Effects of lignin-derived phenolic compounds on xylitol production and key enzyme activities by a xylose utilizing yeast Candida athensensis SB18

Physicochemical Pretreatment of Vietnamosasa pusilla for Bioethanol and Xylitol Production

The consumption of fossil fuels has resulted in severe environmental consequences, including greenhouse gas emissions and climate change. Therefore, transitioning to alternative energy sources, such as cellulosic ethanol, is a promising strategy for reducing environmental impacts and promoting sustainable low-carbon energy. Vietnamosasa pusilla, an invasive weed, has been recognized as a high potential feedstock for sugar-based biorefineries due to its high total carbohydrate content, including glucan (48.1 ± 0.3%) and xylan (19.2 ± 0.4%). This study aimed to examine the impact of NaOH pretreatment-assisted autoclaving on V. pusilla feedstock. The V. pusilla enzymatic hydrolysate was used as a substrate for bioethanol and xylitol synthesis. After treating the feedstock with varying concentrations of NaOH at different temperatures, the glucose and xylose recovery yields were substantially higher than those of the untreated material. The hydrolysate generated by enzymatic hydrolysis was fermented into bioethanol using Saccharomyces cerevisiae TISTR 5339. The liquid byproduct of ethanol production was utilized by Candida tropicalis TISTR 5171 to generate xylitol. The results of this study indicate that the six- and five-carbon sugars of V. pusilla biomass have great potential for the production of two value-added products (bioethanol and xylitol).

Modelling Based Analysis and Optimization of Simultaneous Saccharification and Fermentation for the Production of Lignocellulosic-Based Xylitol

Simultaneous saccharification and fermentation (SSF) configuration offers efficient use of the reactor. In this configuration, both hydrolysis and fermentation processes are conducted simultaneously in a single bioreactor, and the overall processes may be accelerated. However, problems may arise if both processes have different optimum conditions, and therefore process optimization is required. This paper presents a mathematical model over SSF strategy implementation for producing xylitol from the hemicellulose component of lignocellulosic materials. The model comprises the hydrolysis of hemicellulose and the fermentation of hydrolysate into xylitol. The model was simulated for various process temperatures, prior hydrolysis time, and inoculum concentration. Simulation of the developed kinetics model shows that the optimum SSF temperature is 36 °C, whereas conducting prior hydrolysis at its optimum hydrolysis temperature will further shorten the processing time and increase the xylitol productivity. On the other hand, increasing the inoculum size will shorten the processing time further. For an initial xylan concentration of 100 g/L, the best condition is obtained by performing 21-hour prior hydrolysis at 60 °C, followed by SSF at 36 °C by adding 2.0 g/L inoculum, giving 46.27 g/L xylitol within 77 hours of total processing time. Copyright © 2021 by Authors, Published by BCREC Group. This is an open access article under the CC BY-SA License (https://creativecommons.org/licenses/by-sa/4.0). 

Xylitol Production from Exhausted Olive Pomace by Candida boidinii

In this work, the production of xylitol from a hemicellulosic hydrolysate of exhausted olive pomace (EOP), a residue originated in the olive oil production process by Candida boidinii, was assessed. The hydrolysate was obtained by dilute acid pretreatment of EOP at 170 °C and 2% H2SO4 (w/v). A previous detoxification step of the hydrolysate was necessary, and its treatment with activated charcoal and ion-exchange resin was evaluated. Prior to fermentation of the hydrolysate, fermentation tests in synthetic media were performed to determine the maximum xylitol yield and productivity that could be obtained if inhibitory compounds were not present in the medium. In addition, the glucose existing in the media exerted a negative influence on xylitol production. A maximum xylitol yield of 0.52 g/g could be achieved in absence of inhibitor compounds. Fermentation of the hemicellulosic hydrolysate from EOP after detoxification with ion-exchange resin resulted in a xylitol yield of 0.43 g/g.

Isolation and Characterization of the Stress-Tolerant Candida tropicalis YHJ1 and Evaluation of Its Xylose Reductase for Xylitol Production From Acid Pre-treatment Wastewater

A stress-tolerant yeast was isolated from honey using acid hydrolysate generated from sequential acid-/alkali-pretreatment of empty palm fruit bunch fiber (EPFBF). The isolated yeast was identified molecularly, taxonomically, and morphologically as Candida tropicalis YHJ1, and analyzed for application in xylitol production. The isolated yeast showed stress tolerance toward various chemical reagents and could grow with up to 600 g/L xylose in the culture medium. This yeast also had a broad carbohydrate utilization spectrum, and its xylitol yield was greatest in medium supplemented with xylose as the sole carbon source. In batch fermentation for xylitol production, the yeast could convert xylose prepared from acidic EPFBF pretreatment wastewater into xylitol. Interestingly, C. tropicalis YHJ1 xylose reductase, containing a Ser279 residue, exhibited more effective xylitol conversion compared to orthologous Candida enzymes containing Leu279 or Asn279; this improvement was associated with NADPH binding, as predicted through homologous structure modeling and enzyme kinetic analysis. Taken together, these results show a novel stress-tolerant yeast strain that may be applicable to xylitol production from toxic lignocellulosic byproducts.

