Optimisation of media and cultivation conditions for L(+)(S)-lactic acid production by Lactobacillus casei NRRL B-441

Process variables and concentration of carbon in media were optimised for lactic acid production by Lactobacillus casei NRRL B-441. Lactic acid yield was inversely proportional to initial glucose concentration within the experimental area (80-160 g l(-1)). The highest lactic acid concentration in batch fermentation, 118.6 g l(-1), was obtained with 160 g 1(-1) glucose. The maximum volumetric productivity, 4.4 g 1(-1) h(-1) at 15 h, was achieved at an initial glucose concentration of 100 g l(-1). Similar lactic acid concentrations were reached with a fedbatch approach using growing cells, in which case the fermentation time was much shorter. Statistical experimental design and response surface methodology were used for optimising the process variables. The temperature and pH optima for lactic acid production were 35 degrees C, pH 6.3. Malt sprout extract supplemented with yeast extract (4 g l(-1)) appeared to be an economical alternative to yeast extract alone (22 g l(-1)) although the fermentation time was a little longer. The results demonstrated both the separation of the growth and lactic acid production phases and lactic acid production by non-growing cells without any nutrient supplements. Resting L. casei cells converted 120 g l(-1) glucose to lactic acid with 100% yield and a maximum volumetric productivity of 3.5 g l(-1) h(-1).     

Batch propagation of Lactobacillus helveticus for production of lactic acid from lactose concentrated cheese whey with microaeration and nutrient supplementation

Continuous mix batch bioreactors were used to study the kinetic parameters of lactic acid fermentation in microaerated-nutrient supplemented, lactose concentrated cheese whey using Lactobacillus helveticus. Four initial lactose concentrations ranging from 50 to 150 g l^–1 were first used with no microaeration and no yeast extract added to establish the substrate concentration above which inhibition will occur and then the effects of microaeration and yeast extract on the process kinetic parameters were investigated. The experiments were conducted under controlled pH (5.5) and temperature (42 °C) conditions. The results indicated that higher concentrations of lactose had an inhibitory effect as they increased the lag period and the fermentation time; and decreased the specific growth rate, the maximum cell number, the lactose utilization rate, and the lactic acid production rate. The maximum lactic acid conversion efficiency (75.8%) was achieved with the 75 g l^–1 initial lactose concentration. The optimum lactose concentration for lactic acid production was 75 g l^–1 although Lactobacillus helveticus appeared to tolerate up to 100 g l^–1 lactose concentration. Since the lactic acid productivity is of a minor importance compared to lactic acid concentration when considering the economic feasibility of lactic acid production from cheese whey using Lactobacillus helveticus, a lactose concentration of up to 100 g l^–1 is recommended. Using yeast extract and/or microaeration increased the cell number, specific growth rate, cell yield, lactose consumption, lactic acid utilization rate, lactic acid concentration and lactic acid yield; and reduced the lag period, fermentation time and residual lactose. Combined yeast extract and microaeration produced better results than each one alone. From the results it appears that the energy uncoupling of anabolism and catabolism is the major bottleneck of the process. Besides lactic acid production, lactose may also be hydrolysed into glucose and galactose. The -galactosidase activity in the medium is caused by cell lysis during the exponential growth phase. The metabolic activities of Lactobacillus helveticus in the presence of these three sugars need further investigation.     

