Novel process for the coproduction of xylo-oligosaccharides, fermentable sugars, and lignosulfonates from hardwood

Extraction, physicochemical properties, bioactivities and application of natural sweeteners: A review

Natural sweeteners generally refer to a sweet chemical component directly extracted from nature or obtained through appropriate modifications, mainly secondary metabolites of plants. Compared to the first-generation sweeteners represented by sucrose and the second-generation sweeteners represented by sodium cyclamate, natural sweeteners usually have high sweetness, low-calorie content, good solubility, high stability, and rarely toxic side effects. Historically, researchers mainly focus on the function of natural sweeteners as substitutes for sugars in the food industry. This paper reviews the bioactivities of several typical natural sweeteners, including anti-cancer, anti-inflammatory, antioxidant, anti-bacterial, and anti-hyperglycemic activities. In addition, we have summarized the extraction, physicochemical properties, and application of natural sweeteners. The article aimed to comprehensively collate vital information about natural sweeteners and review the potentiality of tapping bioactive compounds from natural products. Hopefully, this review provides insights into the further development of natural sweeteners as therapeutic agents and functional foods.

A coupling strategy combined with acid-hydrothermal and novel DES pretreatment: Enhancing biomethane yield under solid-state anaerobic digestion and efficiently producing xylo-oligosaccharides and recovered lignin from poplar waste

Lignocellulose is an abundant renewable bio-macromolecular complex, which can be used to produce biomethane and other high-value products. The lignin, presents in lignocellulose is typically regarded as an inhibitor of anaerobic digestion. Therefore, it is crucial to develop a novel selective separation strategy to achieve efficient biomethane production and all-component utilization of biomass. Hence, a combination of two-step pretreatment and solid-state anaerobic digestion was employed to enhance the production of biomethane and to generate valuable chemicals from poplar waste. Optimal conditions (4 % acetic acid, 170 °C, and 40 min) resulted in 70.85 % xylan removal, yielding 50.28 % xylo-oligosaccharides. The effect of a strong acid 4-CSA-based novel three-constituent DES on delignification was investigated from 80 °C to 100 °C; the cellulose content of DES pretreated poplar increased from 64.11 % to 80.92 %, and the delignification rate increased from 49.0 % to 90.4 %. However, high delignification of the pretreated poplar (DES-100 and DES-110) led to a rapid accumulation of volatile organic acids during the hydrolysis and acidogenesis stages, resulting in methanogenesis inhibition. The highest biomethane yield of 208 L/kg VS was achieved with DES-80 (49.0 % delignification), representing a 148 % improvement compared over untreated poplar. This approach demonstrates the potential for comprehensive utilization of all components of biomass waste.

Toward Renewable-Based Prebiotics from Woody Biomass: Potential of Tailored Xylo-Oligosaccharides Obtained by Enzymatic Hydrolysis of Beechwood Xylan as a Prebiotic Feed Supplement for Young Broilers

Although antibiotic resistance emerges naturally, this process has been accelerated by the worldwide overuse and misuse of antibiotics. It is essential to find effective alternatives in the broiler industry to improve poultry health while maintaining production efficiency and product safety. In this study, we aimed to evaluate a potential alternative: wood-derived xylo-oligosaccharides (XOS). The objective of this research was to investigate the potential of XOS prepared using enzymatic hydrolysis of beechwood xylan as a prebiotic feed supplement for broilers. A pilot study was conducted to explore the optimal XOS fraction profile by in vitro fermentation. Subsequently, a semi-continuous enzyme membrane reactor was used, allowing for the production of tailored XOS in large quantities. Given the strong bidirectional relationship between intestinal health, nutrition, and intestinal microbiota composition in broilers, an in vivo experiment was performed to explore the potential of XOS as a prebiotic feed supplement by investigating growth performance, feed conversion ratio, caecal short and medium chain fatty acid (SCFA and MCFA) concentration, and microbiological composition of the caecal content. Results from the pilot study indicated that higher enzyme concentrations in the hydrolysis process yield a product that leads to a higher total SCFA and MCFA- and butyric acid production during in vitro fermentation by caecal bacteria. Supplementation of the tailored XOS to the broiler diet (day 1 (d1)-d8 0.13% wt/wt XOS, d9-d15 0.32% XOS) resulted in higher Bifidobacterium counts, beneficial to the health of birds, on d11 and d15.

