Statistical optimization of xylitol production from corncob hemicellulose hydrolysate by Candida tropicalis HDY-02

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.

Improved xylitol production by the novel inhibitor-tolerant yeast Candida tropicalis K2

Production of potential value-added products from different lignocellulosic biomass is becoming more common due to the availability of the feedstocks in abundance and the environment- friendly nature of the microbial production process. Due to the large array of its applications in the pharmaceutical and food sectors, xylitol is considered as potential value-added compound for production. In this study, organic waste samples were collected from various habitats and screened for potential yeast isolates for xylitol production. Among 124 tested isolates, Candida tropicalis K2 showed the highest potential for xylitol production as well as inhibitors tolerance (Furfural, 5-hydroxymethyl furfural and acetic acid) phenotypes. C. tropicalis K2 produced 90 g/L of xylitol in batch fermentation (100 g/L xylose supplemented with 20 g/L of glycerol as co-substrate) with the yield and productivity of 0.90 g/g and 1.5 g/L.h, respectively, at pH 5.5 and 30°C temperature. Together, >10% higher xylitol yield was achieved when glycerol was used as a co-substrate with pure xylose. Moreover, with non-detoxified corncob and Albizia pod hydrolysates, C. tropicalis K2 isolate produced 0.62 and 0.69 g/g of xylitol yields and 1.04 and 0.75 g/L.h xylitol productivities, respectively. Thus, C. tropicalis K2 isolate could be considered as promising candidate for xylitol production from different lignocellulosic biomass.HIGHLIGHTS Candia tropicalis K2 isolate was screened from natural sites of biomass degradation and characterized for xylitol production.Non-detoxified Albizia pod and corncob hydrolysates were explored for xylitol production using selected C. tropicalis K2 isolate.A maximum of 0.90 g/g yield and 1.07 g/L.h xylitol productivity was achieved with pure xylose.A >10% increase in xylitol yield was achieved using glycerol as a co-substrate.

Development of engineered Candida tropicalis strain for efficient corncob-based xylitol-ethanol biorefinery

BACKGROUND

Xylitol has a wide range of applications in the pharmaceuticals, cosmetic, food and beverage industry. Microbial xylitol production reduces the risk of contamination and is considered as environment friendly and sustainable compared to the chemical method. In this study, random mutagenesis and genetic engineering approaches were employed to develop Candida tropicalis strains with reduced xylitol dehydrogenase (XDH) activity to eliminate co-substrate requirement for corn cob-based xylitol-ethanol biorefinery.

RESULTS

The results suggest that when pure xylose (10% w/v) was fermented in bioreactor, the Ethyl methane sulfonate (EMS) mutated strain (C. tropicalis K2M) showed 9.2% and XYL2 heterozygous (XYL2/xyl2Δ::FRT) strain (C. tropicalis K21D) showed 16% improvement in xylitol production compared to parental strain (C. tropicalis K2). Furthermore, 1.5-fold improvement (88.62 g/L to 132 g/L) in xylitol production was achieved by C. tropicalis K21D after Response Surface Methodology (RSM) and one factor at a time (OFAT) applied for media component optimization. Finally, corncob hydrolysate was tested for xylitol production in biorefinery mode, which leads to the production of 32.6 g/L xylitol from hemicellulosic fraction, 32.0 g/L ethanol from cellulosic fraction and 13.0 g/L animal feed.

CONCLUSIONS

This work, for the first time, illustrates the potential of C. tropicalis K21D as a microbial cell factory for efficient production of xylitol and ethanol via an integrated biorefinery framework by utilising lignocellulosic biomass with minimum waste generation.

