Xylitol production from sugarcane bagasse hydrolyzate in fluidized bed reactor. Effect of air flowrate

Cells of Candida guilliermondii immobilized onto porous glass spheres were cultured batchwise in a fluidized bed bioreactor for xylitol production from sugarcane bagasse hemicellulose hydrolyzate. An aeration rate of only 25 mL/min ensured minimum yields of xylose consumption (0.60) and biomass production (0.14 g(DM)/g(Xyl)), as well as maximum xylitol yield (0.54 g(Xyt)/g(Xyl)) and ratio of immobilized to total cells (0.83). These results suggest that cell metabolism, although slow because of oxygen limitation, was mainly addressed to xylitol production. A progressive increase in the aeration rate up to 140 mL/min accelerated both xylose consumption (from 0.36 to 0.78 g(Xyl)/L.h) and xylitol formation (from 0.19 to 0.28 g(Xyt)/L.h) but caused the fraction of immobilized to total cells and the xylitol yield to decrease up to 0.22 and 0.36 g(Xyt)/g(Xyl), respectively. The highest xylitol concentration (17.0 g(Xyt)/L) was obtained at 70 mL/min, but the specific xylitol productivity and the xylitol yield were 43% and 22% lower than the corresponding values obtained at the lowest air flowrate, respectively. The concentrations of consumed substrates and formed products were used in material balances to evaluate the xylose fractions consumed by C. guilliermondii for xylitol production, complete oxidation through the hexose monophosphate shunt, and cell growth. The experimental data collected at variable oxygen level allowed estimating a P/O ratio of 1.35 mol(ATP)/mol(O) and overall ATP requirements for biomass growth and maintenance of 3.4 mol(ATP)/C-mol(DM).     

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.     

Influence of cultivation conditions on xylose-to-xylitol bioconversion by a new isolate of Debaryomyces hansenii

About 270 yeast isolates were screened for xylitol production using xylose as the sole carbon source. The best isolate, Debaryomyces hansenii UFV-170, released 5.84 g L(-1) xylitol from 10 g L(-1) xylose after 24 h, corresponding to a yield of xylitol on consumed substrate (Y(P/S)) of 0.54 g g(-1). This strain was cultivated batch-wise at variable starting concentrations of xylose (S(o)) and biomass (X(o)) and agitation intensity, in order to improve xylitol production and to evaluate, through simple carbon balances, the influence of these conditions on xylose metabolism. Under the best microaerobic conditions (S(o) = 53 g L(-1), X(o) = 1.4 g L(-1), 200 rpm), xylitol production reached 37.0 g L(-1), corresponding to xylitol volumetric productivity of 1.0 g L(-1)h(-1), specific productivity of 0.22 g g(-1)h(-1) and Y(P/S) = 0.76 g g(-1). Almost 83% of xylose was consumed for xylitol production, the rest being consumed for growth, while respiration was negligible. The new isolate appeared to be a promising alternative for industrial xylitol bioproduction.     

Maintenance and growth requirements in the metabolism of Debaryomyces hansenii performing xylose-to-xylitol bioconversion in corncob hemicellulose hydrolyzate

In order to improve the biotechnological production of xylitol, the metabolism of Debaryomyces hansenii NRRL Y-7426 in corncob hemicellulose hydrolyzate has been investigated under different conditions, where either maintenance or growth requirements predominated. For this purpose, the experimental results of two sets of batch bioconversions carried out alternatively varying the starting xylose concentration in the hydrolyzate (65.6 < or = S(0) < or = 154.7 g L(-1)) or the initial biomass level (3.0 < or = X(0) < or = 54.6 g(DM) L(-1)) were used to fit a metabolic model consisting of carbon material and ATP balances based on five main activities, namely fermentative assimilation of pentoses, semi-aerobic pentose-to-pentitol bioconversion, biomass growth on pentoses, catabolic oxidation of pentoses, and acetic acid and NADH regeneration by the electron transport system. Such an approach allowed separately evaluating the main bioenergetic constants of this microbial system, that is, the specific rates of ATP and xylose consumption due to maintenance (m(ATP) = 21.0 mmol(ATP) C-mol(DM) (-1)h(-1); m(Xyl) = 6.5 C-mmol(Xyl) C-mol(DM) (-1)h(-1)) and the true yields of biomass on ATP (Y(ATP) (max) = 0.83 C-mol(DM) mol(ATP) (-1)) and on xylose (Y(Xyl) (max) = 0.93 C-mol(DM) C-mol(Xyl) (-1)). The results of this study highlighted that the system, at very high S(0) and X(0) values, dramatically increased its energy requirements for cell maintenance, owing to the occurrence of stressing conditions. In particular, for S(0) > 130 g L(-1), these activities required an ATP consumption of about 2.1 mol(ATP) L(-1), that is, a value about seven- to eightfold that observed at low substrate concentration. Such a condition led to an increase in the fraction of ATP addressed to cell maintenance from 47% to 81%. On the other hand, the very high percentage of ATP addressed to maintenance (> 96%) at very high cell concentration (X(0) > or = 25 g(DM) L(-1)) was likely due to the insufficient substrate to sustain the growth.     