Efficient co-production of xylo-oligosaccharides and probiotics from corncob by combined lactic acid pretreatment and two-step enzymatic hydrolysis

Lactic acid (LA) is efficient in xylo-oligosaccharides (XOS) production from poplar. However, the role of LA in XOS production from corncob has not been carefully elucidated, and the co-production of probiotics of Bacillus subtilis from corncob residue has not been reported. In this study, LA pretreatment was combined with enzymatic hydrolysis to produce XOS and monosaccharides from corncob. An XOS yield of 69.9% was obtained from corncob by combining 2% LA pretreatment and xylanase hydrolysis. Yields of 95.6% glucose and 54.0% xylose were obtained from corncob residue via cellulase, and the resulting cellulase hydrolysate was used to culture B. subtilis YS01. The resulting viable count of the strain was 6.4×10⁸ CFU/mL, and the glucose and xylose utilization rates were 99.0% and 89.8%, respectively. This study demonstrates a green, efficient, and mild process for producing XOS and probiotics from corncob by combining LA pretreatment and enzymatic hydrolysis.

Improve Enzymatic Hydrolysis of Lignocellulosic Biomass by Modifying Lignin Structure via Sulfite Pretreatment and Using Lignin Blockers

Even traditional pretreatments can partially remove or degrade lignin and hemicellulose from lignocellulosic biomass for enhancing its enzymatic digestibility, the remaining lignin in pretreated biomass still restricts its enzymatic hydrolysis by limiting cellulose accessibility and lignin-enzyme nonproductive interaction. Therefore, many pretreatments that can modify lignin structure in a unique way and approaches to block the lignin’s adverse impact have been proposed to directly improve the enzymatic digestibility of pretreated biomass. In this review, recent development in sulfite pretreatment that can transform the native lignin into lignosulfonate and subsequently enhance saccharification of pretreated biomass under certain conditions was summarized. In addition, we also reviewed the approaches of the addition of reactive agents to block the lignin’s reactive sites and limit the cellulase-enzyme adsorption during hydrolysis. It is our hope that this summary can provide a guideline for workers engaged in biorefining for the goal of reaching high enzymatic digestibility of lignocellulose.

Co-production of amino acid-rich xylooligosaccharide and single-cell protein from paper mulberry by autohydrolysis and fermentation technologies

Background

Autohydrolysis is an extensively investigated pretreatment method due to its environmental friendliness. During autohydrolysis, most xylan from hemicellulose can be converted into xylooligosaccharides (XOS), and cellulose in the autohydrolyzed residues can be transformed into glucose after enzymatic hydrolysis. Both of these are value-added biochemicals in the biorefining process. In this work, paper mulberry (PM), which contains abundant protein, was utilized as a raw material to coproduce XOS and single-cell protein (SCP) through autohydrolysis and fermentation technologies.

Results

The results showed that 8.3 g of XOS and 1.8 g of amino acids could be recovered in the autohydrolysate (based on 100 g raw material) after autohydrolysis (170 °C, 1 h). Moreover, 5.7 g of low-DP XOS along with 1.8 g of amino acids could be further obtained from the autohydrolysate after hydrolysis with endo-β-1-4-xylanase. In addition, 20.1 g of fermentable monosaccharides was recovered after hydrolyzing the autohydrolyzed PM with cellulase, which can be used to produce 4.8 g of SCP after fermentation with Candida utilis.

Conclusion

As a valuable application of PM, a novel process is proposed to coproduce amino acid-rich XOS and SCP through autohydrolysis. The carbohydrate of PM is effectively converted to high value-added products.

The Effect of Ball Milling Time on the Isolation of Lignin in the Cell Wall of Different Biomass

Ball milling technology is the classical technology to isolate representative lignin in the cell wall of biomass for further investigation. In this work, different ball milling times were carried out on hardwood (poplar sawdust), softwood (larch sawdust), and gramineous material (bamboo residues) to understand the optimum condition to isolate the representative milled wood lignin (MWL) in these different biomass species. Results showed that prolonging ball milling time from 3 to 7 h obviously increased the isolation yields of MWL in bamboo residues (from 39.2% to 53.9%) and poplar sawdust (from 15.5% to 35.6%), while only a slight increase was found for the MWL yield of larch sawdust (from 23.4% to 25.8%). Importantly, the lignin substructure of ß-O-4 in the MWL samples from different biomasses can be a little degraded with the increasing ball milling time, resulting in the prepared MWL with lower molecular weight and higher content of hydroxyl groups. Based on the isolation yield and structure features, milling time with 3 and 7 h were sufficient to isolate the representative lignin (with yield over 30%) in the cell wall of bamboo residues and poplar sawdust, respectively, while more than 7 h should be carried out to isolate the representative lignin in larch sawdust.

