Xylitol Bioproduction Research Articles

XylR Overexpression in Escherichia coli Alleviated Transcriptional Repression by Arabinose and Enhanced Xylitol Bioproduction from Xylose Mother Liquor.

Xylitol is a polyol widely used in food, pharmaceuticals, and light industries. It is currently produced through the chemical catalytic hydrogenation of xylose and generates xylose mother liquor as a substantial byproduct in the procedure of xylose extraction. If xylose mother liquor could also be efficiently bioconverted to xylitol, the greenness and atom economy of xylitol production would be largely improved. However, xylose mother liquor contains a mixture of glucose, xylose, and arabinose, raising the issue of carbon catabolic repression in its utilization by microbial conversion. Targeting this challenge, the transcriptional activator XylR was overexpressed in a previously constructed xylitol-producing E. coli strain CPH. The resulting strain CPHR produced 16.61g/L of xylitol in shake-flask cultures from the mixture of corn cob hydrolysate and xylose mother liquor (1:1, v/v) with a xylose conversion rate of 90.1%, which were 2.23 and 2.15 times higher than the starting strain, respectively. Furthermore, XylR overexpression upregulated the expression levels of xylE, xylF, xylG, and xylH genes by 2.08-2.72 times in arabinose-containing medium, suggesting the alleviation of transcriptional repression of xylose transport genes by arabinose. This work lays the foundation for xylitol bioproduction from xylose mother liquor.

Scaling up xylitol bioproduction: Challenges to achieve a profitable bioprocess

Xylitol is a GRAS (Generally Recognized as Safe) polyol commonly used in the food industry and able to promote several benefits to the health. In addition, it can also be used as a building block molecule for the manufacture of different high-value chemicals. Currently, the commercial production of xylitol occurs by chemical route through the catalytic hydrogenation of xylose from lignocellulosic biomass. Since this is an expensive process due to the severe reactional conditions employed, the biotechnological route for xylitol production, which comprises the biological conversion of xylose into xylitol, emerges as a potential lower-cost alternative to obtain this polyol due to the milder process conditions required. However, the biotechnological route still presents important bottlenecks and challenges that impairs the process scaling up. Modern strategies and technologies that can potentially improve xylitol bioproduction include adaptive evolution of microbial strains to enhance their tolerance to inhibitors and the xylose uptake rate during the fermentation step; development of engineered microorganisms to result in higher xylose-to-xylitol bioconversion yields; as well as xylitol purification techniques to improve the recovery yields. Moreover, techno-economic analysis of the overall production chain is essential to identify the process viability for large-scale implementation as well as the steps requiring improvements. These are some key factors discussed in this review, which aims to provide insights for the development of a more economically competitive, less energy demanding and scalable new technology for xylitol production.

Valorization of the sugarcane bagasse and straw hemicellulosic hydrolysate through xylitol bioproduction: effect of oxygen availability and sucrose supplementation as key factors

Sugarcane agroindustry, one of the most important in Brazilian economy, can diversify its product portfolio in a biorefinery context through the use of by-products as feedstocks to produce bioenergy and high-added value chemicals, such as xylitol, which is a sweetener with applications in the food and pharmaceutical industries and with a growing market. Opportunities for the integration of xylitol bioproduction in a sugarcane biorefinery can be based on the valorization of bagasse, straw, and molasses, which are feedstocks with high content of carbohydrates, specifically xylose from the hemicellulosic fraction of the lignocellulosic biomasses, and sucrose from molasses that can act as co-substrate on this bioprocess. Xylitol bioproduction from the hemicellulosic hydrolysate of the sugarcane bagasse and straw mixture was herein investigated. Particularly, the effects of sucrose supplementation to the medium on the fermentative performance of Candida tropicalis depending on its initial concentration and oxygen availability (expressed as KLa), which is one of the most important factors for this bioprocess, are discussed. Xylitol yield (0.86 gg−1) and volumetric productivity (0.51 gL−1 h−1) were concomitantly maximized by using sucrose (0.91 gL−1) and KLa (6.5 h−1) within the optimization range. Furthermore, the addition of sugarcane molasses to the hemicellulosic hydrolysate improved xylitol production, in comparison with the conventional supplementation. Scientific background is given to contribute to supporting the potential integration of xylitol bioproduction to a sugarcane biorefinery, which may enhance both the feasibility of this bioprocess and sugarcane agroindustry valorization based on the applications and growing xylitol market.

