Synthetic Xylose Research Articles

Xylitol biosynthesis and process scale-up from wheat straw hemicellulosic hydrolysate using Candida tropicalis

Due to growing applications of xylitol in pharma and food industries, process development for xylitol biosynthesis from lignocellulosic biomass seems to be a promising strategy. The present study focuses on the process development for facilitating efficient xylose to xylitol bioconversion from wheat straw hemicellulosic hydrolysate using Candida tropicalis ATCC 13803. A methodical attempt to scale up the fermentation bioprocess is demonstrated by maintaining a constant volumetric mass transfer coefficient (kLa) in bioreactors of various scales. Xylitol productivity of 1.47 and 0.58 g/Lh and yield of 1.58 and 0.74 g/g, respectively, were achieved from xylose-rich hydrolysate (41.49 g/L xylose) and synthetic xylose (40.69 g/L) in a 0.5 L bioreactor at an optimum kLa of 0.74 min-1 and 0.68 min-1, respectively. Further, scale-up in 1 L bioreactor using optimum kLa resulted in xylitol productivity (g/Lh) and yield (g/g) of 1.49 & 0.58; 1.57 & 0.74, respectively, from hydrolysate and synthetic xylose. Moreover, scale-up in a 5 L bioreactor using optimum kLa resulted in xylitol yield and productivity of 0.74 g/g and 1.55 g/Lh from synthetic xylose. The successful bioprocess scale-up studies highlight the relevance of lignocellulosic biomass’s valorization to high-valued xylitol.

Bioconversion of hemicellulosic fraction of wheat straw biomass to bioethanol by Scheffersomyces stipitis: A kLa-based scale-up study

The potent utilization of hemicellulosic fraction, which comprises one-third of carbohydrates in lignocellulosic biomass (LCB), is vital to strengthen the bioethanol production from it. Most studies on bioethanol production from LCB focused on cellulosic fraction only, or very low ethanol productivity is reported from hemicellulosic fraction. The present study investigated a process to convert the hemicellulosic fraction of wheat straw biomass to bioethanol effectively using Scheffersomyces stipitis. This work aimed to develop and demonstrate a systematic approach for scaling up the fermentation process, keeping the volumetric oxygen transfer coefficient (kLa) nearly constant in different scale bioreactors. It was found that 99.11±0.13% of synthetic xylose and 99.04±0.10% of lignocellulosic xylose were converted to bioethanol with a yield and productivity of 0.4706±0.0022 g/g and 1.5325±0.0020 g/Lh; 0.4290±0.0030 g/g and 0.7399±0.0043 g/Lh, respectively at an optimum kLa of 0.5776 (0.5735, 0.5817) min−1 and 0.6924 (0.6887, 0.6961) min−1, respectively in a 0.5 L bioreactor. After that, these optimum kLa were validated in 1 L and 5 L bioreactors. The successful kLa validation revealed that the hemicellulosic fraction of lignocellulosic biomass could also be well utilized for bioethanol production.

Xylitol biosynthesis enhancement by Candida tropicalis via medium, process parameter optimization, and co-substrate supplementation