Metabolic behavior of immobilized Candida guilliermondii cells during batch xylitol production from sugarcane bagasse acid hydrolyzate

Candida guilliermondii cells, immobilized in Ca-alginate beads, were used for batch xylitol production from concentrated sugarcane bagasse hydrolyzate. Maximum xylitol concentration (20.6 g/L), volumetric productivity (0.43 g/L. h), and yield (0.47 g/g) obtained after 48 h of fermentation were higher than similar immobilized-cell systems but lower than free-cell cultivation systems. Substrates, products, and biomass concentrations were used in material balances to study the ways in which the different carbon sources were utilized by the yeast cells under microaerobic conditions. The fraction of xylose consumed to produce xylitol reached a maximum value (0.70) after glucose and oxygen depletion while alternative metabolic routes were favored by sub-optimal conditions.     

Xylose metabolism in Debaryomyces hansenii UFV-170. Effect of the specific oxygen uptake rate

The new yeast Debaryomyces hansenii UFV-170 was tested in this work in batch experiments under variable oxygenation conditions. To get additional information on its fermentative metabolism, a stoichiometric network was proposed and checked through a bioenergetic study performed using the experimental data of product and substrate concentrations. The yeast metabolism resulted to be practically inactive under strict oxygen-limited conditions (qO2 = 12.0 mmol(O2) C-mol(DM)(-1) h(-1)), as expected by the impossibility of regenerating NADH2+. Significant fractions of the carbon source were addressed to both respiration and biomass growth under excess oxygen levels (qO2 > or = 55.0 mmol(O2) C-mol(DM)(-1) h(-1)), thus affecting xylitol yield (Y(P/S) = 0.41-0.52 g g(-1)). Semi-aerobic conditions (qO2 = 26.8 mmol(O2) C-mol(DM)(-1) h(-1)) were able to ensure the best xylitol production performance (Pmax = 76.6 g L(-1)), minimizing the fractions of the carbon source addressed either to respiration or biomass production and increasing Y(P/S) up to 0.73 g g(-1). An average P/O ratio of about 1.0 mol(ATP) mol(O)(-1) allowed estimation of the main kinetic-bioenergetic parameters of the biosystem. The overall ATP requirements of biomass were found to be particularly high and dependent on the oxygen availability in the medium as well as on the physiological state of the culture. Under semi-aerobic and aerobic conditions, they varied in the ranges 13.5-15.4 and 9.74-10.2 mol(ATP) C-mol(DM)(-1), respectively, whereas during the best semi-aerobic bioconversion they progressively increased from 5.68 to 24.7 mol(ATP) C-mol(DM)(-1). After a starting phase of adaptation to the medium, the cell achieved a phase of decelerated growth during which its excellent xylose-to-xylitol capacity kept almost constant after 112 h up to the end of the run.     

酵母发酵蔗渣半纤维素水解物生产木糖酶

采用二次正交旋转组合设计研究了蔗渣半纤维素水解过程中硫酸浓度与液 固比对木糖收率的影响。回归分析表明 ,这两个因素与木糖的收率之间存在显著的回归关系。通过回归方程优化水解条件 ,当硫酸浓度 2 .4g L ,液 固 =6 .2 ,在蒸汽压力 2 .5× 10 4Pa的条件下水解 2 .5h ,10 0g蔗渣可水解生成木糖约 2 4g。大孔树脂吸附层析处理蔗渣半纤维素水解物 ,能有效地减少其中的酵母生长抑制物含量 ,显著改善水解物的发酵性能。用大孔树脂在pH 2条件下处理过的蔗渣半纤维素水解物作基质 ,含木糖 2 0 0g L ,产木糖醇酵母菌株CandidatropicalisAS2 .1776发酵 110h耗完基质中的木糖 ,生成木糖醇 12 7g L ,产物转化率 0 .6 4(木糖醇g 木糖g) ,产物生成速率 1.15g L·h .     