A kinetic study of xylose recovery from a hemicellulose-rich biomass for xylitol fermentative production

Hemicellulose in the complex structure of lignocellulosic substances is mainly composed of xylan which is a polymer based on monosaccharide xylose. Using acidic or enzymatic hydrolysis, hemicellulose can be depolymerized into its constituent monomer. The kinetics of hemicellulose depolymerization and decomposition in oat hull was investigated under moderate pressures with catalyst (H2SO4) concentration up to 0.55 N and temperatures of up to 130 °C for a total residence time of 150 min. Different trends of recovery or generation and kinetic mechanisms obtained for the components in the hydrolysate which could be described by different kinetic models, that is, a single-phase kinetic mechanism with product decomposition (two-step sequential reaction) could describe xylose generation. However, generation of arabinose, furfural, and acetic acid followed a single-phase mechanism with no decomposition (one-step reaction). Generation of glucose in the hydrolysate followed a biphasic mechanism due to the fast- and slow-releasing fractions into the liquid phase which was apparently with no decomposition. A pentose recovery of almost 80% was achieved under optimal conditions. Parameters of xylitol bioproduction indicated that a xylitol/xylose conversion yield of 0.80 g/g is achievable from the concentrated hydrolysate with no complementary treatment proving its low toxicity compared to other hemicellulose resources.

Xylitol bioproduction in hemicellulosic hydrolysate obtained from sorghum forage biomass.

This study evaluated the biotechnological production of xylitol from sorghum forage biomass. The yeast Candida guilliermondii was cultivated in hemicellulosic hydrolysates obtained from biomass of three sorghum varieties (A, B, and C). First, the biomass was chemically characterized and subjected to dilute acid hydrolysis to obtain the hemicellulosic hydrolysates which were vacuum-concentrated and detoxified with activated charcoal. The hemicellulosic hydrolysates (initial pH 5.5) were supplemented with nutrients, and fermentations were conducted in 125-mL Erlenmeyer flasks containing 50 mL medium, under 200 rpm, at 30 °C for 96 h. Fermentations were evaluated by determining the parameters xylitol yield (Y P/S ) and productivity (QP), as well as the activities of the enzymes xylose reductase (XR) and xylitol dehydrogenase (XDH). There was no significant difference among the three varieties with respect to the contents of cellulose, hemicellulose, and lignin, although differences were found in the hydrolysate fermentability. Maximum xylitol yield and productivity values for variety A were 0.35 g/g and 0.16 g/L.h(-1), respectively. It was coincident with XR (0.25 U/mg prot) and XDH (0.17 U/mg prot) maximum activities. Lower values were obtained for varieties B and C, which were 0.25 and 0.17 g/g for yield and 0.12 and 0.063 g/L.h(-1) for productivity.

Evaluation of biocomposite-based supports for immobilized-cell xylitol production compared with a free-cell system

This study was conducted to evaluate the importance of aeration in free and immobilized cell systems in an aerated bioreactor for xylitol production from an oat hull hemicellulosic hydrolysate using an integrated process. The aeration rate (AR) or oxygen mass transfer coefficient (kLa) demonstrated a significant role in controlling cell (Candida guilliermondii FTI 20037) regeneration and bioconversion performance in free and immobilized cell systems. In the free cell system, an aeration rate of 1.25vvm corresponding to kLa of 15.81/h resulted in maximum values of product yield (Yp/s: 0.87g/g), productivity (Qp: 0.57g/l/h), and final xylitol concentration (Pf: 55g/l) from the hydrolysate with a 74.5g/l xylose concentration. However, in the aerated immobilized cell system, maximum and almost similar results (almost Pf: 54g/l, Qp: 0.57g/l/h and Yp/s: 0.84g/g) were obtained with aeration rates from 1.25 to 1.5vvm using composites based on polypropylene (PP) and partially delignified fiber (PDF). Composites based on acid treated fiber (ATF) containing a high amount of lignin showed some inhibitory impact on xylose uptake and xylitol formation (Pf: 47g/l and Qp<0.49g/l/h) with the optimal aeration rate of 1.5vvm in the initial cycle of the bioconversion; this inhibition impact could be resolved in the next consecutive cycles. The surface modifier polyethyleneimine (PEI) slightly enhanced cell retention in the immobilized form on the ATF-based cell support. This investigation helps fill in the knowledge gaps existing on the integrated processing of the lignocellulosic biomass for xylitol bioproduction and biorefinery industry; however, more scale-up studies are recommended for commercialization.