Strain improvement and metabolic flux analysis in the wild-type and a mutant Lactobacillus lactis strain for L(+)-lactic acid production

The effects of initial glucose concentration and calcium lactate concentration on the lactic acid production by the parent strain, Lactobacillus lactis BME5-18, were studied. The results of the experiments indicated that glucose and lactate repressed the cell growth and the lactic acid production by Lactobacillus lactis BME5-18. A L(+)-lactic acid overproducing strain, Lactobacillus lactis BME5-18M, was screened by mutagenizing the parent strain with ultraviolet (UV) light irradiation and selecting the high glucose and lactate calcium concentration repression resistant mutant. Starting with a concentration of 100g L(-1) glucose, the mutant produced 98.6 g L(-1) lactic acid after 60 h in flasks, 73.9% higher than that of the parent strain. The L(+)-lactic acid purity was 98.1% by weight based on the amount of total lactic acid. The culture of the parent strain could not be analyzed well by conventional metabolic flux analysis techniques, since some pyruvate were accumulated intracellularly. Therefore, a revised flux analysis method was proposed by introducing intracellular pyruvate pool. Further studies demonstrate that there is a high level of NADH oxidase activity (12.11 mmol mg(-1) min(-1)) in the parent strain. The molecular mechanisms of the strain improvement were proposed, i.e., the high level of NADH oxidase activity was eliminated and the uptake rate of glucose was increased from 82.1 C-mmol (g DW h)(-1) to 98.9 C-mmol (g DW h)(-1) by mutagenizing the parent strain with UV, and therefore the mutant strain converts mostly pyruvate to lactic acid with a higher productivity (1.76 g L(-1) h(-1)) than the parent strain (0.95 g L(-1) h(-1)).     

Enhanced kefiran production by mixed culture of Lactobacillus kefiranofaciens and Saccharomyces cerevisiae

In a batch mixed culture of Lactobacillus kefiranofaciens and Saccharomyces cerevisiae, which could assimilate lactic acid, cell growth and kefiran production rates of L. kefiranofaciens significantly increased, compared with those in pure cultures. The kefiran production rate was 36 mg l(-1) h(-1) in the mixed culture under the anaerobic condition, which was greater than that in the pure culture (24 mg l(-1) h(-1)). Under the aerobic condition, a more intensive interaction between these two strains was observed and higher kefiran production rate (44 mg l(-1) h(-1)) was obtained compared with that under the anaerobic condition. Kefiran production was further enhanced by an addition of fresh medium in the fed-batch mixed culture. In the fed-batch mixed culture, a final kefiran concentration of 5.41 g l(-1) was achieved at 87 h, thereby attaining the highest productivity at 62 mg l(-1) h(-1). Simulation study considered the reduction of lactic acid in pure culture was performed to estimate the additional effect of coculture with S. cerevisiae. Slightly higher cell growth and kefiran production rates in the mixed culture than those expected from pure culture by simulation were observed. These results suggest that coculture of L. kefiranofaciens and S. cerevisiae not only reduces the lactic acid concentration by consumption but also stimulates cell growth and kefiran production of L. kefiranofaciens.     

Kinetic study of the conversion of different substrates to lactic acid using Lactobacillus bulgaricus

Lactic acid fermentation includes several reactions in association with the microorganism growth. A kinetic study was performed of the conversion of multiple substrates to lactic acid using Lactobacillus bulgaricus. Batch experiments were performed to study the effect of different substrates (lactose, glucose, and galactose) on the overall bioreaction rate. During the first hours of fermentation, glucose and galactose accumulated in the medium and the rate of hydrolysis of lactose to glucose and galactose was faster than the convesion of these substrates. Once the microorganism built the necessary enzymes for the substrate conversion to lactic acid, the conversion rate was higher for glucose than for galactose. The inoculum preparation was performed in such a way that healthy young cells were obtained. By using this inoculum, shorter fermentation times with very little lag phase were observed. The consumption patterns of the different substrates converted to lactic acid were studied to determine which substrate controls the overall reaction for lactic acid production. A mathematical model (unstructured Monod type) was developed to describe microorganism growth and lactic acid production. A good fit with a simple equation was obtained. It was found experimentally that the approximate ratio of cell to substrate was 1 to 10, the growth yield coefficient (Y(XS)) was 0.10 g cell/g substrate, the product yield (Y(PS)) was 0.90 g lactic acid/g substrate, and the alpha parameter in the Luedeking-Piret equation was 9. The Monod kinetic parameters were obtained. The saturation constant (K(S)) was 3.36 g/L, and the specific growth rate (microm ) was 1.14 l/h.     