Lignocellulosic xylitol production from corncob using engineered Kluyveromycesmarxianus

Xylitol production from lignocellulose hydrolysate is a sustainable and environment-friendly process. In this study, a systematic process of converting corncob waste into xylitol is described. First, the corncobs are hydrolyzed with acid to a hydrolysate. Second, Kluyveromyces marxianus YZJQ016 derived from K. marxianus YZJ074, constructed by overexpressing ScGAL2-N376F from Saccharomyces cerevisiae, CtXYL1 from Candida tropicalis, and KmZWF1 from K. marxianus, produces xylitol from the hydrolysate. A total of ten xylose reductase genes were evaluated, and CtXYL1 proved best by showing the highest catalytic activity under the control of the KmGAPDH promoter. A 5 L fermenter at 42°C produced 105.22 g/L xylitol using K. marxianus YZJQ016—the highest production reported to date from corncob hydrolysate. Finally, for crystallization of the xylitol, the best conditions were 50% (v/v) methanol as an antisolvent, at 25°C, with purity and yield of 99%–100% and 74%, respectively—the highest yield reported to date.

Sugarcane Bagasse-Based Ethanol Production and Utilization of Its Vinasse for Xylitol Production as an Approach in Integrated Biorefinery

Biorefinery of sugarcane bagasse into ethanol and xylitol was investigated in this study. Ethanol fermentation of sugarcane bagasse hydrolysate was carried out by Saccharomyces cerevisiae. After ethanol distillation, the vinasse containing xylose was used to produce xylitol through fermentation by Candida guilliermondii TISTR 5068. During the ethanol fermentation, it was not necessary to supplement a nitrogen source to the hydrolysate. Approximately 50 g/L of bioethanol was produced after 36 h of fermentation. The vinasse was successfully used to produce xylitol. Supplementing the vinasse with 1 g/L of yeast extract improved xylitol production 1.4-fold. Cultivating the yeast with 10% controlled dissolved oxygen resulted in the best xylitol production and yields of 10.2 ± 1.12 g/L and 0.74 ± 0.04 g/g after 60 h fermentation. Supplementing the vinasse with low fraction of molasses to improve xylitol production did not yield a positive result. The supplementation caused decreases of up to 34% in xylitol production rate, 24% in concentration, and 24% in yield.

Microbial xylitol production

Xylitol is pentahydroxy sugar alcohol, existing in very trace amount in fruits and vegetables, and finds varied application in industries like food, pharmaceuticals, confectionaries, etc. and is of prime importance to health. Owing to its trace occurrence in nature and considerable increase in market demand that exceeds availability, alternate production through biotechnological and chemical approach is in process. Biochemical production involves substrates like lignocellulosic biomasses and industrial effluents and is an eco-friendly process with high dependency on physico-chemical parameters. Although the chemical processes are faster, high yielding and economical, they have a great limitation as usage of toxic chemicals and thus need to be regulated and replaced by an environment friendly approach. Microbes play a major role in xylitol production through a biotechnological process towards the development of a sustainable system. Major microbes working on assimilation of xylose for production of xylitol include Candida tropicalis, Candida maltose, Bacillus subtilis, Debaromyces hansenii, etc. The present review reports all probable microbial xylitol production biochemical pathways encompassing diverse bioprocesses involved in uptake and conversion of xylose sugars from agricultural residues and industrial effluents. A comprehensive report on xylitol occurrence and biotechnological production processes with varied substrates has been encompassed. KEY POINTS: • Xylitol from agro-industrial waste • Microbial xylose assimilation.

Elucidating the effect of solid base on the hydrogenation of C5 and C6 sugars over Pt-Sn bimetallic catalyst at room temperature

Conversion of sugars into sugar alcohols at room temperature with exceedingly high yields are achieved over Pt-Sn/γ-Al2O3 catalyst in the presence of calcined hydrotalcite. pH of the reaction mixture significantly affects the conversion and selectivity for sugar alcohols. Selection of a suitable base is the key to achieve optimum yields. Various solid bases in combination with Pt-Sn/γ-Al2O3 catalysts were evaluated for hydrogenation of sugars. Amongst all combinations, the mixture (1:1 wt/wt) of Pt-Sn/γ-Al2O3 and calcined hydrotalcite showed the best results. Hydrotalcite helps to make the pH of reaction mixture alkaline at which sugar molecules undergo ring opening. The sugar molecule in open chain form has carbonyl group which can be polarized by Sn in Pt-Sn/γ-Al2O3 and Pt facilitates the hydrogenation. In the current work, effect of both; solid base and Sn as a promoter has been studied to improve the yields of sugar alcohols from various C5 and C6 sugars at very mild reaction conditions.

