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1.
The five-carbon sugar d-xylose is a major component of hemicellulose and accounts for roughly one-third of the carbohydrate content of many lignocellulosic
materials. The efficient fermentation of xylose-rich hemicellulose hydrolyzates (prehydrolyzates) represents an opportunity
to improve significantly the economics of large-scale fuel ethanol production from lignocellulosic feedstocks. The National
Renewable Energy Laboratory (NREL) is currently investigating a simultaneous saccharification and cofermentation (SSCF) process
for ethanol production from biomass that uses a dilute-acid pretreatment and a metabolically engineered strain of Zymomonas mobilis that can coferment glucose and xylose. The objective of this study was to establish optimal conditions for cost-effective
seed production that are compatible with the SSCF process design.
Two-level and three-level full factorial experimental designs were employed to characterize efficiently the growth performance
of recombinant Z. mobilis CP4:pZB5 as a function of nutrient level, pH, and acetic acid concentration using a synthetic hardwood hemicellulose hydrolyzate
containing 4% (w/v) xylose and 0.8% (w/v) glucose. Fermentations were run batchwise and were pH-controlled at low levels of
clarified corn steep liquor (cCSL, 1-2% v/v), which were used as the sole source of nutrients. For the purpose of assessing
comparative fermentation performance, seed production was also carried out using a “benchmark” yeast extract-based laboratory
medium. Analysis of variance (ANOVA) of experimental results was performed to determine the main effects and possible interactive
effects of nutrient (cCSL) level, pH, and acetic acid concentration on the rate of xylose utilization and the extent of cell
mass production. Results indicate that the concentration of acetic acid is the most significant limiting factor for the xylose
utilization rate and the extent of cell mass production; nutrient level and pH exerted weaker, but statistically significant
effects. At pH 6.0, in the absence of acetic acid, the final cell mass concentration was 1.4 g dry cell mass/L (g DCM/L),
but decreased to 0.92 and 0.64 g DCM/L in the presence of 0.5 and 1.0% (w/v) acetic acid, respectively. At concentrations
of acetic acid of 0.75 (w/v) or lower, fermentation was complete within 1.5 d. In contrast, in the presence of 1.0% (w/v)
acetic acid, 25% of the xylose remained after 2 d. At a volumetric supplementation level of 1.5–2.0% (v/v), cCSL proved to
be a cost-effective single-source nutritional adjunct that can support growth and fermentation performance at levels comparable
to those achieved using the expensive yeast extract-based laboratory reference medium. 相似文献
2.
Fermentation kinetics of ethanol production from glucose, xylose, and their mixtures using a recombinant Saccharomyces 1400 (pLNH33) are reported. Single-substrate kinetics indicate that the specific growth rate of the yeast and the specific
ethanol productivity on glucose as the substrate was greater than on xylose as a substrate. Ethanol yields from glucose and
xylose fermentation were typically 95 and 80% of the theoretical yield, respectively. The effect of ethanol inhibition is
more pronounced for xylose fermentation than for glucose fermentation. Studies on glucose-xylose mixtures indicate that the
recombinant yeast co-ferments glucose and xylose. Fermentation of a 52.8 g/L glucose and 56.3 g/L xylose mixture gave an ethanol
concentration of 47.9 g/L after 36 h. Based on a theoretical yield of 0.51 g ethanol/g sugars, the ethanol yield from this
experiment (for data up to 24 h) was calculated to be 0.46 g ethanol/g sugar or 90% of the theoretical yield. The specific
growth rate of the yeast on glucose-xylose mixtures was found to lie between the specific growth rate on glucose and the specific
growth rate on xylose. Kinetic studies were used to develop a fermentation model incorporating the effects of substrate inhibition,
product inhibition, and inoculum size. Good agreements were obtained between model predictions and experimental data from
batch fermentation of glucose, xylose, and their mixtures. 相似文献
3.