The present study examines the impact of nitrogen sources (yeast extract, ammonium sulfate peptone, ammonium nitrate, urea, and sodium nitrate), salt solution (0.5 g/L MgSO4, 0.5 g/L KH2PO4, 0.3 g/L CaCl2), trace elements solution (0.1 g/L CuSO4, 0.1 g/L FeSO4, 0.02 g/L MnCl2, 0.02 g/L ZnSO4), operational parameters (temperature, aeration, agitation, initial pH and xylose concentration) and co- substrate supplementation (glucose, fructose, maltose, sucrose, and glycerol) on xylitol biosynthesis by Candida tropicalis ATCC 13803 using synthetic xylose. The significant medium components were identified using the Plackett Burman design followed by central composite designs to obtain the optimal concentration for the critical medium components in shaker flasks. Subsequently, the effect of operational parameters was examined using the One Factor At a Time method, followed by the impact of five co-substrates on xylitol biosynthesis in a 1 L bioreactor. The optimal media components and process parameters are as follows: peptone: 12.68 g/L, yeast extract: 6.62 g/L, salt solution (0.5 g/L MgSO4, 0.5 g/L KH2PO4, and 0.3 g/L CaCl2): 1.23 X (0.62 g/L, 0.62 g/L, and 0.37 g/L respectively), temperature: 30 °C, pH: 6, agitation: 400 rpm, aeration: 1 vvm, and xylose: 50 g/L. Optimization studies resulted in xylitol yield and productivity of 0.71 ± 0.004 g/g and 1.48 ± 0.018 g/L/h, respectively. Glycerol supplementation (2 g/L) further improved xylitol yield (0.83 ± 0.009 g/g) and productivity (1.87 ± 0.020 g/L/h) by 1.66 and 3.12 folds, respectively, higher than the unoptimized conditions thus exhibiting the potential of C. tropicalis ATCC 13803 being used for commercial xylitol production.

Enhancement of Gluconobacter oxydans Resistance to Lignocellulosic-Derived Inhibitors in Xylonic Acid Production by Overexpressing Thioredoxin.

Efficient utilization of lignocellulose is an economically relevant practice for improving the financial prospects of biorefineries. Lignocellulose contains significant levels of xylose that can be converted into valuable xylonic acid. However, some inhibitors of bioconversion processes are produced after pretreatment. Xylonic acid production in bacteria, such as Gluconobacter oxydans, is hindered by poor bacterial tolerance to contaminants. Therefore, in order to enhance bacterial resistance to inhibitors, a recombinant strain of G. oxydans was created by the introduction of the thioredoxin gene. Thioredoxin is a key protein responsible for maintaining cellular redox potential and is critical to the conversion of xylose to xylonate. Overexpression of thioredoxin was confirmed at the enzymatic level, while the recombinant strain showed increased catalytic activity when inhibitors, such as formic acid or p-hydroxybenzaldehyde (PHBA), were added to the synthetic xylose medium (17% and 7% improvement in xylonic acid yield, respectively). To probe the molecular mechanism behind the recombinant strain response to inhibitors, the expression levels of various genes were analyzed by qRT-PCR, which revealed five differentially expressed genes (DEGs) upon exposure to formic acid or PHBA.

Kinetic study on Fermentation of xylose for The Xylitol Production

Xylitol is a natural sugar that has the sweetness level similar to sucrose, but has lower calorie. It is an important sugar alternate for diabetics people. Reduction of xylose is a normally method to produce the xylitol. It Conducted via chemical hydrogenation of xylose at high pressures and temperatures by reacting pure xylose with hydrogen gas using a metal catalyst. This process requires pure xylose as the raw material. Alternatively, the reduction process can be carried out via fermentation. This process does not require high purity of xylose as the raw material, and thus the oil palm empty fruit bunch (EFB) hydrolysate, without any prior pretreatment, can be used. In order to scale up the xylitol production via fermentation, kinetic study of xylitol fermentation including growth and xylitol formation kinetic using the synthetic xylose as substrate will be required. Data used in the kinetic model development were obtained from series of batch fermentations of Debaryomycess hansenii ITB CCR85 varying the initial xylose and glucose concentrations. Yeast growth could be sufficiently modeled using the Monod kinetics, whereas xylitol production could be reasonably well modelled by Luedeking Piret kinetics.

Integrated approach for selecting efficient Saccharomyces cerevisiae for industrial lignocellulosic fermentations: Importance of yeast chassis linked to process conditions

In this work, four robust yeast chassis isolated from industrial environments were engineered with the same xylose metabolic pathway. The recombinant strains were physiologically characterized in synthetic xylose and xylose-glucose medium, on non-detoxified hemicellulosic hydrolysates of fast-growing hardwoods (Eucalyptus and Paulownia) and agricultural residues (corn cob and wheat straw) and on Eucalyptus hydrolysate at different temperatures. Results show that the co-consumption of xylose-glucose was dependent on the yeast background. Moreover, heterogeneous results were obtained among different hydrolysates and temperatures for each individual strain pointing to the importance of designing from the very beginning a tailor-made yeast considering the specific raw material and process.

