Microwave-Assisted Oxalic Acid Pretreatment for the Enhancing of Enzyme Hydrolysis in the Production of Xylose and Arabinose from Bagasse

Sustainable biorefinery approach by utilizing xylose fraction of lignocellulosic biomass

Lignocellulosic biomass (LCB) has been a lucrative feedstock for developing biochemical products due to its rich organic content, low carbon footprint and abundant accessibility. The recalcitrant nature of this feedstock is a foremost bottleneck. It needs suitable pretreatment techniques to achieve a high yield of sugar fractions such as glucose and xylose with low inhibitory components. Cellulosic sugars are commonly used for the bio-manufacturing process, and the xylose sugar, which is predominant in the hemicellulosic fraction, is rejected as most cell factories lack the five‑carbon metabolic pathways. In the present review, more emphasis was placed on the efficient pretreatment techniques developed for disintegrating LCB and enhancing xylose sugars. Further, the transformation of the xylose to value-added products through chemo-catalytic routes was highlighted. In addition, the review also recapitulates the sustainable production of biochemicals by native xylose assimilating microbes and engineering the metabolic pathway to ameliorate biomanufacturing using xylose as the sole carbon source. Overall, this review will give an edge on the bioprocessing of microbial metabolism for the efficient utilization of xylose in the LCB.

Comparison of Alkaline Sulfite Pretreatment and Acid Sulfite Pretreatment with Low Chemical Loading in Saccharification of Poplar

Sulfite pretreatment is a productive process for lignin dissolution in lignocelluloses and to reduce the hydrophobicity of lignin by sulfonation, thus promoting the hydrolyzability of the substrate. Previously, sulfite pretreatment needs high dosages of chemicals and thus results in the high cost of the pretreatment and the great pressure of environmental pollution. To overcome these problems, it was crucial to research whether alkaline sulfite pretreatment (ALS) and acid sulfite pretreatment (ACS) with low chemical loading could enhance the saccharification of poplar. In this work, the results indicated that with low loading of chemicals in sulfite pretreatment, ALS pretreatment (1.6% Na2SO3 and 0.5% NaOH) at 180 °C removed more lignin, resulted in lower hydrophobicity and higher cellulase adsorption capacity of poplar than ACS pretreatment (1.6% Na2SO3 and 0.5% H2SO4) at 180 °C. A satisfying glucose yield of 84.9% and a xylose yield of 76.0% were obtained from poplar after ALS pretreatment with 1.6% Na2SO3 and 0.5% NaOH at 180 °C for 1 h using 10 FPU cellulase/g dry matter, saving sodium sulfite by 60.0% compared to the loading of sulfite in traditional sulfite pretreatment. The strategy developed in this work reduced chemical loading and cellulase loading in alkali sulfite pretreatment for the saccharification of poplar.

An optimum biomass fractionation strategy into maximum carbohydrates conversion and lignin valorization from poplar

Appropriate fractionation of lignocellulosic biomass into useable forms is a key challenge to achieving an economic bioethanol production. In the present study, four different fractionation strategies of hydrothermal-, NaOH-, ethanol-, and NaOH catalyzed ethanol pretreatment were investigated to compare their abilities of cellulose conversion. Results showed that NaOH catalyzed ethanol pretreatment showed a rather high extent of delignification of 85.92%, which also enhanced the retention of cellulose (92.56%) and hemicellulose (76.57%); while other pretreatments tended to produce cellulose fraction which was insufficient to achieve the whole component utilization. After simultaneous saccharification and fermentation at high solids loading, synergistic maximization of xylose (42.47 g/L) and ethanol (85.74 g/L) output was achieved via alkaline ethanol pretreatment. Lignin characterization information showed that alkaline ethanol pretreatment facilitates the cleavage of β-O-4 linkage and further converts into arylglycerol. Moreover, less condensed substructure units with high processing activity were also generated in S- and G- lignin.