Enhanced xylitol production using non-detoxified xylose rich pre-hydrolysate from sugarcane bagasse by newly isolated Pichia fermentans

BACKGROUND

Integrated management of hemicellulosic fraction and its economical transformation to value-added products is the key driver towards sustainable lignocellulosic biorefineries. In this aspect, microbial cell factories are harnessed for the sustainable production of commercially viable biochemicals by valorising C5 and C6 sugars generated from agro-industrial waste. However, in the terrestrial ecosystem, microbial systems can efficiently consume glucose. On the contrary, pentose sugars are less preferred carbon source as most of the microbes lack metabolic pathway for their utilization. The effective utilization of both pentose and hexose sugars is key for economical biorefinery.

RESULTS

Bioprospecting the food waste and selective enrichment on xylose-rich medium led to screening and isolation of yeast which was phylogenetically identified as Pichia fermentans. The newly isolated xylose assimilating yeast was explored for xylitol production. The wild type strain robustly grew on xylose and produced xylitol with > 40% conversion yield. Chemical mutagenesis of isolated yeast with ethyl methanesulphonate (EMS) yielded seven mutants. The mutant obtained after 15 min EMS exposure, exhibited best xylose bioconversion efficiency. This mutant under shake flask conditions produced maximum xylitol titer and yield of 34.0 g/L and 0.68 g/g, respectively. However, under the same conditions, the control wild type strain accumulated 27.0 g/L xylitol with a conversion yield of 0.45 g/g. Improved performance of the mutant was attributed to 34.6% activity enhancement in xylose reductase with simultaneous reduction of xylitol dehydrogenase activity by 22.9%. Later, the culture medium was optimized using statistical design and validated at shake flask and bioreactor level. Bioreactor studies affirmed the competence of the mutant for xylitol accumulation. The xylitol titer and yield obtained with pure xylose were 98.9 g/L and 0.67 g/g, respectively. In comparison, xylitol produced using non-detoxified xylose rich pre-hydrolysate from sugarcane bagasse was 79.0 g/L with an overall yield of 0.54 g/g.

CONCLUSION

This study demonstrates the potential of newly isolated P. fermentans in successfully valorising the hemicellulosic fraction for the sustainable xylitol production.

Ultrasonic enhancement of xylitol production from sugarcane bagasse using immobilized Candida tropicalis MTCC 184

This study investigates ultrasonic enhancement of xylitol production from sugarcane bagasse using C. tropicalis MTCC 184 immobilized on PU foam. Initial xylitol yield of 0.53 g/g xylose improved to 0.65 g/g of xylose (in 36 h fermentation) after optimization of medium and fermentation parameters. Optimum values of experimental parameters for maximum xylitol were: yeast extract = 5.78 g/L, (NH₄)₂SO₄ = 3.22 g/L, KH₂PO₄ = 0.58 g/L, MgSO₄·7H₂O = 0.57 g/L and temperature = 29.3 °C, initial pH = 6.2, agitation rate = 151 rpm and initial xylose concentration = 20.9 g/L. Application of 37 kHz sonication @10% duty cycle during fermentation at optimum conditions resulted in marked intensification of fermentation kinetics. Xylitol yield of 0.66 g/g of xylose has been obtained in ultrasound-assisted fermentation in just 15 h. Fitting of time profiles of substrates and products to kinetic model has highlighted actual physical mechanisms underlying 2-fold faster kinetics induced by sonication.