Production of lactic acid with loofa sponge immobilized Rhizopus oryzae RBU2-10

Lactic acid production by Rhizopus oryzae RBU2-10 immobilized in loofa sponge was evaluated. Shape and texture of loofa sponge, which was obtained from the mature dried fruit of Luffa cylindrica, remained intact after its treatment with buffers of varying pH and following its repeated autoclaving for up to four cycles (121 degrees C, 20 min per cycle). The medium having four pieces of loofa sponge (1.008 cm(3)) per 100 ml medium and inoculated with 3 x1 0(6) spores ml(-1) resulted maximum production (80.75 g l(-1)) of lactic acid in 48 h of fermentation. Repeated batch fermentation for lactic acid production could be carried out for 10 cycles. Remarkably higher levels of productivity (1.66-1.84 g l(-1)h(-1)) was obtained during first five cycles of fermentation with a maximum productivity (1.84 g l(-1)h(-1)) obtained during third cycle of fermentation.     

Ethanol production from concentrated whey permeate using a fed-batch culture ofKluyveromyces fragilis

Summary Fed-batch fermentation of non-supplemented concentrated whey permeate resulted in high ethanol productivity for feeds of lactose for which batch fermentation had a poor performance. At an initial lactose concentration of 100 g/L and a constant lactose feeding rate of 18 g/h we have obtained: ethanol concentration 64 g/L, ethanol productivity 3.3 g/Lh, lactose consumption 100%, ethanol yield 0.47 g/g, and biomass yield 0.058 g/g.Nomenclature St total lactose fed per medium volume in the bioreactor, g/L - Si initial lactose concentration, g/L - F lactpse feeding rate, g/h - P final ethanol concentration, g/L - Y_p/s ethanol yield, g ethanol/g lactose - Y_x/s biomass yield, g biomass/g lactose - X_S lactose consumption, % - Qp overall ethanol volumetric productivity, g/Lh - m maximum specific growth rate, h - qsm maximum specific lactose consumption rate, g/gh - qpm maximum specific ethanol production rate, g/gh     

Biotechnological production of l(+)-lactic acid from wood hydrolyzate by batch fermentation of Enterococcus faecalis

The inhibition of lactic acid fermentation by wood hydrolyzate was decreased (approx. 20%) by adaptation of Enterococcus faecalis RKY1 to wood hydrolyzate-based medium whereby lactic acid productivity and cell growth were enhanced by 0.5 g l(-1) h(-1) and 2.1 g l(-1), respectively. When the diluted or concentrated wood hydrolyzate (equivalent to 25-100 g glucose l(-1)) was supplemented with 15 g yeast extract l(-1), 24-93 g lactic acid l(-1) was produced at a rate between 1.7 g l(-1) h(-1) and 3.2 g l(-1) h(-1).     

Efficient conversion of lactic acid to butanol with pH-stat continuous lactic acid and glucose feeding method by Clostridium saccharoperbutylacetonicum

In order to achieve high butanol production by Clostridium saccharoperbutylacetonicum N1-4, the effect of lactic acid on acetone–butanol–ethanol fermentation and several fed-batch cultures in which lactic acid is fed have been investigated. When a medium containing 20 g/l glucose was supplemented with 5 g/l of closely racemic lactic acid, both the concentration and yield of butanol increased; however, supplementation with more than 10 g/l lactic acid did not increase the butanol concentration. It was found that when fed a mixture of lactic acid and glucose, the final concentration of butanol produced by a fed-batch culture was greater than that produced by a batch culture. In addition, a pH-controlled fed-batch culture resulted in not only acceleration of lactic acid consumption but also a further increase in butanol production. Finally, we obtained 15.5 g/l butanol at a production rate of 1.76 g/l/h using a fed-batch culture with a pH-stat continuous lactic acid and glucose feeding method. To confirm whether lactic acid was converted to butanol by the N1-4 strain, we performed gas chromatography–mass spectroscopy (GC-MS) analysis of butanol produced by a batch culture during fermentation in a medium containing [1,2,3-¹³C₃] lactic acid as the initial substrate. The results of the GC-MS analysis confirmed the bioconversion of lactic acid to butanol.     