Synergistic effect of ultrasound and citric acid treatment on functional, structural and storage properties of hog plum (Spondias mombin L) bagasse

A novel approach in promoting the valorisation of fruit waste as potential bio-ingredients in food applications is gaining research attention in recent times. In the present study, hog plum bagasse (HPB) treated with citric acid alone (0.12 dm3, 99.0% purity) and in synergistic treatment with ultrasound (40 KHz, 400 W, 0.348 W/cm3, 60 min, 80 °C). Treated samples were evaluated for proximate and lignocellulose composition, functional, bioactive, structural, morphology and microbial properties, while the untreated samples served as control. HPB showed varied proximate values with treatment effect. Notably, a significant reduction (42.06%) was observed in fibre content. A similar reduction was observed in the hemicellulose and lignin fraction but improved the yield of the cellulose component. Furthermore, treatment increased bulk density (0.120-0.131 g/cm3), water absorption capacity (5.60-6.35 g/g), swelling power (8.85-9.94 g/g) and solubility index (1.01-2.32%) but reduced oil absorption capacity (7.50-4.15 g/g). All colour parameters were reduced with treatment, while the total phenolic compound and antioxidant capacity of treated bagasse improved by 24.70% and 45.37%. Fourier transform infra-red spectroscopy alterations were observed in the absorption spectra with treatment, while scanning electron microscopestructure in treated samples showed cavity formation. Also, the microbial population was reduced to a non-detectable level after treatment. Ultrasound-assisted treatment of HPB holds a valorisation potential for its food application by relevant agro-based industries.

A fractionation strategy of cellulose, hemicellulose, and lignin from wheat straw via the biphasic pretreatment for biomass valorization

Developing an environmentally friendly and efficient pretreatment to utilize wheat straw is essential to a sustainable future. An acid biphasic system with 2-methyltetrahydrofuran (2-MeTHF) organic solvent and dilute p-toluenesulfonic acid (p-TsOH) were employed for the simultaneous fractionation of three components. Results showed that the biphasic system had excellent cellulose protection and high removal of hemicellulose and lignin. In detail, Under the optimal conditions (0.1 M p-TsOH, 2-MeTHF: H₂O = 1:1 (v:v), 140 °C, 3 h), mostly cellulose retained in the residues (95.69%), 57.18% of lignin was removed and high yield of hemicellulose-based C5 sugars was achieved (77.49%). In the further process of dehydration of pre-hydrolysate dichloromethane (DCM) as an organic phase, the yield of furfural was 80.07% (170 °C-80 min). The saccharification of residue reached 95.82%. p-TsOH/2-MeTHF/H₂O pretreatment was desirable for high selectivity fractionation. Important chemicals for bioenergy including furfural, monosaccharides and lignin are obtained.

Application and prospect of organic acid pretreatment in lignocellulosic biomass separation: A review

As a clean and efficient method of lignocellulosic biomass separation, organic acid pretreatment has attracted extensive research. Hemicellulose or lignin is selectively isolated and the cellulose structure is preserved. Effective fractionation of lignocellulosic biomass is achieved. The separation characteristics of hemicellulose or lignin by different organic acids were summarized. The organic acids of hemicellulose were separated into hydrogen ionized, autocatalytic and α-hydroxy acids according to the separation mechanism. The separation of lignin depends on the dissolution mechanism and spatial effect of organic acids. In addition, the challenges and prospects of organic acid pretreatment were analyzed. The separation of hemicellulose and enzymatic hydrolysis of cellulose were significantly affected by the polycondensation of lignin, which is effectively inhibited by the addition of green additives such as ketones or alcohols. Lignin separation was improved by developing a deep eutectic solvent treatment based on organic acid pretreatment. This work provides support for efficient cleaning of carbohydrate polymers and lignin to promote global carbon neutrality.

The preparation technology and application of xylo-oligosaccharide as prebiotics in different fields: A review

Xylo-oligosaccharide (XOS) is a class of functional oligosaccharides that have been demonstrated with prebiotic activity over several decades. XOS has several advantages relative to other oligosaccharide molecules, such as promoting root development as a plant regulator, a sugar supplement for people, and prebiotics to promote intestinal motility utilization health. Now, the preparation and extraction process of XOS is gradually mature, which can maximize the extraction and avoid waste. To fully understand the recent preparation and application of XOS in different areas, we summarized the various technologies for obtaining XOS (including acid hydrolysis, enzymatic hydrolysis, hydrothermal pretreatment, and alkaline extraction) and current applications of XOS, including in animal feed, human food additives, and medicine. It is hoped that this review will serve as an entry point for those looking into the prebiotic field of research, and perhaps begin to dedicate their work toward this exciting classification of bio-based molecules.