High efficiency and clean separation of eucalyptus components by glycolic acid pretreatment

Glycolic acid has chemical properties similar to those of formic acid. Therefore, similar to formic acid pretreatment, glycolic acid pretreatment has the separation effect of hemicellulose. In this study, eucalyptus hemicellulose was effectively separated by glycolic acid pretreatment. The effects of glycolic acid concentration, temperature and time on the separation of cellulose, hemicellulose and lignin were investigated. The optimum conditions were acid concentration 5.40%, temperature 140 °C, time 3.0 h. The highest yield of xylose was 56.72%. The recovery rate of glycolic acid was 91%. Compared to formic acid, the yield of xylose increased to 10.33% while that of lignin decreased to 11.08%. It showed high selectivity for hemicellulose separation, yielding 65.48% hemicellulose with 72.08% purity. The depolymerization and repolymerization of lignin were inhibited. The integrity of the cellulose structure was preserved. It provides theoretical support for the fractional separation and high-value transformation of lignocellulosic biomass.

Co-production of functional xylo-oligosaccharides and fermentable sugars from corn stover through fast and facile ball mill-assisted alkaline peroxide pretreatment

The aim of this work was to develop a feasible ball mill-assisted alkaline peroxide pretreatment followed by stepwise hydrolysis to improve the yield of xylo-oligosaccharides (XOS) and fermentable sugars. The hydrogen peroxide charge, ball-milling time, and solid-to-liquid ratio affected the compositions, particle sizes, morphology, and crystallinity of the corn stover, directly improving the following hydrolytic efficiency. The optimal pretreatment was with 0.45 g/g (H₂O₂: substrate) and 1:3 solid-to-liquid ratio (w/v) for 1.0 h ball-milling, resulting in 84.29% delignification. Physicochemical properties of the pretreated samples were characterized and their correlations to the enzymatic hydrolysis were revealed. Compared with one-step cellulase hydrolysis, the two-step xylanase-cellulase hydrolysis of the pretreated corn stover showed significant advance in preparing XOS, producing 69.65% (on the base of xylan content in pretreated sample) of XOS, along with 20.55% xylose, 68.94% glucose, and 21.15% gluco-oligosaccharides. The yield of XOS was 2-7 times higher than those in previous studies.

Mechanically Flexible Carbon Aerogel with Wavy Layers and Springboard Elastic Supporting Structure for Selective Oil/Organic Solvent Recovery

Even though compressible carbon aerogels are widely studied for oil/organic solvent recovery, it is challenging to simultaneously achieve excellent mechanical performance and recovery efficiency due to the brittleness of the carbon skeleton. Here a novel strategy is proposed to efficiently fabricate a 3D elastic reduced graphene oxide (RGO)-cross-linked carbon aerogel. Notably, cellulose nanocrystals (CNCs) isolated from plant pulp act as an essential component, and prehydrolysis liquor (PHL), an industrial byproduct in the plant pulping process, serves as the adhesion promoter to achieve enhancement of the strength and flexibility of the carbon aerogel. For the first time, all components (pulp and PHL) of the tree were fully exploited to design a carbon aerogel. The formation of wavy carbon layers with springboard elastic supporting microstructure enables mechanical stretch and shrink as well as avoids interfacial collapse during compression. Benefiting from the unique wavy layer structure and strong interaction, the carbon aerogels are ultralight (4.98 mg cm^-3) and exhibit supercompression (undergoing extreme strain of 95%) and superelasticity (about 100% height retention after 500 cycles at a strain of 50%). Particularly, the carbon aerogel can selectively and quickly adsorb various oily contaminants, exhibiting high oil/organic solvents absorption capacity (reaches up to 276 g g^-1 for carbon tetrachloride) and good recyclability. Finally, practical applications of the carbon aerogel in oil-cleanup and pollution-remediation devices are exhibited. Hence, this versatile and robust functionalized carbon aerogel has promising potential in oil cleanup and pollution remediation.