Biotechnological production of xylitol in a three-phase fluidized bed bioreactor with immobilized yeast cells in Ca-alginate beads

Cells of Candida guilliermondii entrapped in Ca-alginate beads were used for xylitol production, from concentrated hemicellulose hydrolyzate of sugarcane bagasse, in a fluidized bed bioreactor (FBR). The maximum xylitol concentration 28.9 g xylitol/L was obtained at a high aeration rate of 600 mL/min after 70 h of fermentation, indicating that the use of high aeration rate in this system is favored for better oxygen transfer into the immobilized cells. The specific xylitol productivity and the xylitol yield were of 0.4 g xylitol/L.h and 0.58 g xylitol/g xylose respectively. The immobilization efficiency at the end of the fermentation was of 65 %. After 90 h of fermentation xylitol productivity and yield decreased to 0.25 g xylitol/L.h and 0.47 g xylitol/g xylose respectively, indicating the beginning of xylitol consumption by the yeast. The use of FBR system with immobilized cells presented high xylitol yield and productivity.     

Production of xylitol from D-xylose by recombinant Lactococcus lactis

The D-xylose reductase from Pichia stipitis CBS 5773 and the xylose transporter from Lactobacillus brevis ATCC 8287 were expressed in active form in Lactococcus lactis NZ9800. Xylitol production was investigated using non-growing recombinant cells in high cell-density under microaerobic conditions in the presence of xylose and glucose. Besides xylose, the recombinant strain with xylose reductase activity reduced l-arabinose and D-ribose in significant extent to the corresponding pentitols. The ratio of xylitol produced per glucose consumed was almost 10-fold higher under glucose limitation than the ratio in the presence of excess initial glucose. The co-expression of the xylose transporter with the xylose reductase did not increase the efficiency of xylitol production appreciably when compared to the strain in which only the xylose reductase gene was expressed. A fed-batch experiment with high initial xylose concentration (160 gl(-1)) under glucose limitation was carried out using the strain co-expressing xylose reductase and xylose transporter genes. The xylitol yield from xylose was 1.0 mol mol(-1) and the ratio of xylitol produced per glucose consumed was 2.5 mol mol(-1). The volumetric productivity was 2.72 gl(-1)h(-1) at 20 h. Of the xylose initially present, 34% was consumed. Analysis of the fermentation metabolites revealed a shift from homolactic to mixed acid fermentation at early stages of the experiment.     

Furfural, 5-hydroxymethyl furfural, and acetoin act as external electron acceptors during anaerobic fermentation of xylose in recombinant Saccharomyces cerevisiae

The electron acceptors acetoin, acetaldehyde, furfural, and 5-hydroxymethylfurfural (HMF) were added to anaerobic batch fermentation of xylose by recombinant, xylose utilising Saccharomyces cerevisiae TMB 3001. The intracellular fluxes during xylose fermentation before and after acetoin addition were calculated with metabolic flux analysis. Acetoin halted xylitol excretion and decreased the flux through the oxidative pentose phosphate pathway. The yield of ethanol increased from 0.62 mol ethanol/mol xylose to 1.35 mol ethanol/mol xylose, and the cell more than doubled its specific ATP production after acetoin addition compared to fermentation of xylose only. This did, however, not result in biomass growth. The xylitol excretion was also decreased by furfural and acetaldehyde but was unchanged by HMF. Thus, furfural present in lignocellulosic hydrolysate can be beneficial for ethanolic fermentation of xylose. Enzymatic analyses showed that the reduction of acetoin and furfural required NADH, whereas the reduction of HMF required NADPH. The enzymatic activity responsible for furfural reduction was considerably higher than for HMF reduction and also in situ furfural conversion was higher than HMF conversion.     