Bacterial Cellulose Production from agricultural Residues by two Komagataeibacter sp. Strains

Agricultural residues are constantly increasing with increased farming processes, and improper disposal is detrimental to the environment. Majority of these waste residues are rich in lignocellulose, which makes them suitable substrate for bacterial fermentation in the production of value-added products. In this study, bacterial cellulose (BC), a purer and better form of cellulose, was produced by two Komagataeibacter sp. isolated from rotten banana and kombucha drink using corncob (CC) and sugarcane bagasse (SCB) enzymatic hydrolyzate, under different fermentation conditions, that is, static, continuous, and intermittent agitation. The physicochemical and mechanical properties of the BC films were then investigated by Fourier Transformed Infrared Spectroscopy (FTIR), Thermogravimetry analysis, Field Emission Scanning Electron Microscopy (FE-SEM), and Dynamic mechanical analysis. Agitation gave a higher BC yield, with Komagataeibacter sp. CCUG73629 producing BC from CC with a dry weight of 1.6 g/L and 1.4 g/L under continuous and intermittent agitation, respectively, compared with that of 0.9 g/L in HS medium. While BC yield of dry weight up to 1.2 g/L was obtained from SCB by Komagataeibacter sp. CCUG73630 under continuous agitation compared to that of 0.3 g/L in HS medium. FTIR analysis showed BC bands associated with cellulose I, with high thermal stability. The FE-SEM analysis showed that BC fibers were highly ordered and densely packed. Although the BC produced by both strains showed similar physicochemical and morphological properties, the BC produced by the Komagataeibacter sp. CCUG73630 in CC under intermittent agitation had the best modulus of elasticity, 10.8 GPa and tensile strength, 70.9 MPa.

Recent advances in lignocellulose prior-fractionation for biomaterials, biochemicals, and bioenergy

Due to over-consumption of fossil resources and environmental problems, lignocellulosic biomass as the most abundant and renewable materials is considered as the best candidate to produce biomaterials, biochemicals, and bioenergy, which is of strategic significance and meets the theme of Green Chemistry. Highly efficient and green fractionation of lignocellulose components significantly boosts the high-value utilization of lignocellulose and the biorefinery development. However, heterogeneity of lignocellulosic structure severely limited the lignocellulose fractionation. This paper offers the summary and perspective of the extensive investigation that aims to give insight into the lignocellulose prior-fractionation. Based on the role and structure of lignocellulose component in the plant cell wall, lignocellulose prior-fractionation can be divided into cellulose-first strategy, hemicelluloses-first strategy, and lignin-first strategy, which realizes the selective dissociation and transformation of a component in lignocellulose. Ultimately, the challenges and opportunities of lignocellulose prior-fractionation are proposed on account of the existing problems in the biorefining valorization.

Comparison of dilute acid pretreatment of agave assisted by microwave versus ultrasound to enhance enzymatic hydrolysis

A comparison between microwave and ultrasound irradiations in the agave pretreatment using dilute sulfuric acid as catalyst was assessed for the first time. Pretreatments were performed using a Taguchi Orthogonal Array L9 (3⁴) to improve the hemicellulose removal and the agave digestibility. The results showed that under optimal conditions, the hemicellulose removal was superior in the pretreatment assisted with microwave (77.5%) compared to ultrasound (28.2%). Enzymatic hydrolysis yield of agave pretreated with microwave (MW_OC) was 2-fold higher than agave pretreated with ultrasound (US_OC). The relatively mild conditions of pretreatment with MW_OC allowed to obtain a hydrolyzed free of inhibitors with a high glucose concentration (47.7 g/L) at low solids loading (10% w/v). However, these conditions did not have a significant effect over the agave pretreated with ultrasound. The pretreatment assisted with MW_OC allowed to reduce time and temperature of the process compared to pretreatment with conventional heating.