In the production of ethanol from lignocellulosic biomass, the hydrolysis of the acetylated pentosans in hemicellulose during pretreatment produces acetic acid in the prehydrolysate. The National Renewable Energy Laboratory (NREL) is currently investigating a simultaneous saccharification and cofermentation (SSCF) process that uses a proprietary metabolically engineered strain ofZymomonas mobilis that can coferment glucose and xylose. Acetic acid toxicity represents a major limitation to bioconversion, and cost-effective means of reducing the inhibitory effects of acetic acid represent an opportunity for significant increased productivity and reduced cost of producing fermentation fuel ethanol from biomass. In this study, the fermentation performance of recombinant Z.mobilis 39676:pZB4L, using a synthetic hardwood prehydrolysate containing 1% (w/v) yeast extract, 0.2% KH2PO4, 4% (w/v) xylose, and 0.8% (w/v) glucose, with varying amounts of acetic acid was examine. To minimize the concentration of the inhibitory undissociated form of acetic acid, the pH was controlled at 6.0. The final cell mass concentration decreased linearly with increasing level of acetic acid over the range 0-0.75% (w/v), with a 50% reduction at about 0.5% (w/v) acetic acid. The conversion efficiency was relatively unaffected, decreasing from 98 to 92%. In the absence of acetic acid, batch fermentations were complete at 24 h. In a batch fermentation with 0.75% (w/v) acetic acid, about two-thirds of the xylose was not metabolized after 48 h. In batch fermentations with 0.75% (w/v) acetic acid, increasing the initial glucose concentration did not have an enhancing effect on the rate of xylose fermentation. However, nearly complete xylose fermentation was achieved in 48 h when the bioreactor was fed glucose. In the fed-batch system, the rate of glucose feeding (0.5 g/h) was designed to simulate the rate of cellulolytic digestion that had been observed in a modeled SSCF process with recombinant Zymomonas. In the absence of acetic acid, this rate of glucose feeding did not inhibit xylose utilization. It is concluded that the inhibitory effect of acetic acid on xylose utilization in the SSCF biomass-to-ethanol process will be partially ameliorated because of the simultaneous saccharification of the cellulose. 相似文献
4.
The effect of glucose on xylose-xylitol metabolism in fermentation medium consisting of sugarcane bagasse hydrolysate was
evaluated by employing an inoculum of Candida guilliermondii grown in synthetic media containing, as carbon sources, glucose (30 g/L), xylose (30 g/L), or a mixture of glucose (2 g/L)
and xylose (30 g/L). The inoculum medium containing glucose promoted a 2.5-fold increase in xylose reductase activity (0.582
IU/mg prot) and a 2-fold increase in xylitol dehydrogenase activity (0.203 IU/mg prot) when compared with an inoculum-grown medium containing only xylose. The improvement in enzyme activities resulted in higher
values of xylitol yield (0.56 g/g) and productivity (0.46 g/[L·h]) after 48 h of fermentation. 相似文献
5.
Glucose/xylose mixtures (90 g/L total sugar) were evaluated for their effect on ethanol fermentation by a recombinant flocculent Saccharomyces cerevisiae, MA-R4. Glucose was utilized faster than xylose at any ratio of glucose/xylose, although MA-R4 can simultaneously co-ferment both sugars. A high percentage of glucose can increase cell biomass production and therefore increase the rate of glucose utilization (1.224 g glucose/g biomass/h maximum) and ethanol formation (0.493 g ethanol/g biomass/h maximum). However, the best ratio of glucose/xylose for the highest xylose consumption rate (0.209 g xylose/g biomass/h) was 2:3. Ethanol concentration and yield increased and by-product (xylitol, glycerol, and acetic acid) concentration decreased as the proportion of glucose increased. The maximum ethanol concentration was 41.6 and 21.9 g/L after 72 h of fermentation with 90 g/L glucose and 90 g/L xylose, respectively, while the ethanol yield was 0.454 and 0.335 g/g in 90 g/L glucose and 90 g/L xylose media, respectively. High ethanol yield when a high percentage of glucose is available is likely due to decreased production of by-products, such as glycerol and acetic acid. These results suggest that ethanol selectivity is increased when a higher proportion of glucose is available and reduced when a higher proportion of xylose is available. 相似文献
6.