Tracing Sweetgum Lignin's Molecular Properties through Biorefinery Processing

Changes to the molecular properties of lignin over the course of biorefinery processing were investigated by using sweetgum as a feedstock. Hydrothermal pretreatment has been used because it is an economically attractive, green process. Three representative biorefinery lignin preparations were obtained, with about 70 % yield based on raw lignin. The three fractions included soluble lignin adsorbed on resin (XADL), solvent-extracted lignin (HTCELp), and an additional ball-milled residual lignin (HTRELp). By comparing the raw and biorefinery lignin preparations, it can be concluded that lignin undergoes both degradation and condensation throughout the various stages of the hydrothermal-based biorefinery process. The two fractions made soluble by biorefinery processing, XADL and HTCELp, were found to be low-molecular-weight degradation products enriched with free phenolic hydroxyl groups. In addition, about 15 % of noncondensed phenolic units were involved in condensation reactions. Quantitative NMR spectroscopy analysis revealed that at least about 28 % of β-O-4' substructures were cleaved. Hibbert's ketones were identified in XADL and HTRELp, which provided evidence of lignin undergoing acidolysis. The contents of β-5' and β-β' did not change significantly upon biorefinery processing. Finally, episyringaresinol was detected in XADL and HTCELp. It is hoped that these findings will help to further demonstrate the specific effects of biorefinery processing on lignin in hardwood and facilitate its utilization to improve biorefinery economics.

Greener and sustainable production of bioethylene from bioethanol: current status, opportunities and perspectives

The economic value of bioethylene produced from bioethanol dehydration is remarkable due to its extensive usage in the petrochemical industry. Bioethylene is produced through several routes, such as steam cracking of hydrocarbons from fossil fuel and dehydration of bioethanol, which can be produced through fermentation processes using renewable substrates such as glucose and starch. The rise in oil prices, environmental issues due to toxic emissions caused by the combustion of fossil fuel and depletion of fossil fuel resources have led a demand for an alternative pathway to produce green ethylene. One of the abundant alternative renewable sources for bioethanol production is biomass. Bioethanol produced from biomass is alleged to be a competitive alternative to bioethylene production as it is environmentally friendly and economical. In recent years, many studies have investigated catalysts and new reaction engineering pathways to enhance the bioethylene yield and to lower reaction temperature to drive the technology toward economic feasibility and practicality. This paper critically reviews bioethylene production from bioethanol in the presence of different catalysts, reaction conditions and reactor technologies to achieve a higher yield and selectivity of ethylene. Techno-economic and environmental assessments are performed to further development and commercialization. Finally, key issues and perspectives that require utmost attention to facilitate global penetration of technology are highlighted.

Color evolution of poplar wood chips and its response to lignin and extractives changes in autohydrolysis pretreatment

Combining chemi-mechanical pulping with autohydrolysis pretreatment is an efficient and value-added utilization approach for lignocellulosic biomass in paper industry. To further promote the utilization of autohydrolyzed biomass in chemi-mechanical pulping, the color evolution of poplar wood chips in autohydrolysis pretreatment and its chromogenic mechanism were investigated by using CIELab color system, FT-IR, NMR and GPC. The results showed that the total color difference ΔE* increased obviously, which were remarkable as the combined hydrolysis factor (CHF) increased. The lignin content led to a more significant influence on the color of poplar wood chips than the extractives. The autohydrolysis pretreatment with a higher CHF accelerated the degradation and subsequent condensation of lignin, resulting in the formation of chromophoric groups, such as Hibbert ketone, quinones and quinone methides. It is of great significance for biomass refinery and paper industry to reveal the color evolution of poplar wood chips caused by autohydrolysis pretreatment from the point of view of chemical components' structure.

Coproduction of xylooligosaccharides and fermentable sugars from sugarcane bagasse by seawater hydrothermal pretreatment

In this study, natural seawater without additional chemicals was selected to treat sugarcane bagasse for the production of xylooligosaccharides and glucose. This pretreatment not only more effectively conserves freshwater resources than hydrothermal pretreatment and enzymatic hydrolysis, but also decreases corrosion of the equipment relative to techniques utilizing acid and alkaline pretreatment. The maximum yield of 67.12% xylooligosaccharides (of initial xylan), including 11.49% xylobiose, 16.23% xylotriose, 23.82% xylotetraose, and 15.58% xylopentaose was obtained under mild condition (175 °C for 30 min). Moreover, greater amounts of xylotetraose were generated during seawater hydrothermal pretreatment under all conditions, likely because NaCl in seawater cut the hydrogen bonds between xylo-oligomers. In addition, 94.69% cellulose digestibility and 78.58% xylan digestibility were achieved from the solid residue with an enzyme dosage of 30 FPU/g cellulose. Results indicated that seawater hydrothermal pretreatment is a more environmentally-friendly and sustainable technique for producing xylooligosaccharides and fermentable sugars than other methods.

