monod kinetic constant for limiting substrate uptake (mmol L^–1) - K _Lys cell lysis constant (h^–1) - K _S,Glc monod kinetic constant for glucose (mmol L^–1) - K _S,LS monod kinetic constant for limiting substrate (mmol L^–1) - µ cell-specific growth rate (h^–1) - µ _d cell-specific death rate (h^–1) - µ _d,min minimum cell-specific death rate (h^–1) - µ _max maximum cell-specific growth rate (h^–1) - P _Glc membrane permeation coefficient for glucose (dm h^–1) - P _Lac membrane permeation coefficient for lactate (dm h^–1) - P _LS membrane permeation coefficient for limiting substrate (dm h^–1) - q _Glc cell-specific glucose uptake rate (mmol cell^–1 h^–1) - q _Glc,max maximum cell-specific glucose uptake rate (mmol cell^–1 h^–1) - q _Lac cell-specific lactate uptake/production rate (mmol cell^–1 h^–1) - q _Lac,max maximum cell-specific lactate uptake rate (mmol cell^–1 h^–1) - q _LS cell-specific limiting substrate uptake rate (mmol cell^–1 h^–1) - q _LS,max maximum cell-specific limiting substrate uptake rate (mmol cell ^–1 h^–1) - q _Mab cell-specific antibody production rate (mg cell^–1 h^–1) - q _MAb,max maximum cell-specific antibody production rate (mg cell^–1 h^–1) - t time (h) - V _i volume of inner reactor chamber (culture chamber) (L) - V _o volume of outer reactor chamber (dialysis chamber) (L) - X _t total cell concentration (cells L^–1) - X viable cell concentration (cells L^–1) - Y _Lac/Glc kinetic production constant (stoichiometric ratio of lactate production and glucose uptake) (–)     

Batch and continuous production of propionic acid from whey permeate by Propionibacterium acidi-propionici in a three-electrode amperometric culture system

Summary Growth of Propionibacterium acidi-propionici was studied on lactose as substrate and in acid whey permeate in a three-electrode poised-potential system with cobalt sepulchrate as artificial electron donor. In batch culture experiments in a stirred-tank reactor the substrate was fermented completely to propionic acid up to 6.5 g 1^–1 lactose in a supplemented whey permeate medium. No acetic acid was produced during the growth of P. acidi-propionici. An electron flow of 80–100 mA was obtained and the electron balance was 101%. In continuously growing cultures with 3 g 1^–1 of lactose as the substrate, propionate was formed as the only fermentation product up to a dilution rate (D) of 0.04 h^–1. With D>0.04 h^–1 the bacteria immobilized on the working electrode surface. It was examined whether an electron transfer occurred between the platinum working electrode and the immobilized cells. Correspondence to: W. Trösch     

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     

Rhodobacter sphaeroides RV cultivation and hydrogen production in a one- and two-stage chemostat

Rhodobacter sphaeroides RV cultivation and hydrogen production were studied in a one- and two-stage chemostat using lactic acid as substrate. Light saturation was observed when light intensities equal to or above 10 klx were applied. Under light saturation, the two-stage chemostat appeared to be very effective for hydrogen production, allowing complete nitrogen removal by bacterial growth in the first reactor. The hydrogen evolution rate in the second reactor was up to 75 ml H₂ (g dry weight)^–1 h^–1. Accumulation of storage material was observed in the second reactor of the two-stage chemostat under a large carbon excess and limiting light irradiance. The optimal hydraulic residence time was 15 h for both stages, leading to a total hydrogen production about 40% higher than in the one-stage chemostat. Under increasing influent ammonium and yeast extract concentrations, opposite trends of decreasing bacterial activity and increasing concentration resulted in a linear increase of the overall hydrogen production to 1.4–1.6lH₂ (l reactor)^–1 day^–1. Hydrogen production quickly fell when nitrogen was not completely metabolised. The hydrogen evolution rate was also found to depend on lactic acid concentration, and maximum bacterial activity was observed at 100 mM influent lactic acid.     