In pH-controlled batch fermentations with pure sugar synthetic hardwood hemicellulose (1% [w/v] glucose and 4% xylose) and
corn stover hydrolysate (8% glucose and 3.5% xylose) lacking acetic acid, the xyloseutilizing, tetracycline (Tc)-sensitive,
genomically integrated variant of Zymomonas mobilis ATCC 39676 (designated strain C25) exhibited growth and fermentation performance that was inferior to National Renewable
Energy Laboratory's first-generation, Tc-resistant, plasmid-bearing Zymomonas recombinants. With C25, xylose fermentation following glucose exhaustion wasmarkellyslower, and the ethanol yield (based
on sugars consumed) was lower, owing primarily to an increase in lactic acid formation. There was an apparent increased sensitivity
to acetic acid inhibition with C25 compared with recombinants 39676:pZB4L, CP4:pZB5, and ZM4:pZB5. However, strain C25 performed
well in continous ferm entation with nutrient-rich synthetic corn stover medium over the dilution range 0.03–0.06/h, with
a maximum provess ethanol yield at D=0.03/h of 0.46 g/g and a maximum ethanol productivity of 3 g/(L·h). With 0.35% (w/v) acetic acid in the medium, the process
yield at D=0.04/h dropped to 0.32 g/g, and the maximum productivity decreased by 50% to 1.5 g/(L·h). Under the same operating conditions,
rec Zm Zm 4:pZB5 performed better; however, the medium contained 20 mg/L of Tc to constantly maintain selective pressure.
The absence of any need for antibiotics and antiboitic resistance genes makes the chromosomal integrant C25 more com patible
with current regulatory specifications for biocatalysts in large-scale commercial operations. 相似文献
7.
To develop a cost-effective fermentation medium, spent brewer's yeast hydrolysate was evaluated as a nitrogen source for succinic
acid production by Actinobacillus succinogenes NJ113 in glucose-containing media. Autolysis and enzymatic hydrolysis were used to hydrolyze the spent brewer's yeast cells
to release the nutrients. The results showed that enzymatic hydrolysis was a more effective method due to the higher succinic
acid yield and cell growth. However, the incomplete glucose consumption indicated existence of nutrient limitation. Vitamins
were subsequently identified as the main limiting factors for succinic acid production using enzymatically hydrolyzed spent
brewer's yeast as a nitrogen source. After the addition of vitamins, cell growth and succinic acid concentration both improved.
As a result, 15 g/L yeast extract could be successfully replaced with the enzymatic hydrolysate of spent brewer's yeast with
vitamins supplementation, resulting in a production of 46.8 g/L succinic acid from 68 g/L glucose. 相似文献
8.
This work represents a continuation of our investigation into environmental conditions that promote lactic acid synthesis
by Zymomonas mobilis. The characteristic near theoretical yield of ethanol from glucose by Z. mobilis can be compromised by the synthesis of d- and l-lactic acid. The production of lactic acid is exacerbated by the following conditions: pH 6.0, yeast extract, and reduced
growth rate. At a specific growth rate of 0.048/h, the average yield of dl-lactate from glucose in a yeast extract-based medium at pH 6.0 was 0.15 g/g. This represents a reduction in ethanol yield
of about 10% relative to the yield at a growth rate of 0.15/h. Very little lactic acid was produced at pH 5.0 or using a defined
salts medium (without yeast extract) Under permissive and comparable culture conditions, a tetracycline-resistant, d- ldh negative mutant produced about 50% less lactic acid than its parent strain Zm ATCC 39676. d-lactic acid was detected in the cell-free spent fermentation medium of the mutant, but this could be owing to the presence
of a racemase enzyme. Under the steady-state growth conditions provided by the chemostat, the specific rate of glucose consumption
was altered at a constant growth rate of 0.075/h. Shifting from glucose-limited to nitrogen-limited growth, or increasing
the temperature, caused an increase in the specific rate of glucose catabolism. There was good correlation between an increase
in glycolytic flux and a decrease in lactic acid yield from glucose. This study points to a mechanistic link between the glycolytic
flux and the control of end-product glucose metabolism. Implications of reduced glycolytic flux in pentose-fermenting recombinant
Z. mobilis strains, relative to increased byproduct synthesis, is discussed. 相似文献
9.