Acetic acid production from whey lactose by the co-culture ofStreptococcus lactis andClostridium formicoaceticum

Summary Acetic acid was produced from anaerobic fermentation of lactose by the co-culture ofStreptococcus lactis andClostridium formicoaceticum at 35° C and pHs between 7.0 and 7.6. Lactose was converted to lactic acid, and then to acetic acid in this mixed culture fermentation. The overall acetic acid yield from lactose was about 95% at pH 7.6 and 90% at pH 7.0. The fermentation rate was also higher at pH 7.6 than at pH 7.0. In batch fermentation of whey permeate containing about 5% lactose at pH 7.6, the concentration of acetic acid reached 20 g/l within 20 h. The production rate then became very slow due to end-product inhibition and high Na⁺ concentration. About 30 g/l acetate and 20 g/l lactate were obtained at a fermentation time of 80 h. However, when diluted whey permeate containing 2.5% lactose was used, all the whey lactose was converted to acetic acid within 30 h by this mixed culture.     

Simultaneous production of 2,3-butanediol, ethanol and hydrogen with a Klebsiella sp. strain isolated from sewage sludge

A Klebsiella sp. HE1 strain isolated from hydrogen-producing sewage sludge was examined for its ability to produce H(2) and other valuable soluble metabolites (e.g., ethanol and 2,3-butanediol) from sucrose-based medium. The effect of pH and carbon substrate concentration on the production of soluble and gaseous products was investigated. The major soluble metabolite produced from Klebsiella sp. HE1 was 2,3-butanediol, accounting for over 42-58% of soluble microbial products (SMP) and its production efficiency enhanced after increasing the initial culture pH to 7.3 (without pH control). The HE1 strain also produced ethanol (contributing to 29-42% of total SMP) and a small amount of lactic acid and acetic acid. The gaseous products consisted of H(2) (25-36%) and CO(2) (64-75%). The optimal cumulative hydrogen production (2.7 l) and hydrogen yield (0.92molH(2)molsucrose(-1)) were obtained at an initial sucrose concentration of 30gCODl(-1) (i.e., 26.7gl(-1)), which also led to the highest production rate for H(2) (3.26mmolh(-1)l(-1)), ethanol (6.75mmolh(-1)l(-1)) and 2,3-butanediol (7.14mmolh(-1)l(-1)). The highest yield for H(2), ethanol and 2,3-butanediol was 0.92, 0.81 and 0.59molmol-sucrose(-1), respectively. As for the overall energy production performance, the highest energy generation rate was 27.7kJh(-1)l(-1) and the best energy yield was 2.45kJmolsucrose(-1), which was obtained at a sucrose concentration of 30 and 20gCODl(-1), respectively.     

Efficient lactic acid production from high salt containing dairy by-products by Lactobacillus salivarius ssp. salicinius with pre-treatment by proteolytic microorganisms

Lactic acid bacteria have an inefficient proteolytic system. Therefore, cultivation media which may have high protein content are usually supplemented with yeast extract or protein lysates (peptones). These additives might be conveniently replaced by in situ treatment of the cultivation medium with proteolytic enzymes or proteolytic microbes. Lactobacillus salivarius ssp. salicinius, a lactic acid bacterium species that can grow at high salt concentration, was used to ferment lactic acid in cheese whey (with 3 gl(-1) whey protein content) and lactose mother liquor (90 gl(-1) lactose, 9 gl(-1) proteins, 30 gl(-1) minerals). The contribution of protease enzymes or proteolytic microbes to acid production by lactobacilli was examined. Efficient conversion of lactose to lactic acid was obtained in the presence of additional proteolytic activity. Fastest acid production was obtained with the addition of protease enzymes. However, almost equally efficient acid production was obtained by treating the medium with Bacillus megaterium. The results show that fast and complete conversion of lactose to lactic acid can be obtained in dairy by-products without expensive additives.     