Real-Time QCM-D Monitoring of the Adsorption-Desorption of Expansin on Lignin

Expansin has nonhydrolytic disruptive activity and synergistically acts with cellulases to enhance the hydrolysis of cellulose. The adsorption-desorption of expansin on noncellulosic lignin can greatly affect the action of expansin on lignocellulose. In this study, three lignins with different sources (kraft lignin (KL), sodium lignin sulfonate (SLS), and enzymatic hydrolysis lignin (EHL)) were selected as the substrates. The real-time adsorption-desorption of Bacillus subtilis expansin (BsEXLX1) on lignins was monitored using quartz crystal microgravimetry with dissipation (QCM-D). The effects of temperature and Tween 80 on the adsorption-desorption behaviors were also investigated. The results show that BsEXLX1 exhibited high binding ability on lignin and achieved maximum adsorption of 283.2, 273.8, and 266.9 ng cm^-2 at 25 °C on KL, SLS, and EHL, respectively. The maximum adsorption decreased to 148.2-192.8 ng cm^-2 when the temperature increased from 25 to 45 °C. Moreover, Tween 80 competitively bound to lignin and significantly prevented expansin adsorption. After irreversible adsorption of Tween 80, the maximum adsorption of BsEXLX1 greatly decreased to 33.3, 37.2, and 10.3 ng cm^-2 at 25 °C on KL, SLS, and EHL, respectively. Finally, a kinetic model was developed to analyze the adsorption-desorption process of BsEXLX1. BsEXLX1 has a higher adsorption rate constant (k_A) and a lower desorption rate constant (k_D) on KL than on SLS and EHL. The findings of this study provide useful insights into the adsorption-desorption of expansin on lignin.

Structural features, interaction with the gut microbiota and anti-tumor activity of oligosaccharides

Some oligosaccharides are regarded as biological constituents with benefits to human health in an indirect way. They enter the intestinal tract to be fermented by the gut microbiota, causing changes in the abundance and composition of the gut microbiota and producing fermentation products such as short-chain fatty acids (SCFAs). In this review, the structural features and biological activities of eight common natural oligosaccharides were summarized, including human milk oligosaccharides (HMOS), xylo-oligosaccharides (XOS), arabinoxylo-oligosaccharides (AXOS), isomaltooligosaccharides (IMOS), chitin oligosaccharides (NACOS), mannan-oligosaccharides (MOS), galacto-oligosaccharides (GOS) and fructo-oligosaccharides (FOS). Furthermore, XOS were selected to explain the anti-tumor mechanism mediated by gut microbiota. The review aims to reveal primary structural features of natural functional oligosaccharides related to the biological activities and also provide an explanation of the anti-tumor activity of functional oligosaccharides mediated by the gut microbiota.

Aldehydes-Aided Lignin-First Deconstruction Strategy for Facilitating Lignin Monomers and Fermentable Glucose Production from Poplar Wood

In this study, lignin with fine structures and facile enzymatic saccharifying residue were successively dissociated based on the lignin-first biomass deconstruction strategy. In the lignin-first process, aldehyde-protected lignin fractions were firstly isolated by acid-catalyzed dioxane extraction in the presence of formaldehyde (FA) and acetaldehyde (AA) and then analyzed by advanced nuclear magnetic resonance (NMR) spectroscopy and gel permeation chromatography (GPC). The optimized hydrogenolysis of the extracted lignin (LFA and LAA) resulted in a high yield (42.57% and 33.00%) of lignin monomers with high product selectivity (mainly 2,6-dimethoxy-4-propylphenol) (39.93% and 46.61%). Moreover, the cellulose-rich residues were saccharified into fermentable glucose for bioethanol production. The glucose yield of the substrate (RAA) reached to 75.12%, which was significantly higher than that (15.4%) of the substrate (RFA). In short, the lignin-first biomass deconstruction by adding AA is a promising and sustainable process for producing value-added products (energy and fine chemicals) from lignocellulosic biomass.