Scheffersomyces stipitis was cultivated in an optimized, controlled fed-batch fermentation for production of ethanol from glucose–xylose mixture. Effect of feed medium composition was investigated on sugar utilization and ethanol production. Studying influence of specific cell growth rate on ethanol fermentation performance showed the carbon flow towards ethanol synthesis decreased with increasing cell growth rate. The optimum specific growth rate to achieve efficient ethanol production performance from a glucose-xylose mixture existed at 0.1 h ?1. With these optimized feed medium and cell growth rate, a kinetic model has been utilized to avoid overflow metabolism as well as to ensure a balanced feeding of nutrient substrate in fed-batch system. Fed-batch culture with feeding profile designed based on the model resulted in high titer, yield, and productivity of ethanol compared with batch cultures. The maximal ethanol concentration was 40.7 g/L. The yield and productivity of ethanol production in the optimized fed-batch culture was 1.3 and 2 times higher than those in batch culture. Thus, higher efficiency ethanol production was achieved in this study through fed-batch process optimization. This strategy may contribute to an improvement of ethanol fermentation from lignocellulosic biomass by S. stipitis on the industrial scale. 相似文献
10.
The combined effects of inhibitors present in lignocellulosic hydrolysates was studied using a multivariate statistical approach.
Acetic acid (0–6 g/L), formic acid (0–4.6 g/L) and hydroquinone (0–3 g/L) were tested as model inhibitors in synthetic media
containing a mixture of glucose, xylose, and arabinose simulating concentrated hemicellulosic hydrolysates. Inhibitors were
consumed sequentially (acetic acid, formic acid, and hydroquinone), alongside to the monosaccharides (glucose, xylose, and
arabinose). Xylitol was always the main metabolic product. Additionally, glycerol, ethanol, and arabitol were also obtained.
The inhibitory action of acetic acid on growth, on glucose consumption and on all product formation rates was found to be
significant ( p≤0.05), as well as formic acid inhibition on xylose consumption and biomass production. Hydroquinone negatively affected biomass
productivity and yield, but it significantly increased xylose consumption and xylitol productivity. Hydroquinone interactions,
either with acetic or formic acid or with both, are also statistically signficant. Hydroquinone seems to partially lessen
the acetic acid and amplify formic acid effects. The results clearly indicate that the interaction effects play an important
role on the xylitol bioprocess. 相似文献
11.
Efficient utilization of the pentosan fraction of hemicellulose from lignocellulosic feedstocks offers an opportunity to increase
the yield and to reduce the cost of producing fuel ethanol. During prehydrolysis (acid hydrolysis or autohydrolysis of hemicellulose),
acetic acid is formed as a consequence of the deacetylation of the acetylated moiety of hemicellulose. Recombinant Escherichia
coli B (ATCC 11303), carrying the plasmid pLO1297 with pyruvate decarboxylase and alcohol dehydrogenase II genes from Zymomonas
mobilis (CP4), converts xylose to ethanol with a product yield that approaches theoretical maximum. Although other pentose-utilizing
microorganisms are inhibited by acetic acid, the recombinant E. coli displays a high tolerance for acetic acid. In xylose
fermentations with a synthetic medium (Luria broth), where the pH was controlled at 7, neither yield nor productivity was
affected by the addition of 10.7 g/L acetic acid. Nutrient-supplemented, hardwood (aspen) hemicellulose hydrolysate (40.7
g/L xylose) was completely fermented to ethanol (16.3 g/L) in 98 h. When the acetic acid concentration was reduced from 5.6
to 0.8 g/L, the fermentation time decreased to 58 h. Overliming, with Ca(OH) 2 to pH 10, followed by neutralization to pH 7 with sulfuric acid and removal of insolubles, resulted in a twofold increase
in volumetric productivity. The maximum productivity was 0.93 g/L/h. The xylose-to-ethanol conversion efficiency and productivity
in Ca(OH) 2-treated hardwood prehydrolysate, fortified with only mineral salts, were 94% and 0.26 g/L/h, respectively. The recombinant
E. coli exhibits a xylose-to-ethanol conversion efficiency that is superior to that of other pentose-utilizing yeasts currently
being investigated for the production of fuel ethanol from lignocellulosic materials. 相似文献
12.