Xylitol production by Saccharomyces cerevisiae overexpressing different xylose reductases using non-detoxified hemicellulosic hydrolysate of corncob.

Xylitol production was compared in fed batch fermentation by Saccharomyces cerevisiae strains overexpressing xylose reductase (XR) genes from Candida tropicalis, Pichia stipitis, Neurospora crassa, and an endogenous gene GRE3. The gene encoding a xylose specific transporter (SUT1) from P. stipitis was cloned to improve xylose transport and fed batch fermentation was used with glucose as a cosubstrate to regenerate NADPH. Xylitol yield was near theoretical for all the strains in fed batch fermentation. The highest volumetric (0.28 gL−1 h−1) and specific (34 mgg−1 h−1) xylitol productivities were obtained by the strain overexpressing GRE3 gene, while the control strain showed 7.2 mgg−1 h−1 specific productivity. The recombinant strains carrying XR from C. tropicalis, P. stipitis, and N. crassa produced xylitol with lower specific productivity of 14.3, 6.8, and 6.3 mgg−1 h−1, respectively, than GRE3 overexpressing strain. The glucose fed as cosubstrate was converted to biomass and ethanol, while xylose was only converted to xylitol. The efficiency of ethanol production was in the range of 38–45 % of the theoretical maximum for all the strains. Xylitol production from the non-detoxified corncob hemicellulosic hydrolysate by recombinant S. cerevisiae was reported for the first time. Xylitol productivity was found to be equivalent in the synthetic xylose as well as hemicellulosic hydrolysate-based media showing no inhibition on the S. cerevisiae due to the inhibitors present in the hydrolysate. A systematic evaluation of heterologous XRs and endogenous GRE3 genes was performed, and the strain overexpressing the endogenous GRE3 gene showed the best xylitol productivity.

Rapid ethanol production at elevated temperatures by engineered thermotolerant Kluyveromyces marxianus via the NADP(H)-preferring xylose reductase-xylitol dehydrogenase pathway

Conversion of xylose to ethanol by yeasts is a challenge because of the redox imbalances under oxygen-limited conditions. The thermotolerant yeast Kluyveromyces marxianus grows well with xylose as a carbon source at elevated temperatures, but its xylose fermentation ability is weak. In this study, a combination of the NADPH-preferring xylose reductase (XR) from Neurospora crassa and the NADP+-preferring xylitol dehydrogenase (XDH) mutant from Scheffersomyces stipitis (Pichia stipitis) was constructed. The xylose fermentation ability and redox balance of the recombinant strains were improved significantly by over-expression of several downstream genes. The intracellular concentrations of coenzymes and the reduced coenzyme/oxidized coenzyme ratio increased significantly in these metabolic strains. The byproducts, such as glycerol and acetic acid, were significantly reduced by the disruption of glycerol-3-phosphate dehydrogenase (GPD1). The resulting engineered K. marxianus YZJ088 strain produced 44.95g/L ethanol from 118.39g/L xylose with a productivity of 2.49g/L/h at 42°C. Additionally, YZJ088 realized glucose and xylose co-fermentation and produced 51.43g/L ethanol from a mixture of 103.97g/L xylose and 40.96g/L glucose with a productivity of 2.14g/L/h at 42°C. These promising results validate the YZJ088 strain as an excellent producer of ethanol from xylose through the synthetic xylose assimilation pathway.