Effect of various pretreatments on improving cellulose enzymatic digestibility of tobacco stalk and the structural features of co-produced hemicelluloses

Hereon, tobacco stalk was deconstructed by lyophilization, ball-milling, ultrasound-assisted alkali extraction, hydrothermal pretreatment (HTP), and alkali presoaking, respectively, followed by dilute alkali cooking to both improve its enzymatic digestibility and isolate the hemicellulosic streams. It was found that a maximum cellulose saccharification rate of 93.5% was achieved from the integrated substrate by ball-milling and dilute alkali cooking, which was 4.4-fold higher than that from the raw material. Interestingly, in this case, 76.9% of hemicelluloses were simultaneously recovered during the integrated treatment. Structural determination indicated that the hemicelluloses released from tobacco stalk by dilute alkali cooking were mixed polysaccharides, and the (1 → 4)-linked β-D-Xylp backbone branched with L-Araf units at O-2/O-3 and 4-O-Me-α-D-GlcpA units at O-2 of the xylose residues was the main structure. In comparison, ultrasound-assisted alkali extraction, ball-milling, and HTP favored the extraction of hemicelluloses with less branched structure and lower molecular weights in the following alkali cooking.

Production of xylooligosaccharides and monosaccharides from hydrogen peroxide-acetic acid-pretreated poplar by two-step enzymatic hydrolysis

The severe pretreatment of poplar makes xylan difficult to utilize efficiently. In this work, poplar was pretreated by hydrogen peroxide-acetic acid (HPAC) with H₂SO₄ as catalyst to remove lignin, and the solid residues were used to produce xylooligosaccharides (XOS) and monosaccharides by two-step xylanase and cellulase hydrolysis. The results indicated that higher H₂SO₄ concentrations in the HPAC pretreatment of poplar afforded stronger lignin removal ability. An increased XOS yield of 19.8% was obtained from 200 mM H₂SO₄-catalyzed poplar by xylanase and the XOS purity was high, with a very low xylose/XOS ratio of 0.14. Higher glucose (75.2%) and xylose (61.4%) yields were obtained from the HPAC-pretreated poplar using 50 mM H₂SO₄ as catalyst. Finally, 16.9 g XOS and 296.4 g glucose were produced from 1 kg poplar by xylanase and cellulase. This study provides a method for producing functional XOS and monosaccharides from poplar using a simple reduced-pollution strategy.

Comparative study on different pretreatment on enzymatic hydrolysis of corncob residues

Under the situation of increasingly severe challenge of energy consumption, it is of great importance to make full use of bioresources such as forestry and agricultural residues. Herein, the corncob residues generated after processing corncob were enzymatically hydrolyzed to yield fermentable sugars. To overcome the recalcitrance of corncob residues, three kinds of pretreatment methods, i.e., sulfonation, PFI refining, and wet grinding, were applied; their effects on enzymatic hydrolysis and main characteristics of corncob residues substrate were investigated. The results showed that the enzymatic digestibility of the substrate was greatly enhanced by employing each method. The wet grinding exhibited obvious advantages, e.g., the conversion yield of cellulose to glucose and glucose concentration reached 96.7% and 32.2 g/L after 59 h of enzymatic hydrolysis, respectively. The improvement in enzymatic hydrolysis was mainly attributed to the altered characteristics of the substrate such as swelling ability, specific surface area, and particle size and distribution.

Improving enzymatic saccharification of hardwood through lignin modification by carbocation scavengers and the underlying mechanisms

In this work, the beneficial effect of carbocation scavenger additives on hardwood pretreatment was revealed by significantly improved biomass saccharification: cellulose hydrolysis yield was increased by over 15% after steam pretreatment of poplar, while that was enhanced by more than 48% after dilute acid pretreatment. Besides, the relative contributions of lignin towards enzyme binding and physical barrier effect for proposed mechanisms were quantified. Results indicated that the addition of carbocation scavenger, 2-naphthol-7-sulfonate, resulted in acid groups incorporation of 62.36 mmol/kg to lignin, which mitigated enzyme non-productive binding. Moreover, enlarged biomass porosity and reduced surface lignin coverage were detected through BET and XPS analysis, respectively, which mostly related to the diminished physical barrier effect of lignin. As a result, the lignin inhibitions were significantly suppressed through the addition of carbocation scavenger, giving rise to significantly improved enzymatic hydrolysis of hardwood.