Continuous production of ammonium lactate by Streptococccus cremoris in a three-stage reactor

Batch and continuous fermentation studies were performed to optimize the production of ammonium lactate from whey to optimize the production of ammonium lactate from whey permeate. The product known as fermented ammoniated condensed whey permeate (FACWP) is a very promising animal feed. After an initial screening of four strains which produce predominantly L(+)- lactic acid, the desired isomer [D(-)-lactic acid is toxic], Streptococcus cremoris 2487 was chosen for further study. In batch mode, pH between 6.0 and 6.5 and 35 degrees C provided optimum incubation conditions. To stimulate a plug flow reactor, three CSTRs (continuous stirred tank reactors) were connected in tandem. For a 7.5-h retention time, 1.6-fold and 1.3-fold higher productivities were obtained for three-stage than for the single- and two-stage reactors, respectively. Various retentions times were examined (5, 7.5, and 10 h; 5g/L yeast extract). Although maximum lactate productivity occurred at a 5-h residence time (5.38 g/L H. 75% lactose utilization), lactose utilization was more complete at 7.5 h (4.38 g/L h productivity, 91% lactose utilization and a productivity, 91% lactose utilization). Retention time was increased to 15 h to obtain 95.9% lactose utilization and a productivity of 2.42g/L h for 2g/L yeast extract. Based on this lower yeast extract concentration, it was determined that ammonium lactate production and subsequent concentration by 11-fold would yield a product (FACWP) 17% more than soybean meal (crude protein contents are equivalent, 44%) at current market prices.     

Influence of Culture Parameters on Biological Hydrogen Production by Clostridium saccharoperbutylacetonicum ATCC 27021

Summary Various medium components (carbon and nitrogen sources, iron, inoculum size) and environmental factors (initial pH and the agitation speed) were evaluated for their effects on the rate and the yield of hydrogen production by Clostridium saccharoperbutylacetonicum. Among the carbon sources assessed, cells grown on disaccharides (lactose, sucrose and maltose) produced on the average more than twice (2.81 mol-H2/mol sugar) as much hydrogen as monosaccharides (1.29 mol-H2/mol sugar), but there was no correlation between the carbon source and the production rate. The highest yield (2.83 mol/mol) was obtained in lactose and sucrose but the highest production rate (1.75 mmol/h) in sucrose. Using glucose as carbon source, yeast extract was the best nitrogen source. A parallel increase between the production rate and the yield was obtained by increasing glucose concentration up to 40 g/l (1.76 mol-H2/mol, 3.39 mmol/h), total nitrogen as yeast extract up to 0.1% (1.41 mol/mol, 1.91 mmol/h) and agitation up to 100 rev/min (1.66 mol-H2/mol, 1.86 mmol/h). On the other hand, higher production rates were favoured in preference to the yield at a neutral initial pH 7 (2.27 mmol/h), 1000 mg iron/l or more (1.99 mmol/h), and a larger inoculum size, 10%, (2.36 mmol/h) whereas an initial alkaline pH of 8.5 (1.72 mol/mol), a lower iron concentration of 25 mg/l (1.74 mol/mol) and smaller inoculum size, 1%, (1.85 mol/mol) promoted higher yield over production rate.     