Ethanol fermentation characteristics of Pichia stipitis yeast from sugar beet pulp hydrolysate: Use of new detoxification methods

Abstract This study investigates the ethanol production characteristics of adapted Pichia stipitis yeast in growth media containing various sugar sources, namely, synthetic xylose, sulfuric acid hydrolyzed sugar beet pulp and sugar beet pulp hydrolysate–synthetic xylose mixture. In synthetic growth media, the xylose concentration range of 10–75 g l−1 did not exhibit substrate inhibition and at an initial xylose concentration of 75.1 g l−1, 37.1 g l−1 ethanol was obtained after 75 h with productivity of 0.494 g l−1 h−1. Prior to fermentation, the inhibitory components of the sugar beet pulp hydrolysate was detoxified using three different treatments: commercial activated charcoal with overliming, activated charcoal obtained from sugar beet pulp with overliming and fly ash treatment. Phenolics and furan compounds were significantly adsorbed with activated charcoal treatments and overliming with CaO showed to enhance their reduction. A significant amount of furan was removed by fly ash treatment due to its high CaO content. Neither case demonstrated a noticeable decrease in acetic acid concentration. Ethanol concentrations and productivities ranged between 10.8–12.2 g l−1 and 0.119–0.286 g l−1 h−1 respectively, over 50–121 h, in the detoxified hydrolysate media which contained 48.2 g l−1 of total reducing sugar. Increase in inoculum concentration also improved the ethanol production yield.

Metabolic engineering of Arabidopsis for butanetriol production using bacterial genes

1,2,4-butanetriol (butanetriol) is a useful precursor for the synthesis of the energetic material butanetriol trinitrate and several pharmaceutical compounds. Bacterial synthesis of butanetriol from xylose or arabinose takes place in a pathway that requires four enzymes. To produce butanetriol in plants by expressing bacterial enzymes, we cloned native bacterial or codon optimized synthetic genes under different promoters into a binary vector and stably transformed Arabidopsis plants. Transgenic lines expressing introduced genes were analyzed for the production of butanetriol using gas chromatography coupled to mass spectrometry (GC–MS). Soil-grown transgenic plants expressing these genes produced up to 20µg/g of butanetriol. To test if an exogenous supply of pentose sugar precursors would enhance the butanetriol level, transgenic plants were grown in a medium supplemented with either xylose or arabinose and the amount of butanetriol was quantified. Plants expressing synthetic genes in the arabinose pathway showed up to a forty-fold increase in butanetriol levels after arabinose was added to the medium. Transgenic plants expressing either bacterial or synthetic xylose pathways, or the arabinose pathway showed toxicity symptoms when xylose or arabinose was added to the medium, suggesting that a by-product in the pathway or butanetriol affected plant growth. Furthermore, the metabolite profile of plants expressing arabinose and xylose pathways was altered. Our results demonstrate that bacterial pathways that produce butanetriol can be engineered into plants to produce this chemical. This proof-of-concept study for phytoproduction of butanetriol paves the way to further manipulate metabolic pathways in plants to enhance the level of butanetriol production.

Avaliação de um nova levedura da biodiversidade brasileira, Scheffersomyces shehatae UMFG-HM 52.2, para a conversão de pentoses em bioetanol

Hemicellulose, a heterogeneous polymer of various sugars linked with xylose backbone, is the second most available carbohydrate polymer on Earth, after cellulose. Hemicellulose can be hydrolysed by thermochemical or enzyme mediated action into a mixed sugar solution primarily composed by xylose. Bioconversion of hemicellulose derived sugars into ethanol is an important criterion to get the satisfactory overall bioethanol yield from lignocellulosic materials. However, the development of hemicellulose conversion into ethanol requires additional challenges. Only few microorganisms are able to ferment pentose sugars into ethanol. Xylose-utilizing microorganisms have shown low rates of sugar assimilation, conversion and poor tolerance to ethanol. These factors often prevent their use in technological processes at large scale. In this line, the search for new microorganisms that efficiently assimilate pentoses, particularly xylose, is a priority within the context of the integral use of lignocellulosic biomass into ethanol. This study showed the improved fermentation of synthetic xylose into ethanol under batch cultivations by a new strain of Scheffersomyces shehatae (UMFG-HM 52.2), isolated from Brazilian Atlantic rain forest ecosystem. Ethanol production performance of this yeast was evaluated in synthetic medium (using supplemented synthetic xylose as carbon source) in 125mL Erlenmeyer flaks at 200 rpm contained 50 mL of medium incubated for 72 h at 30°C. After 18 h of incubation, ethanol yield (YP/S) of 0.39 g/g and productivity (Qp) of 0.79 g/L was obtained. This study showed the potential of S. shehatae UMFG-HM 52.2 as a promising candidate in process to produce ethanol from pentose sugars.