In this study, the fed-batch fermentation technique was applied to improve the yield of l-threonine produced by Escherichia coli TRFC. Various fermentation substrates and conditions were investigated to identify the optimal carbon source, its concentration
and C/N ratio in the production of l-threonine. Sucrose was found to be the optimal initial carbon source and its optimal concentration was determined to be 70 g/L
based on the results of fermentations conducted in a 5-L jar fermentor using a series of fed-batch cultures of E. coli TRFC. The effects of glucose concentration and three different feeding methods on the production of l-threonine were also investigated in this work. Our results showed that the production of l-threonine by E. coli was enhanced when glucose concentration varied between 5 and 20 g/L with DO-control pulse fed-batch method. Furthermore,
the C/N ratio was a more predominant factor than nitrogen concentration for l-threonine overproduction and the optimal ratio of ammonium sulfate to sucrose ( g/ g) was 30. Under the optimal conditions, a final l-threonine concentration of 118 g/L was achieved after 38 h with the productivity of 3.1 g/L/h (46% conversion ratio from
glucose to threonine). 相似文献
13.
The fermentation characteristics of two recombinant strains of Zymomonas mobilis, viz. CP4 (pZB5) and ZM4 (pZB5), capable of converting both glucose and xylose to ethanol, have been characterized in batch
and continuous culture studies. The strain ZM4 (pZB5) was found to be capable of converting a mixture of 65 g/L glucose and
65 g/L xylose to 62 g/L ethanol in 48h with a yield of 0.46 g/g. Higher sugar concentrations resulted in incompletexylose
utilization (80h) presumably owing to ethanol inhibition of xylose assimilation or metabolism. The fermentation results with
ZM4 (pZB5) show a significant improvement over results published previously for recombinant yeasts and other bacteria capable
of glucose and xylose utilization. 相似文献
14.
This study examined the continuous cofermentation performance characteristics of a dilute-acid “prehydrolysate-adapted” recombinant
Zymomonas 39676:pZB4L and builds on the pH-stat batch fermentations with this recombinant that we reported on last year. Substitution
of yeast extract by 1% (w/v) corn steep liquor (CSL) (50% solids) and Mg (2 mM) did not alter the coferm entation performance.
Using declared assumptions, the cost of using CSL and Mg was estimated to be 12.5c/gal of ethanol with a possibility of 50%
cost reduction using fourfold less CSL with 0.1% diammonium phosphate. Because of competition for a common sugar transporter
that exhibits a higher affinity for glucose, utilization of glucose was complete whereas xylose was always present in the
chemostat effluent. The ethanol yield, based on sugar used, was 94% of theoretical maximum. Altering the sugar ratio of the
synthetic dilute acid hardwood prehydrolysate did not appear to significantly change the pattern of xylose utilization. Using
a criterion of 80% sugar utilization for determining the maximum dilution rate ( D
max), changing the composition of the feed from 4% xylose to 3%, and simultaneously increasing the glucose from 0.8 to 1.8% shifted
D
max from 0.07 to 0.08/h. With equal amounts of both sugars (2.5%), D
max was 0.07/h. By comparison to a similar investigation with rec Zm CP4:pZB5 with a 4% equal mixture of xylose and glucose,
we observed that at pH 5.0, the D
max was 0.064/h and shifted to 0.084/h at pH 5.75. At a level of 0.4% (w/v) acetic acid in the CSL-based medium with 3% xylose
and 1.8% glucose at pH 5.75, the D
max for the adapted recombinant shifted from 0.08 to 0.048/h, and the corresponding maximum volumetric ethanol productivity decreased
45%, from 1.52 to 0.84 g/(L·h). Under these conditions of continuous culture, linear regression of a Pirt plot of the specific
rate of sugar utilization vs D showed that 4 g/L of acetic acid did not affect the maximum growth yield (0.030 g dry cell mass/g sugar), but did increase
the maintenance coefficient twofold, from 0.46 to 1.0 g of sugar/(g of cell·h). 相似文献
15.