Lactic acid production by batch fermentation of whey permeate: A mathematical model

Summary The batch fermentation of whey permeate to lactic acid was improved by supplementing the broth with enzyme-hydrolyzed whey protein. A mathematical model based on laboratory results predicts to a 99% confidence limit the kinetics of this fermentation. Cell growth, acid production and protein and sugar use rates are defined in quantifiable terms related to the state of cell metabolism. The model shows that the constants of the Leudeking-Piret model are not true constants, but must vary with the medium composition, and especially the peptide average molecular weight. The kinetic mechanism on which the model is based also is presented.Nomenclature K _i lactic acid inhibition constant (g/l) - K _pr protein saturation constant during cell growth (g/l) - K _pr protein saturation constant during maintenance (g/l) - K _s lactose saturation constant (g/l) - [LA] lactic acid concentration (g/l) - [PR] protein concentration (g/l) - [S] lactose concentration (g/l) - t time (h) - [X] cell mass concentration (g/l) - , fermentation constants of Leudeking and Piret - specific growth rate (l/h) - Y _{g, LA/S} acid yield during cell growth (g acid/g sugar) - Y _{m, LA/S} acid yield during maintenance (g acid/g sugar) - Y _x/pr yield (g cells/g protein) - specific sugar use rate during cell growth (g sugar/h·g cell) - specific sugar use rate during maintenance (g sugar/h·cell)     

SSF production of lactic acid from cellulosic biosludges

The use of cellulosic biosludges generated in a Kraft pulp mill was investigated as substrate for lactic acid production by simultaneous saccharification and fermentation (SSF). The effect of the operation mode (batch or fedbatch), the initial liquid to solid ratio (12 or 30 g/g) and the nutrient supplementation (MRS components or none) on several parameters including lactic acid concentration, volumetric productivity and product yields, were evaluated. When the operation was carried out in fedbatch mode with nutrient supplementation and using a LSR(0)=12 g/g, a broth containing 42 g/L was obtained after 48 h with a volumetric productivity of 0.87 g/L h and a product yield of 37.8 g lactic acid/100 g biosludges. In a similar experiment carried out without nutrient supplementation, a lactic acid concentration of 39.4 g/L was obtained after 48 h with a volumetric productivity of 0.82 g/L h and a product yield of 35.5 g L-lactic acid/100 g biosludges.     

Evaluation of plastic-composite supports in repeated fed-batch biofilm lactic acid fermentation by Lactobacillus casei

A customized stirred-tank biofilm reactor was designed for plastic-composite supports (PCS). In repeated-batch studies, the PCS-biofilm reactors outperformed the suspended-cell reactors by demonstrating higher lactic acid productivities (2.45 g l(-1) h(-1) vs 1.75 g l(-1) h(-1)) and greater glucose consumption rates (3.27 g l(-1) h(-1) vs 2.09 g l(-1) h(-1)). In the repeated fed-batch studies, reactors were spiked periodically with concentrated glucose (75%) to maintain a concentration of approximately 80 g of glucose l(-1) in the bioreactor. In suspended-cell fermentations with 10 g of yeast extract (YE) l(-1) and zero, one, two, and three glucose spikes, the lactic acid productivities were 2.64, 1.58, 0.80, and 0.62 g l(-1) h(-1), respectively. In comparison, biofilm reactors with 7 g of YE l(-1) and zero, one, two, and three glucose spikes achieved lactic acid productivities of 4.20, 2.78, 0.66, and 0.94 g l(-1) h(-1), respectively. The use of nystatin (30 U ml(-1)) subdued the contaminating yeast population with no effect on the lactic acid productivity of the biofilm reactors, but it did affect productivity in the suspended-cell bioreactor. Overall, in repeated fed-batch fermentations, the biofilm reactors consistently outperformed the suspended-cell bioreactors, required less YE, and produced up to 146 g of lactic acid l(-1) with 7 g of YE l(-1), whereas the suspended-cell reactor produced 132 g l(-1) with 10 g of YE l(-1).     