Xylitol production from non-detoxified and non-sterile lignocellulosic hydrolysate using low-cost industrial media components

Immobilized Candida tropicalis cells in freeze dried calcium alginate beads were used for production of xylitol from lignocellulosic waste like corn cob hydrolysate without any detoxification and sterilization of media. Media components for xylitol fermentation were screened by statistical methods. Urea, KH₂PO₄ and initial pH were identified as significant variables by Plackett-Burman (PB) design. Significant medium components were optimized by response surface methodology (RSM). Predicted xylitol yield by RSM model and experimental yield was 0.87 and 0.79 g/g, respectively. Optimized conditions (urea 1.5 g/L, KH₂PO₄ 1.9 g/L, xylose 55 g/L, pH 6.7) enhanced xylitol yield by 32% and xylose consumption by twofold over those of basal media. In addition, the immobilized cells were reused five times at shake flask level with optimized medium without affecting the xylitol productivity and yield. Xylitol production was successfully scaled up to 7.5 L stirred tank reactor using optimized media. Thus, the optimized condition with non-detoxified pentose hydrolysate from corn cob lignocellulosic waste with minimal nutrients without any sterilization opens up the scope for commercialization of the process.

Xylitol production from waste xylose mother liquor containing miscellaneous sugars and inhibitors: one-pot biotransformation by Candida tropicalis and recombinant Bacillus subtilis

Background

The process of industrial xylitol production is a massive source of organic pollutants, such as waste xylose mother liquor (WXML), a viscous reddish-brown liquid. Currently, WXML is difficult to reuse due to its miscellaneous low-cost sugars, high content of inhibitors and complex composition. WXML, as an organic pollutant of hemicellulosic hydrolysates, accumulates and has become an issue of industrial concern in China. Previous studies have focused only on the catalysis of xylose in the hydrolysates into xylitol using one strain, without considering the removal of other miscellaneous sugars, thus creating an obstacle to subsequent large-scale purification. In the present study, we aimed to develop a simple one-pot biotransformation to produce high-purity xylitol from WXML to improve its economic value.

Results

In the present study, we developed a procedure to produce xylitol from WXML, which combines detoxification, biotransformation and removal of by-product sugars (purification) in one bioreactor using two complementary strains, Candida tropicalis X828 and Bacillus subtilis Bs12. At the first stage of micro-aerobic biotransformation, the yeast cells were allowed to grow and metabolized glucose and the inhibitors furfural and hydroxymethyl furfural (HMF), and converted xylose into xylitol. At the second stage of aerobic biotransformation, B. subtilis Bs12 was activated and depleted the by-product sugars. The one-pot process was successfully scaled up from shake flasks to 5, 150 L and 30 m³ bioreactors. Approximately 95 g/L of pure xylitol could be obtained from the medium containing 400 g/L of WXML at a yield of 0.75 g/g xylose consumed, and the by-product sugars glucose, l-arabinose and galactose were depleted simultaneously.

Conclusions

Our results demonstrate that the one-pot procedure is a viable option for the industrial application of WXML to produce value-added chemicals. The integration of complementary strains in the biotransformation of hemicellulosic hydrolysates is efficient under optimized conditions. Moreover, our study of one-pot biotransformation also provides useful information on the combination of biotechnological processes for the biotransformation of other compounds.

Ethanol and xylitol production by fermentation of acid hydrolysate from olive pruning with Candida tropicalis NBRC 0618

Olive tree pruning biomass has been pretreated with pressurized steam, hydrolysed with hydrochloric acid, conditioned and afterwards fermented using the non-traditional yeast Candida tropicalis NBRC 0618. The main aim of this study was to analyse the influence of acid concentration on the hydrolysis process and its effect on the subsequent fermentation to produce ethanol and xylitol. From the results, it could be deduced that both total sugars and d-glucose recovery were enhanced by increasing the acid concentration tested; almost the whole hemicellulose fraction was hydrolysed when 3.77% was used. It has been observed a sequential production first of ethanol, from d-glucose, and then xylitol from d-xylose. The overall ethanol and xylitol yields ranged from 0.27 to 0.38kgkg(-1), and 0.12 to 0.23kgkg(-1) respectively, reaching the highest values in the fermentation of the hydrolysates obtained with hydrochloric acid 2.61% and 1.11%, respectively.

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.