Mixed fermentation of Aspergillus niger and Candida shehatae to produce bioethanol with ionic-liquid-pretreated bagasse

In this study, bagasse was pretreated with ionic liquid (IL) 1-butyl-3-methylimidazolium chloride ([Bmim]Cl) and 1% NaOH solution for initial activation of bagasse. A mixed fermentation of treated bagasse by Aspergillus niger and Candida shehatae showed the optimal conditions with the addition of C. shehatae 12 h later at a 1:1 proportion to A. niger. To further improve the ethanol production and obtain optimal fermentation conditions, a Plackett-Burman design was applied to screen the significant formulation and process variables. The optimal ethanol fermentation conditions with IL pretreated bagasse were determined using response surface methodology by Box-Behnken design. Three variables "initial pH, (NH4)2SO4, fermentation time" were regarded as significant factors in the optimization study. The resulting optimum fermentation conditions for bioethanol was identified as: initial pH of 5.89, (NH4)2SO4 concentration of 0.40 g/50 mL, and fermentation time of 3.60 days. The verification experimental ethanol concentration was 8.14 g/L, which agreed with the predicted value. An enhancement of approximately 153.58% compared with initial fermentation conditions in ethanol production was found using optimized conditions. It demonstrated that optimization methodology had a positive effect on the improvement of ethanol production. Under the optimal fermentation medium and conditions, the ethanol production with IL-pretreated bagasse and untreated bagasse was 8.14 g/L and 5.03 g/L, respectively, which exhibited 62% increase, compared to initial conditions with production of 3.21 g/L and 2.67 g/L, respectively, which displayed 20% increase. Both under optimal and original fermentation conditions, compared to the fermentation medium with untreated bagasse, all the results indicated that IL-pretreated bagasse resulted in higher ethanol production than untreated bagasse, demonstrating that IL-pretreated bagasse successfully increased the ethanol production in the mixed fermentation by A. niger and C. shehatae.

Fumaric acid production using renewable resources from biodiesel and cane sugar production processes

The microbial production of fumaric acid by Rhizopus arrhizus NRRL 2582 has been evaluated using soybean cake from biodiesel production processes and very high polarity (VHP) sugar from sugarcane mills. Soybean cake was converted into a nutrient-rich hydrolysate via a two-stage bioprocess involving crude enzyme production via solid state fermentations (SSF) of either Aspergillus oryzae or R. arrhizus cultivated on soybean cake followed by enzymatic hydrolysis of soybean cake. The soybean cake hydrolysate produced using crude enzymes derived via SSF of R. arrhizus was supplemented with VHP sugar and evaluated using different initial free amino nitrogen (FAN) concentrations (100, 200, and 400 mg/L) in fed-batch cultures for fumaric acid production. The highest fumaric acid concentration (27.3 g/L) and yield (0.7 g/g of total consumed sugars) were achieved when the initial FAN concentration was 200 mg/L. The combination of VHP sugar with soybean cake hydrolysate derived from crude enzymes produced by SSF of A. oryzae at 200 mg/L initial FAN concentration led to the production of 40 g/L fumaric acid with a yield of 0.86 g/g of total consumed sugars. The utilization of sugarcane molasses led to low fumaric acid production by R. arrhizus, probably due to the presence of various minerals and phenolic compounds. The promising results achieved through the valorization of VHP sugar and soybean cake suggest that a focused study on molasses pretreatment could lead to enhanced fumaric acid production.

Strategies for enhancing microbial tolerance to inhibitors for biofuel production: A review

Using lignocellulosic biomass for the production of renewable biofuel provides a sustainable and promising solution to the crisis of energy and environment. However, the processes of biomass pretreatment and biofuel fermentation bring a variety of inhibitors to microbial strains. These inhibitors repress microbial growth, decrease biofuel yields and increase fermentation costs. The production of biofuels from renewable lignocellulosic biomass relies on the development of tolerant and robust microbial strains. In recent years, the advancement of tolerance engineering and evolutionary engineering provides powerful platform for obtaining host strains with desired tolerance for further metabolic engineering of biofuel pathways. In this review, we summarized the inhibitors derived from biomass pretreatment and biofuel fermentation, the mechanisms of inhibitor toxicity, and the strategies for enhancing microbial tolerance.