Influence of operational parameters in the flocculation and production ofLactobacillus plantarum

Summary The influence of different operational parameters, such as the dilution rate (D) and the bleeding rate (B), in the production of a flocculent strain ofLactobacillus plantarum was studied. The effect of the dilution rate was demonstrated to be related to the lactic acid concentration inside the reactor. The effect of the bleeding rate was shown to be critical in the stabilization of the operation (due to a better pH control). It also allowed a continuous recovery of cells outside the reactor. Viability testing of the lactic starter cultures showed that operation with cell purge increased the viability of the starter cultures obtained.Nomenclature B Bleeding rate, h^–1 - D Dilution rate, h^–1 - F Feed flow rate, L h^–1 - I Feed velocity, m h^–1 - Specific growth rate, h^–1 - v Lactic acid specific productivity, g g^–1 h^–1 - P Product concentration (lactic acid), g L^–1 - P _out Product concentration leaving the system, g L^–1 - Q _b Bleeding flow rate, L h^–1 - R Recirculation velocity, m h^–1 - S Substract concentration, g L^–1 - t Time, h - T _p Time of ascensional flow (length of the column/total ascensional velocity), h - T _r Residence time (1/D), h - V Volume of the reactor, L - X Cell concentration, g L^–1 - X _out Cell concentration leaving the system, g L^–1     

A trickle bed reactor for ferrous sulphate oxidation using Thiobacillus ferrooxidans

Summary A trickle bed reactor was used to improve ferrous sulphate oxidation rate with Thiobacillus ferrooxidans immobilised in BSPs (Biomass Support Particles). A maximum iron(II) oxidation rate of 4.4 gL^–1h^–1 was observed at a dilution rate D = 0.9 h^–1. The ability of the reactor to operate under non-aseptic conditions due to the chemoautotrophy and acidophilia of the bacterium makes industrial application promising.     

Factors which influence the sedimentation characteristics of Torulopsis glabrata after growth in a recirculation system

Summary Some environmental affects on cell aggregation described in the literature are briefly summarized. By means of a biomass recirculation culture (Contact system), using the yeast Torulopsis glabrata, the aggregation behavior of cells in static and in dynamic test systems is described. Sedimentation times required to obtain 50 g · l^–1 yeast dry matter in static systems were always higher than in dynamic ones.In addition to, influencing the biomass yield, the specific growth rate of the yeast also affected cell aggregation. The specific growth rate and therefore the aggregation could be regulated by the biomass recirculation rate as well as by the sedimenter volume.Abbreviations f_o Overflow flow rate (l·h^–1) - f_R Recycle flow rate (l·h^–1) - f_t0t Total flow rate through the fermenter (l·h^–1) - g Gram - h Hour - D_R Fermenter dilution rate due to recycle (h^–1) - D_S Fermeter dilution rate due to substrate (h^–1) - D_tot Total fermenter dilution rate (h^–1) - l Liter - Specific growth rate (h^–1) - P_F Fermenter productivity (g·l^–1·h^–1) - P_FS Overall productivity (g·l^–1·h^–1) - R_pM Rates per minute - RS Residual sugar content in the effluent with respect to the substrate concentration (%) - Y Yield of biomass with respect to sugar concentration (%) - Sed 50 Sedimentation time to reach a YDM of 50 g·l^–1 (min) - V Volume (l) - V_F Fermenter volume (l) - V_Sed Sedimenter volume (l) - VVM Volumes per volume and minute - X_F YDM in the fermenter (g·l^–1) - X_F YDM in the recycle (g·l^–1) - X_S Yeast dry matter due to substrate concentration (g·l^–1) - YDM Yeast dry matter (g·l^–1)     

Potential of Thiobacillus ferrooxidans for waste gas purification. Part 1. Kinetics of continuous ferrous iron oxidation

Kinetic data of ferrous iron oxidation by Thionacillus ferrooxidans were determined. The aim was to remove H₂S (<0.5 ppm) from waste gas by a process proposed earlier. Kinetic data necessary for industrial scale-up were investigated in a chemostat airlift reactor (dilution rate 0.02–0.12 h^–1; pH 1.3). Due to the low pH, ferric iron precipitation and wall growth could be avoided. The maximum ferrous iron oxidation rate of submersed bacteria was 0.77 g 1^–1 h^–1, the maximum specific growth rate about 0.12 h^–1 and the yield coefficient was found to be 0.007 g g^–1 Fe²⁺. The specific O₂ demand of an exponentially growing, ironoxidizing batch culture was 1.33 mg O₂ mg^–1 biomass h^–1. The results indicate that a pH of 1.3 has no negative influence on the kinetics of iron oxidation and growth. Correspondence to: W. Schäfer-Treffenfeldt     

Effects of growth temperature on maximal specific growth rate,yield, maintenance,and death rate in glucose-limited continuous culture of the thermophilic Bacillus caldotenax