A strain of <Emphasis Type="Italic">Meyerozyma guilliermondii</Emphasis> isolated from sugarcane juice is able to grow and ferment pentoses in synthetic and bagasse hydrolysate media

The search for new microbial strains that are able to withstand inhibitors released from hemicellulosic hydrolysis and are also still able to convert sugars in ethanol/xylitol is highly desirable. A yeast strain isolated from sugarcane juice and identified as Meyerozyma guilliermondii was evaluated for the ability to grow and ferment pentoses in synthetic media and in sugarcane bagasse hydrolysate. The yeast grew in xylose, arabinose and glucose at the same rate at an initial medium pH of 5.5. At pH 4.5, the yeast grew more slowly in arabinose. There was no sugar exhaustion within 60 h. At higher xylose concentrations with a higher initial cell concentration, sugar was exhausted within 96 h at pH 4.5. An increase of 350 % in biomass was obtained in detoxified hydrolysates, whereas supplementation with 3 g/L yeast extract increased biomass production by approximately 40 %. Ethanol and xylitol were produced more significantly in supplemented hydrolysates regardless of detoxification. Xylose consumption was enhanced in supplemented hydrolysates and arabinose was consumed only when xylose and glucose were no longer available. Supplementation had a greater impact on ethanol yield and productivity than detoxification; however, the product yields obtained in the present study are still much lower when compared to other yeast species in bagasse hydrolysate. By the other hand, the fermentation of both xylose and arabinose and capability of withstanding inhibitors are important characteristics of the strain assayed.     

Variables that affect xylitol production from sugarcane bagasse hydrolysate in a zeolite fluidized bed reactor

The operational conditions for xylitol production by fermentation of sugarcane bagasse hydrolysate in a fluidized bed reactor with cells immobilized on zeolite were evaluated. Fermentations were carried out under different conditions of air flowrate (0.0125-0.0375 vvm), zeolite mass (100-200 g), initial pH (4-6), and xylose concentration (40-60 g/L), according to a 2(4) full factorial design. The air flowrate increase resulted in a metabolic deviation from product to biomass formation. On the other hand, the pH increase favored both the xylitol yield (Y(P/S)) and volumetric productivity (Q(P)), and the xylose concentration increase positively influenced the xylitol concentration. The best operational conditions evaluated were based on the use of an air flowrate of 0.0125 vvm, 100 g of zeolite, pH 6, and xylose concentration of 60 g/L. Under these conditions, 38.5 g/L of xylitol were obtained, with a Y(P/S) of 0.72 g/g, Q(P) of 0.32 g/L.h, and cell retention of 25.9%.     

Xylitol formation by Candida boidinii in oxygen limited chemostat culture

Summary Production of xylitol by Candida boidinii NRRL Y-17213 occurs under conditions of an oxygen limitation. The extent to which substrate is converted to xylitol and its coproducts (ethanol, other polyols, acetic acid), and the relative flow rates of substrate to energetic and biosynthetic pathways is controlled by the degree of oxygen limitation.With decrease in oxygen concentration in the inlet gas, for a constant dilution rate of 0.05 1/h. the specific oxygen uptake rate decreased from 1.30 to 0.36 mmol/gh Xylitol was not produced at specific oxygen uptake rates above 0.91 mmol/gh. Upon shift to lower oxygen rates, specific xylitol production rate increased more rapidly than specific ethanol production rate:Nomenclature D dilution rate (1/h) - DOT dissolved oxygen tension (%) - m_o2 maintenance coefficient (mmol O₂/g cell mass h) - q_o2 specific oxygen uptake rate (mmol O₂/g cell mass h) - q_s specific xylose uptake rate (g xylose/g cell mass h) or (mmol xylose/g cell mass h) - q_x specific xylitol production rate (g xylitol/ g cell mass h) or (mmol xylitol/ g cell mass h) - q_e specific ethanol production rate (g ethanol/ g cell mass h) or (mmol ethanol/ g cell mass h) - q_CO2 specific carbon dioxide production rate (mmol CO₂/g cell mass h) - S xylose concentration (g/1) - Y_cm/s cell mass yield coefficient, (g cell mass/mmol xylose) or (g cell mass/ g xylose consumed) - Y_cm/O2 cell mass yield coefficient, (g cell mass/mmol O₂) - Y_X/S xylitol yield coefficient (g xylitol/g xylose consumed) - Y_x/O2 xylitol yield coefficient (g xylitol/mmol O₂) - Y_e/s ethanol yield coefficient (g ethanol/g xylose consumed) - OUR oxygen uptake rate (mmol O₂/1h) - specific growth rate (1/h)     