Production of xylitol and bio-detoxification of cocoa pod husk hemicellulose hydrolysate by Candida boidinii XM02G

The use of cocoa pod husk hemicellulose hydrolysate (CPHHH) was evaluated for the production of xylitol by Candida boidinii XM02G yeast isolated from soil of cocoa-growing areas and decaying bark, as an alternative means of reusing this type of waste. Xylitol was obtained in concentrations of 11.34 g.L^-1, corresponding to a yield (Y_p/s) of 0.52 g.g^-1 with a fermentation efficiency (ε) of 56.6%. The yeast was tolerant to inhibitor compounds present in CPHHH without detoxification in different concentration factors, and was able to tolerate phenolic compounds at approximately 6 g.L^-1. The yeast was also able to metabolize more than 99% (p/v) of furfural and hydroxymethylfurfural present in the non-detoxified CPHHH without extension of the cell-growth lag phase, showing the potential of this microorganism for the production of xylitol. The fermentation of cocoa pod husk hydrolysates appears to provide an alternative use which may reduce the impact generated by incorrect disposal of this waste.

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.

By-products resulting from lignocellulose pretreatment and their inhibitory effect on fermentations for (bio)chemicals and fuels

Lignocellulose might become an important feedstock for the future development of the biobased economy. Although up to 75 % of the lignocellulose dry weight consists of sugar, it is present in a polymerized state and cannot be used directly in most fermentation processes for the production of chemicals and fuels. Several methods have been developed to depolymerize the sugars present in lignocellulose, making the sugars available for fermentation. In this review, we describe five different pretreatment methods and their effect on the sugar and non-sugar fraction of lignocellulose. For several pretreatment methods and different types of lignocellulosic biomass, an overview is given of by-products formed. Most unwanted by-products present after pretreatment are dehydrated sugar monomers (furans), degraded lignin polymers (phenols) and small organic acids. Qualitative and quantitative effects of these by-products on fermentation processes have been studied. We conclude this review by giving an overview of techniques and methods to decrease inhibitory effects of unwanted by-products.

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 ozone pretreatment on hydrogen production from barley straw

Application of ozone technology to lignocellulosic biohydrogen production was explored with a barley straw. Ozone pretreatment effectively degraded the straw lignin and increased reducing sugar yield. A simultaneous enzyme hydrolysis and dark fermentation experiment was conducted using a mixed anaerobic consortium together with saccharification enzymes. Both untreated and ozonated samples produced hydrogen. Compared to the untreated group, hydrogen produced by the groups ozonated for 15, 30, 45 and 90 min increased 99%, 133%, 166% and 94%, respectively. Some inhibitory effect on hydrogen production was observed with the samples ozonated for 90 min, and the inhibition was on the fermentative microorganisms, not the saccharification enzymes. These results demonstrate that production of biohydrogen from barley straw, a lignocellulosic biomass, can be significantly enhanced by ozone pretreatment.

Xylans are a valuable alternative resource: production of D-xylose, D-lyxose and furfural under microwave irradiation

The influence of microwave irradiation on hydrolysis of xylan and simultaneous epimerization of the D-xylose to D-lyxose has been studied. An acidic solution of xylan was treated with catalytic amount of sodium molybdate and the composition of the reaction mixture was analyzed. Short reaction times of hydrolysis and subsequent epimerization reaction provided an equilibrium reaction mixture of D-xylose and D-lyxose (1.6:1) without significant formation of undesirable side products. Obtained pentoses can be reduced to the corresponding alditols (D-xylitol and D-lyxitol) in very good yields (88% and 85%) or can be further dehydrated to furfural (53%). Combined use of Mo(VI) catalyst and microwave irradiation allows better conversions and substantial reduction of reaction times (400-fold) compared to that obtained by conventional heating. Studied stereospecific transformation of xylan proceeds with high selectivity, short reaction times and very good yields that makes this approach attractive also for preparative purposes.

A novel pathway construction in Candida tropicalis for direct xylitol conversion from corncob xylan

In this study, an integrated xylitol production pathway, directly using xylan as the substrate, was constructed in Candida tropicalis BIT-Xol-1 which could efficiently convert xylose into xylitol. In order to consolidate this bioprocessing, a β-1,4-xylanase gene (atn) and a β-xylosidase gene (atl) were cloned from Aspergillus terreus, and were constructed onto episomal plasmid pAUR123. Additionally, combination of the individual atn and atl expression cassette was also cloned onto pAUR123. After transforming, the positive C. tropicalis transformants co-expressing xylanase and xylosidase produced larger hydrolysis zones than those expressing xylanase alone, when incubated on xylan-congo red plates. The engineered C. tropicalis/pAUR-atn-atl-3 (C. tropicalis PNL3) secrete heterologous xylanase and xylosidase simultaneously, with the activities of 48.17 and 11.56 U/mL, respectively. The xylitol yields by C. tropicalis PNL3 utilizing xylan and corncob were 77.1% and 66.9%, respectively. The integrated pathway of xylitol production was feasible and efficient in utilization of xylan-rich renewable biomass via combining saccharification and transformation of xylan in engineered C. tropicalis.