Co-catalysis of magnesium chloride and ferrous chloride for xylo-oligosaccharides and glucose production from sugarcane bagasse

Inorganic salt treatment is a novel, high-yield, and environmentally friendly approach for the production of xylo-oligosaccharides from Sugarcane bagasse with degree of polymerization of 2-5. A xylo-oligosaccharides yield of 53.79% was obtained with 0.1 M MgCl₂ treatment at 180 °C/10 min, and 41.89% with 0.1 M FeCl₂ treatment at 140 °C/30 min. The xylo-oligosaccharides yield from the co-catalysis of 0.05 M FeCl₂ + 0.05 M MgCl₂ reached 54.68% (29.34% xylobiose and 20.94% xylotriose) at 140 °C/30 min. The co-catalysis not only effectively improved the xylobiose and xylotriose contents but also increased the total yield of xylo-oligosaccharides under mild reaction conditions. Additionally, the glucose yield observed from the solid residue after inorganic salt treatment was 71.62% by enzymatic hydrolysis. Mg²⁺ and Fe²⁺ are essential for good human health without separation from the system, therefore, the inorganic salt treatment can be potentially applied in the co-production of xylo-oligosaccharides and glucose.

A detailed overview of xylanases: an emerging biomolecule for current and future prospective

Xylan is the second most abundant naturally occurring renewable polysaccharide available on earth. It is a complex heteropolysaccharide consisting of different monosaccharides such as l-arabinose, d-galactose, d-mannoses and organic acids such as acetic acid, ferulic acid, glucuronic acid interwoven together with help of glycosidic and ester bonds. The breakdown of xylan is restricted due to its heterogeneous nature and it can be overcome by xylanases which are capable of cleaving the heterogeneous β-1,4-glycoside linkage. Xylanases are abundantly present in nature (e.g., molluscs, insects and microorganisms) and several microorganisms such as bacteria, fungi, yeast, and algae are used extensively for its production. Microbial xylanases show varying substrate specificities and biochemical properties which makes it suitable for various applications in industrial and biotechnological sectors. The suitability of xylanases for its application in food and feed, paper and pulp, textile, pharmaceuticals, and lignocellulosic biorefinery has led to an increase in demand of xylanases globally. The present review gives an insight of using microbial xylanases as an “Emerging Green Tool” along with its current status and future prospective.

Integrated process for the coproduction of fermentable sugars and lignin adsorbents from hardwood

This work proposed an integrated process based on alkali-sulfite (AlkSul) pretreatment to coproduce fermentable sugars and lignin adsorbents from hardwood. Different from conventional liquid hot water (LHW) pretreatment, this pretreatment improved cellulose accessibility through selective lignin removal and modification, resulting in significantly enhanced biomass saccharification. Over 75% of the original cellulose and hemicellulose was released and could be recovered as fermentable sugars after pretreatment and subsequent enzymatic hydrolysis. Meanwhile, lignin residues from pretreatment hydrolysate and enzymatic hydrolysate showed lead ions adsorption capacities of 156.25 and 68.49 mg/g, respectively, indicating both streams of lignin residues were favorable adsorbents for heavy metal ions. The improved adsorption capacity of lignin residues was primarily due to the lignin modification as sulfur-containing functional groups incorporation during the integrated pretreatment. Results demonstrated the integrated alkali-sulfite pretreatment improved biomass saccharification, while coproducing lignin adsorbents for wastewater treatment, which can promote the sustainability of lignocellulosic biorefinery.

An eco-friendly biorefinery strategy for xylooligosaccharides production from sugarcane bagasse using cellulosic derived gluconic acid as efficient catalyst

A novel approach was proposed for the production of xylooligosaccharides by direct pre-hydrolysis using gluconic acid as catalyst. Maximum xylooligosaccharides (degree of polymerization 2-6) yield of 53.2% could be obtained in 60 min through 5% gluconic acid hydrolysis of sugarcane bagasse at 150 °C. Furthermore, the yield of glucose from solids following gluconic acid hydrolysis treatment was 86.2% after fed-batch enzymatic hydrolysis with 10% solids loading. Results indicated that gluconic acid pretreatment combined with enzymatic hydrolysis could be successfully applied to sugarcane bagasse substrate. Subsequently, glucose could be efficiently bio-oxidized to gluconic acid by Gluconobacter oxydans ATCC 621H with 93.1% yield, and sugarcane bagasse derived gluconic acid has been proved to be an effective catalyst for xylooligosaccharides production. In this study, xylooligosaccharides production from sugarcane bagasse by gluconic acid hydrolysis demonstrated a great potential with respect to the production of these probiotics around the world.