Xylitol bioproduction from wheat straw: Process engineering and technical feasibility

Xylitol bioproduction from wheat straw: Process engineering and technical feasibility

Profiles of xylose reductase, xylitol dehydrogenase and xylitol production under different oxygen transfer volumetric coefficient values

AbstractBACKGROUND: Xylitol is a sugar alcohol (polyalcohol) with many interesting properties for pharmaceutical and food products. It is currently produced by a chemical process, which has some disadvantages such as high energy requirement. Therefore microbiological production of xylitol has been studied as an alternative, but its viability is dependent on optimisation of the fermentation variables. Among these, aeration is fundamental, because xylitol is produced only under adequate oxygen availability. In most experiments with xylitol‐producing yeasts, low oxygen transfer volumetric coefficient (KLa) values are used to maintain microaerobic conditions. However, in the present study the use of relatively high KLa values resulted in high xylitol production. The effect of aeration was also evaluated via the profiles of xylose reductase (XR) and xylitol dehydrogenase (XD) activities during the experiments.RESULTS: The highest XR specific activity (1.45 ± 0.21 U mgprotein−1) was achieved during the experiment with the lowest KLa value (12 h−1), while the highest XD specific activity (0.19 ± 0.03 U mgprotein−1) was observed with a KLa value of 25 h−1. Xylitol production was enhanced when KLa was increased from 12 to 50 h−1, which resulted in the best condition observed, corresponding to a xylitol volumetric productivity of 1.50 ± 0.08 gxylitol L−1 h−1 and an efficiency of 71 ± 6.0%.CONCLUSION: The results showed that the enzyme activities during xylitol bioproduction depend greatly on the initial KLa value (oxygen availability). This finding supplies important information for further studies in molecular biology and genetic engineering aimed at improving xylitol bioproduction. Copyright © 2008 Society of Chemical Industry

Xylitol production from wheat straw hemicellulosic hydrolysate: hydrolysate detoxification and carbon source used for inoculum preparation

Wheat straw hemicellulosic hydrolysate was used for xylitol bioproduction. The use of a xylose-containing medium to grow the inoculum did not favor the production of xylitol in the hydrolysate, which was submitted to a previous detoxification treatment with 2.5% activated charcoal for optimized removal of inhibitory compounds.

Clarification of a wheat straw‐derived medium with ion‐exchange resins for xylitol crystallization

AbstractBACKGROUND: Xylitol bioproduction from lignocellulosic residues comprises hydrolysis of the hemicellulose, detoxification of the hydrolysate, bioconversion of the xylose, and recovery of xylitol from the fermented hydrolysate. There are relatively few reports on xylitol recovery from fermented media. In the present study, ion‐exchange resins were used to clarify a fermented wheat straw hemicellulosic hydrolysate, which was then vacuum‐concentrated and submitted to cooling in the presence of ethanol for xylitol crystallization.RESULTS: Sequential adsorption into two anion‐exchange resins (A‐860S and A‐500PS) promoted considerable reductions in the content of soluble by‐products (up to 97.5%) and in medium coloration (99.5%). Vacuum concentration led to a dark‐colored viscous solution that inhibited xylitol crystallization. This inhibition could be overcome by mixing the concentrated medium with a commercial xylitol solution. Such a strategy led to xylitol crystals with up to 95.9% purity. The crystallization yield (43.5%) was close to that observed when using commercial xylitol solution (51.4%).CONCLUSION: The experimental data demonstrate the feasibility of using ion‐exchange resins followed by cooling in the presence of ethanol as a strategy to promote the fast recovery and purification of xylitol from hemicellulose‐derived fermentation media. Copyright © 2008 Society of Chemical Industry

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.

Semi-continuous xylitol bioproduction in sugarcane bagasse hydrolysate: effect of nutritional supplementation

Xylose-to-xylitol bioconversion by Ca-alginate entrapped Candida guilliermondii cells in sugarcane bagasse hemicellulosic hydrolysate was carried out in erlenmeyer flasks using the repeated-batch mode of fermentation. The hydrolysate was supplemented or not with ammonium sulfate and/or rice bran extract at the beginning of each repeated batch. Altogether, six sets of three repeated-batches were carried out, the immobilized cells being reused at the end of each batch. The best results were achieved when the hydrolysate was supplemented with both nutrients in all the three repeated batches, which provided xylitol productions of 25.9, 46.8, 48.7 gL-1, productivities of 0.27, 0.49, 0.51 gL-1h-1, and yields of 0.45, 0.58, 0.55 gg-1, respectively. In the absence of nutrients, the xylitol production, productivity and yield did not exceed 12.1 gL-1, 0.13 gL-1h-1 and 0.30 gg-1, respectively.

Repeated-Batch Xylitol Bioproduction Using Yeast Cells Entrapped in Polyvinyl Alcohol–Hydrogel

Xylose-to-xylitol conversion was investigated in a bench-scale bioreactor using Candida guilliermondii cells entrapped within polyvinyl alcohol-hydrogel beads in a system operated in repeated-batch mode with cell recycling. Yeast-viable cells were immobilized in the support using the freezing-thawing method. Bioconversion assays were performed in a stirred tank reactor operated at 400-rpm agitation speed, 30 degrees C temperature, and 1.04-vvm air flow rate. The system was explored during six successive cycles, and a small decrease in the conversion performance in the fifth cycle was observed, but the biocatalytic activity of the microorganism was recovered in the sixth cycle after washing the particles. During the process, the hydrogel beads maintained their shape and size without appreciable deterioration. Xylitol production, yield factor, and volumetric productivity increased with progressive recycling of cells and achieved their maximum values (P(F) = 39.7 g l(-1); Y(P/S) = 0.77 g g(-1); Q(P) = 0.53 g l(-1) h(-1), respectively) after the third cell recycling, probably because of cells' adaptation to the medium.