Summary The influence of temperature on the growth of the theromophilic Bacillus caldotenax was investigated using chemostat techniques and a chemically defined minimal medium. All determined growth constants, that is maximal specific growth rate, yield and maintenance, were temperature dependent. It was striking that the very large maintenance requirement was about 10 times higher than for mesophilic cells under equivalent conditions. A death rate, which was very substantial at optimal and supraoptimal growth temperatures, was estimated by comparing the maintenance for substrate and oxygen. There was no indication for a thermoadaptation as postulated by Haberstich and Zuber (1974).Symbols D Dilution rate (h^–1) - D_c=_max Critical dilution rate (h^–1) - E Temperature characteristic (J mol^–1) - k Organism constant - k_d Death rate coefficient (h^–1) - k_m Maintenance substrate coefficient estimated from M_O (h^–1) - M_O Maintenance respiration, mmol O₂ per g dry biomass and h (mmol g^–1h^–1) - M_O Maintenance respiration, taking k_d into account - m_S Maintenance substrate coefficient, g glucose per g dry biomass and h (h^–1) - OD Optical density at 546 nm - QO₂ Specific O₂-uptake rate (mmol g^–1h^–1) - Q _O2 ^V Specific O₂-uptake rate for viable portion of biomass (mmol g^–1 h^–1) - Q_S Specific glucose uptake rate (h^–1) - Q _S ^V Specific glucose uptake rate for viable portion of biomass (h^–1) - R Gas constant 8.28 J mol^–1K^–1 - S Substrate concentration in reactor (g l^–1) - S_O Influent substrate concentration (g l^–1) - T_max Maximal growth temperature (°C) - T_min Minimal growth temperature (°C) - X Dry biomass (g l^–1) - X_tOt=X Dry biomass containing dead and viable cells - X_v Viable portion of biomass - Y _O ^m Potential yield for O₂ corrected for maintenance respiration (g mol^–1) - Y _S ^m Potential yield for substrate corrected for maintenance requirement, g biomass per g glucose (–) - Specific growth rate (h^–1) - _max Maximal specific growth rate (h^–1)     

L-Sorbose production in a continuous fermenter with and without cell recycle

The kinetics of continuous l-sorbose fermentation using Acetobacter suboxydans with and without cell recycle (100%) were investigated at dilution rates (D) of 0.05, 0.10, 0.15 and 0.3 h^–1. The biomass and sorbose concentrations for continuous fermentation without recycle increased as the dilution rate was increased from 0.05 to 0.10 h^–1. A maximum biomass concentration of 8.44 g l^–1 and sorbose concentration of 176.90 g l^–1 were obtained at D=0.10 h^–1. The specific rate of sorbose production and volumetric sorbose productivity at this dilution rate were 2.09 g g^–1 h^–1 and 17.69 g l^–1 h^–1. However, on further increasing the dilution rate to 0.3 h^–1, both biomass and sorbose concentrations decreased to 2.93 and 73.20 g l^–1 respectively, mainly due to washout of the reactor contents. However, the specific rate of sorbose formation and volumetric sorbose productivity at this dilution rate increased to 7.49 g g^–1 h^–1 and 21.96 g l^–1 h^–1 respectively. Continuous fermentation with 100% cell recycle served to further enhance the concentration of biomass and sorbose to 28.27 and 184.32 g l^–1 respectively (in the reactor at a dilution rate of 0.05 h^–1). Even though, there was a decline in the biomass and sorbose concentrations to 6.8 and 83.40 g l^–1 at a dilution rate of 0.3 h^–1, the specific rates of sorbose formation and volumetric sorbose productivity increased to 3.67 g g^–1h^–1 and 25.02 g l^–1 h^–1.     

Production of acetic acid by Acetogenium kivui

Summary The formation of acetic acid by the thermophilic nonsporeforming homoacetogenic bacterium Acetogenium kivui was studied under various conditions. In pH-controlled batch fermentation at pH 6.4 this bacterium was able to produce up to 625 mM of acetic acid from glucose within 50–60 h. The value of _max obtained was about 0.17 h^-1, the yield was about 2.55 mol of acetic acid per mol of glucose utilized. In continuous fermentation both substrate concentration and dilution rate (D) influenced the yield of acetate and the stationary concentration: a glucose concentration of 67 mM at D=0.09 h^-1 resulted in 2.82 mol acetate/mol glucose and 190 mM acetate at a production rate of 17.1 mM/1 h. When the dilution rate was increased the production rate reached a maximal value of 43.2 mM/1 h at D=0.32 h^-1. At a glucose concentration of 195 mM the dependence of yield upon dilution rate followed a similar pattern and an acetate concentration of 420 mM could be obtained. Enzymatic studies indicate that in A. kivui pyruvate ferredoxin-oxidoreductase and acetate kinase are inhibited at acetate concentrations higher than 800 mM. Based on these results a fed-batch fermentation was developed, which allowed to produce more than 700 mM acetic acid within 40–50 h.Dedicated to Prof. Dr. H. J. Rehm on the occasion of his 60th birthday     