One-pot fractionation of corn stover with peracetic acid and maleic acid

Lignocellulose fractionation is a primary treatment to enhance cellulose accessibility and multi-component use. Herein, the development of a one-step fractionation is reported for cellulose enrichment from corn stover using a low concentration of peracetic acid combined with maleic acid (PAM). The effects of pretreatment parameters on the contents of cellulose, hemicellulose, and lignin were investigated. After cooking for 1 h at 130 °C with 1.5 wt% peracetic acid and 3 wt% maleic acid, 86.83% of corn stover cellulose remained in the solid residue while 88.21% of hemicellulose and 87.77% of lignin dissolved into the aqueous liquid. Hemicellulose was primarily hydrolyzed into xylose with 84.58% recovered during the PAM process. The cellulose-rich residue was enzymatically hydrolyzed with a glucose yield of 89.65%, which was two to three times that of untreated substrate. Generally, the proposed process offers a promising approach for efficient fractionation of lignocellulose under mild and environmental-friendly conditions.

Lignocellulosic Biomass as a Substrate for Oleaginous Microorganisms: A Review

Microorganisms capable of accumulating lipids in high percentages, known as oleaginous microorganisms, have been widely studied as an alternative for producing oleochemicals and biofuels. Microbial lipid, so-called Single Cell Oil (SCO), production depends on several growth parameters, including the nature of the carbon substrate, which must be efficiently taken up and converted into storage lipid. On the other hand, substrates considered for large scale applications must be abundant and of low acquisition cost. Among others, lignocellulosic biomass is a promising renewable substrate containing high percentages of assimilable sugars (hexoses and pentoses). However, it is also highly recalcitrant, and therefore it requires specific pretreatments in order to release its assimilable components. The main drawback of lignocellulose pretreatment is the generation of several by-products that can inhibit the microbial metabolism. In this review, we discuss the main aspects related to the cultivation of oleaginous microorganisms using lignocellulosic biomass as substrate, hoping to contribute to the development of a sustainable process for SCO production in the near future.

State of the art of straw treatment technology: Challenges and solutions forward

Straw as an agricultural byproduct has been recognized as a potential resource. However, open-field straw burning is still the main mean in many regions of the world, which causes the wasting of resource and air pollution. Recently, many technologies have been developed for energy and resource recovery from straw, of which the biological approach has attracted growing interests because of its economically viable and eco-friendly nature. However, pretreatment of straw prior to biological processes is essential, and largely determines the process feasibility, economic viability and environmental sustainability. Thus, this review attempts to offer a critical and holistic analysis of current straw pretreatment technologies and management practices. Specifically, an integrated biological processes coupled with microbial degradation and enzymatic hydrolysis was proposed, and its potential benefits, limitations and challenges associated with future large-scale straw treatment were also elaborated, together with the perspectives and directions forward.

Lignocellulosic biomass: Acid and alkaline pretreatments and their effects on biomass recalcitrance - Conventional processing and recent advances

Lignocellulosic biomass is one of the most abundant organic resources worldwide and is a promising source of renewable energy and bioproducts. It basically consists of three fractions, cellulose, hemicelluloses and lignin, which confer a recalcitrant structure. As such, pretreatment steps are required to make each fraction available for further use, with acidic, alkaline and combined acidic-alkaline treatments being the most common techniques. This review focuses on recent strategies for lignocellulosic biomass pretreatment, with a critical discussion and comparison of their efficiency based on the composition of the materials. Mild pretreatments usually allow the recovery of the three biomass fractions for further transformation and valorisation. An insight is provided of newly developed technologies from recently filed patents on lignocellulosic biomass pretreatment and the transformation of agro-industrial residues into high value-added products, such as biofuels and organic acids.