Simultaneous acetic acid separation and monosaccharide concentration by reverse osmosis

This study aimed to investigate the feasibility and efficiency of simultaneous acetic acid separation and sugar concentration in model lignocellulosic hydrolyzates by reverse osmosis. The effects of operation parameters such as pH, temperature, pressure and feed concentration on the solute retentions were examined with a synthetic xylose–glucose–acetic acid model solution. Results showed that the monosaccharides were almost completely rejected at above 20bar, while the acetic acid retention increased with the increase in pH and pressure, and decreased with the temperature increase. The maximum separation factors of acetic acid over xylose and glucose reached as high as 211.5 and 228.4 at pH 2.93 (the initial pH of model lignocellulosic hydrolyzates), 40°C and 20bar. Furthermore, the concentration and diafiltration process were employed at optimal operation conditions. Consequently, a high sugar concentration and a beneficially lower acetic acid concentration were simultaneously achieved by reverse osmosis.

Separation of acetic acid from monosaccharides by NF and RO membranes: Performance comparison

Lignocellulosic materials can be acid-hydrolyzed into fermentable sugars for ethanol production. However, inhibitory compounds such as carboxylic acid, furans and phenolic substances, are also generated in this process. It is necessary to separate these inhibitors from sugars or reduce the concentration of the inhibitors prior to the fermentation process. Our work focused on the separation of acetic acid from monosaccharides, due to its high concentration and strong toxicity for subsequent fermentation. The separation performance of acetic acid from monosaccharides by NF/RO membranes was compared to evaluate the feasibility of simultaneous acetic acid separation and sugars concentration, by using synthetic xylose–glucose–acetic acid model solution. The influences of operation conditions such as pH, pressure, temperature and feed concentration on the retention of solutes were investigated, while the stimulation effect of acetic acid at low concentration on fermentation process was also discussed. Consequently, extremely high separation factor of acetic acid over monosaccharides of a RO membrane RO98pHt was reported, and the separation factor of this membrane is significantly greater than the separation factor of NF membranes that have been considered as the standard membranes for this application so far. Considering that the sugars should be retained as much as possible during detoxification process, and the low concentration of acetic acid has a promoting contribution to ethanol production, RO98pHt is the best membrane for simultaneous acetic acid separation and sugar concentration. Therefore, we suggest that RO process instead of NF process should be used for this application.

Improved inhibitor tolerance in xylose-fermenting yeast Spathaspora passalidarum by mutagenesis and protoplast fusion

The xylose-fermenting yeast Spathaspora passalidarum showed excellent fermentation performance utilizing glucose and xylose under anaerobic conditions. But this yeast is highly sensitive to the inhibitors such as furfural present in the pretreated lignocellulosic biomass. In order to improve the inhibitor tolerance of this yeast, a combination of UV mutagenesis and protoplast fusion was used to construct strains with improved performance. Firstly, UV-induced mutants were screened and selected for improved tolerance towards furfural. The most promised mutant, S. passalidarum M7, produced 50% more final ethanol than the wild-type strain in a synthetic xylose medium containing 2 g/l furfural. However, this mutant was unable to grow in a medium containing 75% liquid fraction of pretreated wheat straw (WSLQ), in which furfural and many other inhibitors were present. Hybrid yeast strains, obtained from fusion of the protoplasts of S. passalidarum M7 and a robust yeast, Saccharomyces cerevisiae ATCC 96581, were able to grow in 75% WSLQ and produce around 0.4 g ethanol/g consumed xylose. Among the selected hybrid strains, the hybrid FS22 showed the best fermentation capacity in 75% WSLQ. Phenotypic and partial molecular analysis indicated that S. passalidarum M7 was the dominant parental contributor to the hybrid. In summary, the hybrids are characterized by desired phenotypes derived from both parents, namely the ability to ferment xylose from S. passalidarum and an increased tolerance to inhibitors from S. cerevisiae ATCC 96581.