Long-term (149 d) continuous fermentation was used to adapt a xylose-fermenting recombinant Zymomonas mobilis, strain 39676:pZB 4L, to conditioned (overlimed) dilute-acid yellow poplar hemicellulose hydrolyzate (“prehydrolyzate”).
An “adapted” variant was isolated from a chemostat operating at a dilution rate of 0.03/h with a 50% (v/v) prehydrolyzate,
corn steep liquor, and sugar-supplemented medium, at pH 5.75. The level of xylose and glucose in the medium was kept constant
at 4% (w/v) and 0.8% (w/v), respectively. These sugar concentrations reflect the composition of the undiluted hardwood prehydrolyzate.
The level of conditioned hardwood prehydrolyzate added to the medium was increased in 5% increments startingata level of 10%.
At the upper level of 50% prehydrolyzate, the acetic-acid concentration was about 0.75% (w/v). The adapted variant exhibited
improved xylose-fermentation performance in a pure-sugar, synthetic hardwood prehydrolyzate medium containing 4% xylose (w/v),
0.8% (w/v) glucose, and acetic acid in the range 0.4–1.0% (w/v). The ethanol yield was 0.48–0.50 g/g; equivalent to a sugar-to-ethanol
conversion efficiency of 94–96% of theoretical maximum. The maximum growth yield and maintenance energy coefficients were
0.033 g dry cell mass (DCM)/g sugars and 0.41 g sugars/g DCM/h, respectively. The results confirm that long-term continuous
adaptation is a useful technique for effecting strain improvement with respect to the fermentation of recalcitrant feedstocks. 相似文献
16.
The fermentation characteristics of a recombinant strain of Zymomonas mobilis ZM4(pZB5) capable of converting both glucose and xylose to ethanol have been further investigated. Previous studies have
shown that the strain ZM4(pZB5) was capable of converting a mixture o 65 g/L of glucose and 65 g/L of xylose to 62 g/L of
ethanol in 48 h with an overall yield of 0.46 g/g. Higher sugar concentrations (e.g., 75/75 g/L) resulted in incomplete xylose
utilization (80 h). In the present study, further kinetic evaluations at high sugar levels are reported. Acetate inhibition
studies and evaluation of temperature and pH effects indicated increased maximum specific uptake rates of glucose and xylose
under stressed conditions with increased metabolic uncoupling. A high-productivity system was developed that involved a membrane
bioreactor with cell recycling. At sugar concentrations of approx 50/50 g/L of glucose/xylose, an ethanol concentration of
50 g/L, an ethanol productivity of approx 5 g/(L·h), and a yield ( Y
p/s) of 0.50 g/g were achieved. Decreases in cell viability were found in this system after attainment of an initial steady state
(40–60 h); a slow bleed of concentrated cells may be required to overcome this problem. 相似文献
17.
Using the simultaneoussaccharification and fermentation (SSF) technique, pulp mill solid waste cellulose was converted into
glucose using cellulase enzyme and glucose into lacticacid using NRRL B445. SSF experiments were conducted at various pH levels,
temperatures, and nutrient concentrations, and the lactic acid yield ranged from 86 to 97%. The depletion of xylose in SSF
was further investigated by inoculating NRRL B445 into a xylose-only medium. On prolonged incubation, depletion of xylose
with lactic acid production was observed. An experimental procedure with a nonglucose medium was developed to eliminate the
lag phase. From xylose fermentation, Lactobacillus delbrueckii yielded 88–92% lactic acid and 2–12% acetic acid. 相似文献
18.