Use of different carbon sources in cultivation of recombinant Pichia pastoris for angiostatin production

To improve the growth of recombinant Pichia pastoris with a phenotype of Mut^S and expression of angiostatin, the effects of glycerol, sorbitol, acetate and lactic acid which were, respectively, added together with methanol in the expression phase, were studied in a 5-l fermentor. Methanol concentration was automatically controlled at 5 g/l by a methanol monitor and control system, while the feeding of the other carbon source was manually adjusted. The angiostatin production level was 108 mg/l when glycerol was added at an initial rate of 2.3 g/h and gradually increased to 9.9 g/h within an induction period of 96 h. The angiostatin concentration was 141 mg/l as sorbitol was used, while only 52 mg/l were obtained on acetate. The highest angiostatin production of 191 mg/l was achieved as lactic acid was used; whose feeding rate was gradually increased from 2.6 to 11.3 g/h. Lactic acid accumulated during the induction phase and reached 6.3 g/l at the end of fermentation. However, the accumulation of lactic acid did not interfere with angiostatin production, indicating that lactic acid to be a non-repressive carbon source. The average productivity and specific productivity of angiostatin obtained on lactic acid and methanol were, respectively, 2.96 and 0.044 mg/(g h), 1.7- and 2.5-fold of those obtained in the fermentation fed with glycerol and methanol.     

Synthesis of galacto-oligosaccharides by β-galactosidase from Aspergillus oryzae using partially dissolved and supersaturated solution of lactose

The effect of enzyme to substrate ratio, initial lactose concentration and temperature has been studied for the kinetically controlled reaction of lactose transgalactosylation with Aspergillus oryzae β-galactosidase, to produce prebiotic galacto-oligosaccharides (GOS). Enzyme to substrate ratio had no significant effect on maximum yield and specific productivity. Galacto-oligosaccharide syntheses at very high lactose concentrations (40, 50 and 60%, w/w, lactose monohydrate) were evaluated at different temperatures (40, 47.5 and 55°C). Within these ranges, lactose could be found as a supersaturated solution or a heterogeneous system with precipitated lactose, resulting in significant effect on GOS synthesis. An increase in initial lactose concentration produced a slight increase in maximum yield as long as lactose remained dissolved. Increase in temperature produced a slight decrease in maximum yield and an increase in specific productivity when supersaturation of lactose occurred during reaction. Highest yield of 29 g GOS/100 g lactose added was obtained at a lactose monohydrate initial concentration of 50% (w/w) and 47.5°C. Highest specific productivity of 0.38 g GOSh(-1) mg enzyme(-1) was obtained at lactose monohydrate initial concentration of 40% (w/w) and 55°C, where a maximum yield of 27 g GOS/100 g lactose added was reached. This reflects the complex interplay between temperature and initial lactose concentration on the reaction of synthesis. When lactose precipitation occurred, values of yields and specific productivities lower than 22 g GOS/100 g lactose added and 0.03 gGOSh(-1) mg enzyme(-1) were obtained, respectively.     

Bacterial cellulose production under oxygen-enriched air at different fructose concentrations in a 50-liter, internal-loop airlift reactor

Bacterial cellulose (BC) production by Acetobacter xylinum subsp. sucrofermentans BPR2001 was carried out in a 50-1 internal-loop airlift reactor in air at an initial fructose concentration of 40 g/l. The BC production rate was 0.059 g/l per h. When oxygen-enriched air was supplied instead of air, the BC production rate increased to 0.093 g/l per h, and the BC yield was enhanced from 11% in air to 18%. When the initial fructose concentrations were varied from 30 to 70 g/l, the highest BC yield (35%) the highest production rate (0.22 g/l x per h), and the highest concentration of BC produced (10.4 g/l) were observed at 60-70 g/l fructose. From the carbon mass balance calculated at the final stage of cultivation, it was observed that enhanced BC production was reflected as a decrease in both CO2 evolution and the concentration of other unknown substances, suggesting the efficient utilization of energy for BC synthesis despite O2 limitation.