Comparative study on different strategies involved for xylitol purification from culture media fermented by Candida tropicalis

Xylitol, a sugar substitute, is a high value product for pharmaceutical and food industries and its purification being of commercial importance. In the present study, the purification of xylitol obtained through Candida tropicalis by fermentation using synthetic xylose and corn cob hemicellulosic hydrolysate as substrates were studied for liquid–liquid extraction (21.72 g/l xylitol extracted in 1:5 (v/v) of ethyl acetate) and precipitation (67.44% xylitol recovery along with certain impurities). By this method xylitol recovery is difficult and expensive for large scale processes. Therefore, activated charcoal treatment followed by vaccum concentration and crystallization method for xylitol extraction was evaluated. The optimized conditions obtained for activated charcoal treatment followed by vaccum concentration and crystallization method were 15.0 g/l of charcoal concentration at 30 °C for 1 h with 10 times super saturation of initial concentration and crystallization temperature of −20 °C for initiation and then at 8 °C yielding 43.97%. After 4 cycles of crystallization, 76.20% and 68.06% xylitol crystallization yield was obtained in 50 ml and 5.0 l of the synthetic xylose fermentation broth by adapted strain of C. tropicalis respectively. The effect of solvents on the crystalline structure of xylitol showed prismatic structure in the presence of ethanol and orthorhombic needles in the presence of tetrahydrofuran. The purity of the xylitol was characterized using 13C and 1H nuclear magnetic resonance, mass spectroscopy, and optical rotation, confirming 98.99% purity in a pure crystallized form.

Effects of pretreatment methods for hazelnut shell hydrolysate fermentation with Pichia Stipitis to ethanol

In this study, we investigated the use of hazelnut shell as a renewable and low cost lignocellulosic material for bioethanol production for the first time. High lignin content of hazelnut shell is an important obstacle for such a biotransformation. Biomass hydrolysis with acids yields reducing sugar with several inhibitors which limit the fermentability of sugars. The various conditioning methods for biomass and hydrolysate were performed to overcome the toxicity and their effects on the subsequent fermentation of hazelnut shell hydrolysate by Pichia stipitis were evaluated with shaking flasks experiments. Hazelnut shells hydrolysis with 0.7M H(2)SO(4) yielded 49 gl(-1) total reducing sugars and fermentation inhibitors in untreated hydrolysate. First, it was shown that several hydrolysate detoxification methods were solely inefficient in achieving cell growth and ethanol production in the fermentation of hazelnut shell hydrolysates derived from non-delignified biomass. Next, different pretreatments of hazelnut shells were considered for delignification and employed before hydrolysis in conjunction with hydrolysate detoxification to improve alcohol fermentation. Among six delignification methods, the most effective pretreatment regarding to ethanol concentration includes the treatment of shells with 3% (w/v) NaOH at room temperature, which was integrated with sequential hydrolysate detoxification by overliming and then treatment with charcoal twice at 60 degrees C. This treatment brought about a total reduction of 97% in furans and 88.4% in phenolics. Almost all trialed treatments caused significant sugar loss. Under the best assayed conditions, ethanol concentration of 16.79gl(-1) was reached from a hazelnut shell hyrolysate containing initial 50g total reducing sugar l(-1) after partial synthetic xylose supplementation. This value is equal to 91.25% of ethanol concentration that was obtained from synthetic d-xylose under same conditions. The present study demonstrates that Pichia stipitis is able to grow and ferment sugars to ethanol in detoxified hazelnut hydrolysate derived from delignified biomass.