The effect of pH, time of fermentation, and xylose and glucose concentration on xylitol production, cell growth, xylose reductase
(XR), and xylitol dehydrogenase (XD) activities of Candida guilliermondii FTI 20037 were determined. For attaining XR and XD activities of 129-2190 U/mg of protein and 24-917 U/mg of protein, respectively,
the cited parameters could vary as follows: initial pH: 3.0-5.0; xylose: 15-60 g/L; glucose: 0-5 g/L; and fermentation time:
12-24 h. Moreover, the high XR and XD activities occurred when the xylitol production by the yeast was less than 19.0 g/L. 相似文献
19.
Iogen Corporation of Ottawa, Canada, has recently built a 50 t/d biomass-to-ethanol demonstration plant adjacent to its enzyme
production facility. Iogen has partnered with the University of Toronto to test the C6/C5 cofermentation performance characteristics
of National Renewable Energy Laboratory's metabolically engineered Zymomonas mobilis using its biomass hydrolysates. In this study, the biomass feedstock was an agricultural waste, namely oat hulls, which was
hydrolyzed in a proprietary two-stage process involving pretreatment with dilute sulfuric acid at 200–250°C, followed by cellulase
hydrolysis. The oat hull hydrolysate (OHH) contained glucose, xylose, and arabinose in a mass ratio of about 8:3:0.5. Fermentation
media, prepared from diluted hydrolysate, were nutritionally amended with 2.5 mL/L of corn steep liquor (50% solids) and 1.2
g/L of diammonium phosphate. The estimated cost for large-scale ethanol production using this minimal level of nutrient supplementation
was 4.4c/gal of ethanol. This work examined the growth and fermentation performance of xyloseutilizing, tetracycline-resistant,
plasmid-bearing, patented, recombinant Z. mobilis cultures: CP4:pZB5, ZM4:pZB5, 39676:pZB4L, and a hardwood prehydrolysate-adapted variant of 39676:pZB4L (designated asthe
“adapted” strain). In pH-stat batch fermentations with unconditioned OHH containing 6% (w/v) glucose, 3% xylose, and 0.75%
acetic acid, rec Zm ZM4:pZB5 gave the best performance with a fermentation time of 30h, followed by CP4:pZB5 at 48h, with
corresponding volumetric productivities of 1.4 and 0.89 g/(L·h), respectively. Based on the available glucose and xylose,
the process ethanol yield for both strains was 0.47 g/g (92% conversion efficiency). At 48 h, the process yield for rec Zm
39676:pZB4L and the adapted strain was 0.32 and 0.34 g/g, respectively. None of the test strains was able to fermentarabinose.
Acetic acid tolerance appeared to be a major determining factor in cofermentation performance. 相似文献
20.
Lactic acid is used as a food additive for flavor and preservation and a precursor in the development of poly-lactic acid,
a product used to make biodegradable plastics and textiles. Rhizopus oryzae NRRL 395 is known to be a strain that produces optically pure l-(+)-lactic acid. The morphology of Rhizopus cultures is complex, forming filamentous, clumps, and pellet mycelia. Different morphology growth has significant effects
on lactic acid production. In bioreactors, the filamentous or clump mycelia increase the viscosity of the medium, wrap around
impellers, and block the nutrient transportation, leading to a decrease in production efficiency and bioreactor performance.
Growing fungi in pellet form can significantly improve these problems. In this study, factors that affect lactic acid production
in pelletized flask cultures using R. oryzae NRRL 395 were investigated in detail. Completely randomized designs were used to determine the influence of culture temperature,
time, concentration of glucose, and inoculum size. Lactic acid fermentation using clump and pellet morphologies were performed
in a 5 L fermentor at the optimal values obtained from flask culture. Finally, fed-batch culture was used to enhance the lactate
concentration in broth. The final lactate concentration of fed-batch culture reached 92 g/L. The data presented in the article
can provide useful information on optimizing lactic acid production using alternative source materials. 相似文献
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