The reduction of xylose to xylitol by Candida guilliermondii and Candida parapsilosis: Incidence of oxygen and pH

Summary The ability of C. guilliermondii and C. parapsilosis to ferment xylose to xylitol was evaluated under different oxygen transfer rates in order to enhance the xylitol yield. In C. guilliermondii, a maximal xylitol yield of 0.66 g/g was obtained when oxygen transfer rate was 2.2 mmol/l.h. Optimal conditions to produce xylitol by C. parapsilosis (0.75 g/g) arose from cultures at pH 4.75 with 0.4 mmoles of oxygen/l.h. The response of the yeasts to anaerobic conditions has shown that oxygen was required for xylose metabolism.Nomenclature max maximum specific growth rate (per hour) - qSmax maximum specific rate of xylose consumption (g xylose per g dry biomass per hour) - qpmax maximum specific productivity of xylitol (g xylitol per g dry biomass per hour) - Qp average volumetric productivity of xylitol (g xylitol per liter per hour) - Y_P/S xylitol yield (g xylitol per g substrate utilized) - Y_P'/S glycerol yield (g glycerol per g substrate utilized) - Y_X/S biomass yield (g dry biomass per g substrate utilized)     

Decreased xylitol formation during xylose fermentation in Saccharomyces cerevisiae due to overexpression of water-forming NADH oxidase

The recombinant xylose-fermenting Saccharomyces cerevisiae strain harboring xylose reductase (XR) and xylitol dehydrogenase (XDH) from Scheffersomyces stipitis requires NADPH and NAD(+), creates cofactor imbalance, and causes xylitol accumulation during growth on d-xylose. To solve this problem, noxE, encoding a water-forming NADH oxidase from Lactococcus lactis driven by the PGK1 promoter, was introduced into the xylose-utilizing yeast strain KAM-3X. A cofactor microcycle was set up between the utilization of NAD(+) by XDH and the formation of NAD(+) by water-forming NADH oxidase. Overexpression of noxE significantly decreased xylitol formation and increased final ethanol production during xylose fermentation. Under xylose fermentation conditions with an initial d-xylose concentration of 50 g/liter, the xylitol yields for of KAM-3X(pPGK1-noxE) and control strain KAM-3X were 0.058 g/g xylose and 0.191 g/g, respectively, which showed a 69.63% decrease owing to noxE overexpression; the ethanol yields were 0.294 g/g for KAM-3X(pPGK1-noxE) and 0.211 g/g for the control strain KAM-3X, which indicated a 39.33% increase due to noxE overexpression. At the same time, the glycerol yield also was reduced by 53.85% on account of the decrease in the NADH pool caused by overexpression of noxE.     

Effects of NADH-preferring xylose reductase expression on ethanol production from xylose in xylose-metabolizing recombinant Saccharomyces cerevisiae

Efficient conversion of xylose to ethanol is an essential factor for commercialization of lignocellulosic ethanol. To minimize production of xylitol, a major by-product in xylose metabolism and concomitantly improve ethanol production, Saccharomyces cerevisiae D452-2 was engineered to overexpress NADH-preferable xylose reductase mutant (XR(MUT)) and NAD?-dependent xylitol dehydrogenase (XDH) from Pichia stipitis and endogenous xylulokinase (XK). In vitro enzyme assay confirmed the functional expression of XR(MUT), XDH and XK in recombinant S. cerevisiae strains. The change of wild type XR to XR(MUT) along with XK overexpression led to reduction of xylitol accumulation in microaerobic culture. More modulation of the xylose metabolism including overexpression of XR(MUT) and transaldolase, and disruption of the chromosomal ALD6 gene encoding aldehyde dehydrogenase (SX6(MUT)) improved the performance of ethanol production from xylose remarkably. Finally, oxygen-limited fermentation of S. cerevisiae SX6(MUT) resulted in 0.64 g l?1 h?1 xylose consumption rate, 0.25 g l?1 h?1 ethanol productivity and 39% ethanol yield based on the xylose consumed, which were 1.8, 4.2 and 2.2 times higher than the corresponding values of recombinant S. cerevisiae expressing XR(MUT), XDH and XK only.     