Control of recombinant human endostatin production in fed-batch cultures of Pichia pastoris using the methanol feeding rate

Endostatin is a 20 kDa carboxyl-terminal fragment of collagen XVIII that strongly inhibits angiogenesis and tumor growth. The methylotrophic yeast, Pichia pastoris, is a robust expression system that can be used to study methods to improve the yields of rhEndostatin. We expressed rhEndostatin in P. pastoris under the control of the alcohol oxidase 1 (aox 1) promoter (Mut⁺ phenotype) as a model, and used a cell biomass of about 50 g l^–1 dry cell wt as a starting point for the induction phase and varied the methanol feed rate at 8 ml l^–1 h^–1, 11 ml l^–1 h^–1 and 15 ml l^–1 h^–1. While the cell growth rate was proportional to the rate of methanol delivery, protein production rate was not. These findings could be used to guide parameters for large-scale production of recombinant proteins in the P. pastoris system.     

Production of vitamin B12 in an upflow anaerobic filter continuous reactor using Acetobacterium sp.

The accumulation of biofilm by Acetobacterium sp. during continuous culture in an upflow anaerobic filter (UAF) growing on methanol-formate was the result of space velocity and inlet concentrations of substrate and Co⁺². To achieve good development of biofilm, a space velocity of 0.38 h^–1, inlet substrate concentrations of 125 mM of both methanol and formate, and Co⁺² at 0.16 mM were required. Cell productivities in the effluent of the UAF-reactor were about 6-fold higher than in chemostat cultures (0.20 g l^–1 h^–1 for UAF and 0.035 g l^–1 h^–1 for chemostat) (previous studies), and the maximum vitamin B₁₂ specific concentration was 5.1 mg g cell^–1.     

Continuous ethanol production by Zymomonas mobilis in a fluidized bed reactor. Part II: Process development for the fermentation of hydrolysed B-starch without sterilization

A continuous fluidized bed reactor operation system has been developed for ethanol production by Zymomonas mobilis using hydrolysed B-starch without sterilization. The operation system consists of two phases. In the first phase macroporous glass carriers in a totally mixed fluidized bed reactor were filled up totally with a monoculture of Z. mobilis by fast computer-controlled colonization, so that in the subsequent production phase no contaminants, especially lactic-acid bacteria, could penetrate into the carrier beads. In the production phase the high concentration of immobilized Z. mobilis cells in the fluidized bed reactor permits unsterile fermentation of hydrolysed B-starch to ethanol at short residence times. This results in wash-out conditions for contaminants from the substrate. Long-term experimental studies (more than 120 days) of unsterile fermentation of hydrolysed B-starch in the laboratory fluidized bed reactor (2.2 l) demonstrated stable operation up to residence times of 5 h. A semi-technical fluidized bed reactor plant (cascade of two fluidized bed reactors, each 55 l) was operated stably at a mean residence time of 4.25 h. Glucose conversion of 99% of the unsterile hydrolysed B-starch was achieved at 120 g glucose/l^–1 in the substrate, resulting in an ethanol concentration of 50 g·l^–1 and an ethanol space-time yield of 13 g·l^–1·h^–1. This is a factor of three compared to ethanol fermentation of hydrolysed B-starch with Z. mobilis in a continuous stirred tank reactor, which can only be operated stably under sterile conditions. Correspondence to: D. Weuster-Botz     

d(−)lactic acid production by suspended and aggregated continuous cultures of Bacillus laevolacticus

Summary A new method of d(–)lactic acid production based on the aggregated form of growth of Bacillus laevolacticus in continuous culture in an anaerobic gaslift reactor is presented. With glucose as the substrate a bacterial dry weight of 25 g·1^–1 and a lactic acid production rate of 13 g·1^–1·h^–1 was attained. In conventional glucose-limited chemostat cultures elevated levels of lactic acid stimulated specific lactic acid production while the formation rates of other end-products remained unaffected. In glucose-limited aggregated cultures lactic acid positively influenced the aggregation of cells, improving the volumetric production rate. It is concluded that lactic acid itself is a positive effector in the optimisation of lactic acid production with aggregated cultures of B. laevolacticus.Offprint requests to: J. P. de Boer     