Continuous Xylitol Production from Synthetic Xylose Solutions by Candida guilliermondii: Influence of pH and Temperature

AbstractThe effects of pH and temperature on continuous xylitol production by Candida guilliermondii have been studied under semiaerobic conditions. The specific productivites obtained at different pH have been used in the model proposed by Villadsen et al. with the aim to get information on the enzyme likely limiting xylitol production (xylose reductase). The kinetic results obtained at different temperatures have been utilized to estimate the thermodynamic parameters either of the reduction of xylose to xylitol (ΔH# = 62 kJ mol–1; ΔS# = –0.12 kJ mol–1 K–1) or of the biocatalyst thermal inactivation (ΔHD° = 90 kJ mol–1; ΔSD° = 0.30 kJ mol–1 K–1). Finally, the well‐known empirical procedure of Sizer has been compared with the so‐called “thermodynamic approach” from the viewpoint of their ability to explain the physical meaning of the related thermodynamic parameters.

Influence of temperature and pH on xylitol production from xylose by Debaryomyces hansenii.

The production of xylitol from concentrated synthetic xylose solutions (S(o) = 130-135 g/L) by Debaryomyces hansenii was investigated at different pH and temperature values. At optimum starting pH (pH(o) = 5.5), T = 24 degrees C, and relatively low starting biomass levels (0.5-0.6 g(x)/L), 88% of xylose was utilized for xylitol production, the rest being preferentially fermented to ethanol (10%). Under these conditions, nearly 70% of initial carbon was recovered as xylitol, corresponding to final xylitol concentration of 91.9 g(P)/L, product yield on substrate of 0.81 g(P)/g(S), and maximum volumetric and specific productivities of 1.86 g(P)/L x h and 1.43 g(P)/g(x) x h, respectively. At higher and lower pH(o) values, respiration also became important, consuming up to 32% of xylose, while negligible amounts were utilized for cell growth (0.8-1.8%). The same approach extended to the effect of temperature on the metabolism of this yeast at pH(o) = 5.5 and higher biomass levels (1.4-3.0 g(x)/L) revealed that, at temperatures ranging from 32-37 degrees C, xylose was nearly completely consumed to produce xylitol, reaching a maximum volumetric productivity of 4.67 g(P)/L x h at 35 degrees C. Similarly, both respiration and ethanol fermentation became significant either at higher or at lower temperatures. Finally, to elucidate the kinetic mechanisms of both xylitol production and thermal inactivation of the system, the related thermodynamic parameters were estimated from the experimental data with the Arrhenius model: activation enthalpy and entropy were 57.7 kJ/mol and -0.152 kJ/mol x K for xylitol production and 187.3 kJ/mol and 0.054 kJ/mol x K for thermal inactivation, respectively.

Comparison of different pretreatments in ethanol fermentation using corn cob hemicellulosic hydrolysate with Pichia stipitis and Candida shehatae

A hemicellulosic hydrolysate was prepared with 0.3 M H2SO4 at 98 °C for 1 h. The total initial reducing sugar was maintained at 45 g l−1 by synthetic xylose supplementation. The seven detoxification methods were employed including either the single addition of solid CaO (to pH 10 or 6) or its combinations with zeolite shaking. Over-liming gave the hydrolysate that was most completely fermented by Pichia stipitis and Candida shehatae at 30 °C, pH 6, among the tested methods.

Inhibition of the fermentation of oak hemicellulose acid-hydrolysate by minor sugars

Synthetic xylose media and detoxified oak hemicellulose acid-hydrolysates with different starting xylose contents were fermented batchwise by two strains of Pachysolen tannophilus. Maximum productivities were calculated from the experimental data of ethanol concentration using a graphical procedure. The kinetic parameters calculated for the fermentations of both carbon sources indicate that a competitive inhibition is exerted by the minor sugars (arabinose, rhamnose, and galactose) that are metabolised only slowly or not at all.