Xylitol bioproduction from wheat straw: hemicellulose hydrolysis and hydrolyzate fermentation

AbstractA 22 central composite design with five center points was performed to estimate the effects of temperature (120, 130 and 140 °C) and acid loading (100, 150 and 200 mg g−1) on the yield of monomeric xylose recovery from wheat straw hemicellulose (YS/RM). Under the best hydrolysis condition (140 °C and 200 mg g−1), a YS/RM of 0.26 g g−1 was achieved. After vacuum concentration and detoxification by pH alteration and active charcoal adsorption, the hydrolyzate was used as source of xylose for xylitol bioproduction in a stirred tank reactor. A xylitol production of 30.8 g L−1 was achieved after 54 h−1 of fermentation, resulting in a productivity (QP) of 0.57 g L−1 h−1 and bioconversion yield (YP/S) of 0.88 g g−1. The maximum specific rates of xylose consumption and xylitol production were 0.19 and 0.15 g g−1 h−1, respectively. Copyright © 2006 Society of Chemical Industry

Yeast Immobilization in LentiKats®: A New Strategy for Xylitol Bioproduction from Sugarcane Bagasse

A new PVA-hydrogel matrix for yeast cell immobilization for xylitol bioproduction from sugarcane bagasse was studied. Five repeated-batch fermentation runs were carried out in medium based on sugarcane bagasse hemicellulosic hydrolysate with reuse of the entrapped biocatalyst. The system performance as well as the metabolic behaviour of cells entrapped into the matrix were evaluated. The biocatalyst remained stable and exhibited a similar fermentative profile in all the successive batches, demonstrating the viability of the system. At the end of the run, an average xylitol production was observed of 35.1 g l−1 and average xylitol yield and productivity of 0.58 g g−1 and 0.49 g l−1 h−1, respectively.

Effects of Sulfuric Acid Loading and Residence Time on the Composition of Sugarcane Bagasse Hydrolysate and Its Use as a Source of Xylose for Xylitol Bioproduction

A 2(2) full factorial design was employed to evaluate the effects of sulfuric acid loading and residence time on the composition of sugarcane bagasse hydrolysate obtained in a 250-L reactor. The acid loading and the residence time were varied from 70 to 130 mg acid per gram of dry bagasse and from 10 to 30 min, respectively, while the temperature (121 degrees C) and the bagasse loading (10%) were kept constant. Both the sulfuric acid loading and the residence time influenced the concentrations of xylose and inhibitors in the hydrolysate. The highest xylose concentration (22.71 g/L) was achieved when using an acid loading of 130 mg/g and a residence time of 30 min. These conditions also led to increased concentrations of inhibiting byproducts in the hydrolysate. All of the hydrolysates were vacuum-concentrated to increase the xylose concentration, detoxified by pH alteration and adsorption into activated charcoal, and used for xylitol bioproduction in a stirred tank reactor. Neither the least (70 mg/g, 10 min) nor the most severe (130 mg/g, 30 min) hydrolysis conditions led to the best xylitol production (37.5 g/L), productivity (0.85 g/L h), and yield (0.78 g/g).

Optimization of Acid Hydrolysis of Sugarcane Bagasse and Investigations on its Fermentability for the Production of Xylitol by &lt;I&gt;Candida guilliermondii

The dilute-acid hydrolysis of sugarcane bagasse was optimized using a statistical experimental design resulting in hydrolysates containing 57.25 g/L of xylose, which were fermented with a high inoculum concentration (10 g/L of the yeast Candida guilliermondii IM/UFRJ 50088). The addition of urea reduced the time of conversion (tC) to 75 h (without nitrogen source addition tC > 127 h), and, consequently, improving the rates of xylitol bioproduction. Fermentator experiments, using the optimized conditions, resulted in enhanced conversion rates, reducing tC to 30 h. The stability of the yeast in the hydrolysate was also verified in a 480-h cultivation.

Influence of pH on the xylose reductase activity of Candida guilliermondii during fed-batch xylitol bioproduction

The influence of pH on the xylose reductase (XR) activity was studied. The fed-batch fermentation using exponential feeding rate was employed to produce xylitol. The feeding started when the xylose concentration in the fermenter reached 50 g/l and the pH was adjusted to 2.5, 4.0 or 6.0. The best results for XR activity (0.567 U/mg(protein)) and xylitol volumetric productivity (1.06 g/l.h) were achieved with pH 6.0.