Continuous ethanol production by Zymomonas mobilis in a fluidized bed reactor. Part I. Kinetic studies of immobilization in macroporous glass beads

A model has been developed to calculate the ethanol production in a well-mixed fluidized bed reactor. This model takes into account diffusion and the reaction inside porous glass beads and the activity of suspended cells in the fluidized bed reactor. The associated model parameters have been determined from the literature and by kinetic studies with Zymomonas mobilis in a continuous stirred tank reactor. The model permits good predictions of steady-state data in a fluidized bed reactor at residence times longer than 1–1.5 h. The immobilization of Z. mobilis in a fluidized bed reactor results in high ethanol space-time yields of more than 50 g·^–1·h^–1 at a glucose conversion of 80% (glucose in substrate: 120 gl^–1). At 99% conversion a space-time yield of 30 g·^–1·^–1 can be achieved when two fluidized bed reactors operate as cascade.     

Protein production from whey usingPenicillium cyclopium; growth parameters and cellular composition

Summary The growth parameters ofPenicillium cyclopium have been evaluated in a continuous culture system for the production of fungal protein from whey. Dilution rates varied from 0.05 to 0.20 h^–1 under constant conditions of temperature (28°C) and pH (3.5). The saturation coefficients in the Monod equation were 0.74 g l^–1 for lactose and 0.14 mg l^–1 for oxygen, respectively. For a wide range of dilution rates, the yield was 0.68 g g^–1 biomass per lactose and the maintenance coefficient 0.005 g g^–1 h^–1 lactose per biomass, respectively. The maximum biomass productivity achieved was 2 g l^–1 h^–1 biomass at dilution rates of 0.16–0.17 h^–1 with a lactose concentration of 20 g l^–1 in the feed. The crude protein and total nucleic acid contents increased with a dilution rate, crude protein content varied from 43% to 54% and total nucleic acids from 6 to 9% in the range of dilution rates from 0.05 to 0.2 h^–1, while the Lowry protein content was almost constant at approximately 37.5% of dry matter.Nomenclature (mg l^–1) C_o initial concentration of dissolved oxygen - (h^–1) D dilution rate - (mg l^–1) K₀₂ saturation coefficient for oxygen - (g l^–1) K_s saturation coefficient for substrate - (g g^–1 h^–1) lactose per biomass) m maintenance energy coefficient - (mM g^–1 h^–1O₂ per biomass) Q₀₂ specific oxygen uptake rate - (g l^–1) S residual substrate concentration at steady state - (g l^–1) S_o initial substrate concentration in feed - (min) t_1/2 time when C_o is equal to C_o/2 - (g l^–1) X biomass concentration - (g l^–1) X biomass concentration at steady state - (g g^–1 biomass per lactose) Y_G yield coefficient for cell growth - (g g^–1 biomass per lactose) Y_x/s overall yield coefficient - (h^–1) specific growth rate     

Xylulose fermentation by Saccharomyces cerevisiae and xylose-fermenting yeast strains

Xylulose fermentation by four strains of Saccharomyces cerevisiae and two strains of xylose-fermenting yeasts, Pichia stipitis CBS 6054 and Candida shehatae NJ 23, was compared using a mineral medium at a cell concentration of 10 g (dry weight)/l. When xylulose was the sole carbon source and fermentation was anaerobic, S. cerevisiae ATCC 24860 and CBS 8066 showed a substrate consumption rate of 0.035 g g cells^–1 h^–1 compared with 0.833 g g cells^–1 h^–1 for glucose. Bakers' yeast and S. cerevisiae isolate 3 consumed xylulose at a much lower rate although they fermented glucose as rapidly as the ATCC and the CBS strains. While P. stipitis CBS 6054 consumed both xylulose and glucose very slowly under anaerobic conditions, C. shehatae NJ 23 fermented xylulose at a rate of 0.345 g g cells^–1 h^–1, compared with 0.575 g g cells^–1 h^–1 for glucose. For all six strains, the addition of glucose to the xylulose medium did not enhance the consumption of xylulose, but increased the cell biomass concentrations. When fermentation was performed under oxygen-limited conditions, less xylulose was consumed by S. cerevisiae ATCC 24860 and C. shehatae NJ 23, and 50%–65% of the assimilated carbon could not be accounted for in the products determined.