The influence of aeration and hemicellulosic sugars on xylitol production by Candida tropicalis

The influence of other hemicellulosic sugars (arabinose, galactose, mannose and glucose), oxygen limitation, and initial xylose concentration on the fermentation of xylose to xylitol was investigated using experimental design methodology. Oxygen limitation and initial xylose concentration had considerable influences on xylitol production by Canadida tropicalis ATCC 96745. Under semiaerobic conditions, the maximum xylitol yield was 0.62 g/g substrate, while under aerobic conditions, the maximum volumetric productivity was 0.90 g/l h. In the presence of glucose, xylose utilization was strongly repressed and sequential sugar utilization was observed. Ethanol produced from the glucose caused 50% reduction in xylitol yield when its concentration exceeded 30 g/l. When complex synthetic hemicellulosic sugars were fermented, glucose was initially consumed followed by a simultaneous uptake of the other sugars. The maximum xylitol yield (0.84 g/g) and volumetric productivity (0.49 g/l h) were obtained for substrates containing high arabinose and low glucose and mannose contents.     

The effect of xylose reductase genes on xylitol production by industrial Saccharomyces cerevisiae in fermentation of glucose and xylose

The co-production of xylitol and ethanol from agricultural straw has more economic advantages than the production of ethanol only. Saccharomyces cerevisiae, the most widely used ethanol-producing yeast, can be genetically engineered to ferment xylose to xylitol. In the present study, the effects of xylose-specificity, cofactor preference, and the gene copy number of xylose reductase (XR; encoding by XYL1 gene) on xylitol production of S. cerevisiae were investigated. The results showed that overexpression of XYL1 gene with a lower xylose-specificity and a higher NADPH preference favored the xylitol production. The copy number of XYL1 had a positive correlation with the XR activity but did not show a good correlation with the xylitol productivity. The overexpression of XYL1 from Candida tropicalis (CtXYL1) achieved a xylitol productivity of 0.83 g/L/h and a yield of 0.99 g/g-consumed xylose during batch fermentation with 43.5 g/L xylose and 17.0 g/L glucose. During simultaneous saccharification and fermentation (SSF) of pretreated corn stover, the strain overexpressing CtXYL1 produced 45.41 g/L xylitol and 50.19 g/L ethanol, suggesting its application potential for xylitol and ethanol co-production from straw feedstocks.     

Metabolic engineering of the initial stages of xylose catabolism in yeasts for construction of efficient producers of ethanol from lignocelluloses

Plant biomass possesses a huge potential as a source for biofuel production. The main components of biomass are glucose and five-carbon sugar xylose. The yeast Saccharomyces cerevisiae that is used for industrial ethanol production from glucose is unable to xylose fermentation. Therefore a microorganism capable for efficient fermentation of both glucose and xylose has to be found in nature or constructed for economically feasible biomass conversion to ethanol. The active xylose fermentation could be performed by increasing the efficiency of initial stages of xylose metabolism. In this review the enzymes of initial stages of xylose metabolism in yeasts (xylose reductase, xylitol dehydrogenase, xylulokinase) and bacteria (xylose isomerase and xylulokinase) are characterized. The ways for construction of yeast strains capable of efficient alcoholic xylose fermentation are discussed.     

Xylitol production from high xylose concentration: evaluation of the fermentation in bioreactor under different stirring rates

AIMS: To investigate the production of xylitol by the yeast Candida guilliermondii FTI 20037, in a bioreactor, from rice straw hemicellulosic hydrolysate with a high xylose concentration. METHODS AND RESULTS: Batch fermentation was carried out with rice straw hemicellulosic hydrolysate containing about 85 g xylose l(-1), in a stirred-tank bioreactor at 30 degrees C, under aeration of 1.3 vvm (volume of air per volume of medium per min) and different stirring rates (200, 300 and 500 rev min(-1)). The bioconversion of xylose into xylitol by the yeast depended on the stirring rate, the maximum xylitol yield (YP/S = 0.84 g g(-1)) being achieved at 300 rev min-1, with no need to pretreat the hydrolysate for purification. CONCLUSIONS: To determine the most adequate oxygen transfer rate is fundamental to improving the xylose-to-xylitol bioconversion by C. guilliermondii. SIGNIFICANCE AND IMPACT OF THE STUDY: For the microbial production of xylitol to be economically viable, the initial concentration of xylose in the lignocellulosic hydrolysate should be as high as possible, as with high substrate concentrations it is possible to increase the final product concentration. Nevertheless, there are few reports on the use of high xylose concentrations. Considering a process in bioreactor, from rice straw hemicellulosic hydrolysate, this is an innovator work.