Direct and efficient xylitol production from xylan by Saccharomyces cerevisiae through transcriptional level and fermentation processing optimizations

In this study, four engineered Saccharomyces cerevisiae carrying xylanase, β-xylosidase and xylose reductase genes by different transcriptional regulations were constructed to directly convert xylan to xylitol. According to the results, the high-copy number plasmid required a rigid selection for promoter characteristics, on the contrast, the selection of promoters could be more flexible for low-copy number plasmid. For cell growth and xylitol production, glucose and galactose were found more efficient than other sugars. The semi-aerobic condition and feeding of co-substrates were taken to improve the yield of xylitol. It was found that the strain containing high-copy number plasmid had the highest xylitol yield, but it was sensitive to the change of fermentation. However, the strain carrying low-copy number plasmid was more adaptable to different processes. By optimization of the transcriptional regulation and fermentation processes, the xylitol concentration could be increased of 1.7 folds and the yield was 0.71 g xylitol/g xylan.

Effect of selected aldehydes found in the corncob hemicellulose hydrolysate on the growth and xylitol fermentation of Candida tropicalis

The effects of four aldehydes (furfural, 5-hydroxymethylfurfural, vanillin and syringaldehyde), which were found in the corncob hemicellulose hydrolysate, on the growth and xylitol fermentation of Candida tropicalis were investigated. The results showed that vanillin was the most toxic aldehyde for the xylitol fermentation, followed by syringaldehyde, furfural and 5-hydroxymethylfurfural. Moreover, the binary combination tests revealed that furfural amplified the toxicity of other aldehydes and the toxicities of other binary combinations without furfural were simply additive. Based on the fermentation experiments, it was demonstrated that the inhibition of aldehydes could be alleviated by prolonging the fermentation incubation, increasing the initial cell concentration, enhancing the initial pH value and minimizing the furfural levels in the hydrolysate evaporation process. The strategies that we proposed to suppress the inhibitory effects of the aldehydes successfully avoided the complicated and costly detoxifications. Our findings could be potentially adopted for the industrial xylitol fermentation from hydrolysates.

Enhanced Production of Xylitol from Corncob by Pachysolen tannophilus Using Response Surface Methodology

Optimization of the culture medium and process variables for xylitol production using corncob hemicellulose hydrolysate by Pachysolen tannophilus (MTTC 1077) was performed with statistical methodology based on experimental designs. The screening of nine nutrients for their influence on xylitol production was achieved using a Plackett-Burman design. Peptone, xylose, MgSO₄·7H₂O, and yeast extract were selected based on their positive influence on xylitol production. The selected components were optimized with Box-Behnken design using response surface methodology (RSM). The optimum levels (g/L) were peptone: 6.03, xylose: 10.62, MgSO₄·7H₂O: 1.39, yeast extract: 4.66. The influence of various process variables on the xylitol production was evaluated. The optimal levels of these variables were quantified by the central composite design using RSM, for establishment of a significant mathematical model with a coefficient determination of R² = 0.91. The validation experimental was consistent with the prediction model. The optimum levels of process variables were temperature (36.56°C), pH (7.27), substrate concentration (3.55 g/L), inoculum size (3.69 mL), and agitation speed (194.44 rpm). These conditions were validated experimentally which revealed an enhanced xylitol yield of 0.80 g/g.

Microbial and bioconversion production of D-xylitol and its detection and application

D-Xylitol is found in low content as a natural constituent of many fruits and vegetables. It is a five-carbon sugar polyol and has been used as a food additive and sweetening agent to replace sucrose, especially for non-insulin dependent diabetics. It has multiple beneficial health effects, such as the prevention of dental caries, and acute otitis media. In industry, it has been produced by chemical reduction of D-xylose mainly from photosynthetic biomass hydrolysates. As an alternative method of chemical reduction, biosynthesis of D-xylitol has been focused on the metabolically engineered Saccharomyces cerevisiae and Candida strains. In order to detect D-xylitol in the production processes, several detection methods have been established, such as gas chromatography (GC)-based methods, high performance liquid chromatography (HPLC)-based methods, LC-MS methods, and capillary electrophoresis methods (CE). The advantages and disadvantages of these methods are compared in this review.