Preparation of Lignocellulose-Based Activated Carbon Paper as a Manganese Dioxide Carrier for Adsorption and in-situ Catalytic Degradation of Formaldehyde

Formaldehyde is a colorless, highly toxic, and flammable gas that is harmful to human health. Recently, many efforts have been devoted to the application of activated carbon to absorb formaldehyde. In this work, lignocellulose-based activated carbon fiber paper (LACFP) loaded with manganese dioxide (MnO₂) was fabricated for the adsorption and in-situ catalytic degradation of formaldehyde. LACFP was prepared by two-stage carbonization and activation of sisal hemp pulp-formed paper and was then impregnated with manganese sulfate (MnSO₄) and potassium permanganate (KMnO₄) solutions; MnO₂ then formed by in situ growth on the LACFP base by calcination. The catalytic performance of MnO₂-loaded LACFP for formaldehyde was then investigated. It was found that the suitable carbonization conditions were elevating the temperature first by raising it at 10°C/min from room temperature to 280°C, then at 2°C/min from 280 to 400°C, maintaining the temperature at 400°C for 1 h, and then increasing it quickly from 400 to 700°C at 15°C/min. The conditions used for activation were similar to those for carbonization, with the temperature additionally being held at 700°C for 2 h. The conditions mentioned above were optimized to maintain the fiber structure and shape integrity of the paper, being conducive to loading with catalytically active substances. Regarding the catalytic activity of MnO₂-loaded LACFP, the concentration of formaldehyde decreased by 59 ± 6 ppm and the concentration of ΔCO₂ increased by 75 ± 3 ppm when the reaction proceeded at room temperature for 10 h. The results indicated that MnO₂-loaded LACFP could catalyze formaldehyde into non-toxic substances.

Enhanced Enzymatic Hydrolysis of Pennisetum alopecuroides by Dilute Acid, Alkaline and Ferric Chloride Pretreatments

In this study, effects of different pretreatment methods on the enzymatic digestibility of Pennisetum alopecuroides, a ubiquitous wild grass in China, were investigated to evaluate its potential as a feedstock for biofuel production. The stalk samples were separately pretreated with H₂SO₄, NaOH and FeCl₃ solutions of different concentrations at 120 °C for 30 min, after which enzymatic hydrolysis was conducted to measure the digestibility of pretreated samples. Results demonstrated that different pretreatments were effective at removing hemicellulose, among which ferric chloride pretreatment (FCP) gave the highest soluble sugar recovery (200.2 mg/g raw stalk) from the pretreatment stage. In comparison with FCP and dilute acid pretreatment (DAP), dilute alkaline pretreatment (DALP) induced much higher delignification and stronger morphological changes of the biomass, making it more accessible to hydrolysis enzymes. As a result, DALP using 1.2% NaOH showed the highest total soluble sugar yield through the whole process from pretreatment to enzymatic hydrolysis (508.5 mg/g raw stalk). The present work indicates that DALP and FCP have the potential to enhance the effective bioconversion of lignocellulosic biomass like P. alopecuroides, hence making this material a valuable and promising energy plant.

A new approach to recycle oxalic acid during lignocellulose pretreatment for xylose production

BACKGROUND

Dilute oxalic acid pretreatment has drawn much attention because it could selectively hydrolyse the hemicellulose fraction during lignocellulose pretreatment. However, there are few studies focusing on the recovery of oxalic acid. Here, we reported a new approach to recycle oxalic acid used in pretreatment via ethanol extraction.

RESULTS

The highest xylose content in hydrolysate was 266.70 mg xylose per 1 g corncob (85.0% yield), which was achieved using 150 mmol/L oxalic acid under the optimized treatment condition (140 °C, 2.5 h). These pretreatment conditions were employed to the subsequent pretreatment using recycled oxalic acid. Oxalic acid in the hydrolysate could be recycled according to the following steps: (1) water was removed via evaporation and vacuum drying, (2) ethanol was used to extract oxalic acid in the remaining mixture, and (3) oxalic acid and ethanol were separated by reduced pressure evaporation. The total xylose yields could be stabilized by intermittent adding oxalic acid, and the yields were in range of 46.7-64.3% in this experiment.

CONCLUSIONS

This sustainable approach of recycling and reuse of oxalic acid has a significant potential application for replacing traditional dilute mineral acid pretreatment of lignocellulose, which could contribute to reduce CO₂ emissions and the cost of the pretreatment.