The effects of exogenous ash on the autohydrolysis and enzymatic hydrolysis of wheat straw

The effects of exogenous ash (EA) from harvest wheat straw and its internal components on wheat straw autohydrolysis efficiency and subsequent enzymatic hydrolysis were investigated. Results showed that when EA and its insoluble mineral components were included in the autohydrolysis, the enzymatic efficiencies of pretreated residues were significantly reduced from 84.9% to 66.3% and 58.4%, respectively. This was found to be largely attributable to the buffering of free H⁺ in the pretreatment medium which took place due to the ash. Specifically, the insoluble mineral fraction of said ash exerted strongest buffering capacity in EA. Furthermore, this decrease was found to linearly correlate with decreases to substrate enzymatic accessibility and hydrophobicity. These results demonstrate that the penalties of ash upon autohydrolysis are borne of specific fractions comprising the ash, making the case for ash removal processes or supplementation of processes with additives that will counter the negative effects of ash.

A sustainable process for procuring biologically active fractions of high-purity xylooligosaccharides and water-soluble lignin from Moso bamboo prehydrolyzate

BACKGROUND

Prehydrolyzate, which is from the prehydrolysis process in dissolving pulps industry, contains various sugar-derived and lignin compounds such as xylooligosaccharides (XOS), gluco-oligosaccharides, xylose, glucose, and soluble lignin (S-L). The XOS has several beneficial effects on human physiology. XOS and S-L in prehydrolyzate are difficult to efficiently fractionate due to their similar molecular weights and water solubility. In this work, we proposed a sustainable and green process using polystyrene divinylbenzene (PS-DVB) resin to simultaneously separate and recover XOS and S-L. Enzymatic hydrolysis with endo-1,4-β-xylanase and fermentation with P. stipites were sequentially applied to purify XOS to minimize xylose content as well as amplify contents of xylobiose and xylotriose. In addition, 2D-HSQC NMR was used to analyze the structural characteristics of XOS and S-L. Furthermore, the biological abilities of antioxidants and prebiotics of these fractions were investigated by scavenging radicals and cultivating intestinally beneficial bacterias, respectively.

RESULTS

Results showed that PS-DVB resin could simultaneously separate XOS and solubilized lignin with excellent yields of 93.2% and 85.3%, respectively. The obtained XOS after being purified by enzymatic hydrolysis and fermentation contained 57.7% of xylobiose and xylotriose. 10.4% amount of inherent xylan was found in the S-L fraction obtained by PS-DVB resin separation. 2D-HSQC NMR revealed that lignin carbohydrate complexes existed in both XOS and S-L as covalent linkages between lignin and 4-O-methylglucuronoarabinoxylan. The biological application results showed that the antioxidant capacity of S-L was stronger than XOS, while XOS was superior in promoting growth of intestinal Bifidobacteria adolescentis and stimulating production of short-chain fatty acids by Lactobacillus acidophilus.

CONCLUSIONS

The proposed strategy of sequentially combining hydrophobic resin separation, enzymatic hydrolysis, and fermentation was successfully demonstrated and resulted in simultaneous production of high-quality XOS and solubilized lignin. These biomass-derived products in prehydrolyzate can be regarded as value-adding prebiotics and antioxidants.

Integrative process for sugarcane bagasse biorefinery to co-produce xylooligosaccharides and gluconic acid

An integrated and green process for co-producing xylooligosaccharides (XOS) and gluconic acid (GA), was developed by utilizing sugarcane bagasse as starting material. In this study, the highest XOS yield of 39.1% obtained from the prehydrolysis was achieved with 10% acetic acid at 150 °C for 45 min. Subsequently, 88.6% conversion of cellulose was achieved in a fed-batch enzymatic hydrolysis using a solid loading of 15%. Results of glucose fermentation suggested that inherent regulatory system of strain Gluconobacter oxydans ATCC 621H boosted GA accumulation without the requirement of pH control, leading to a good 96.3% of GA yield. Great performance of this strain offer an economically feasible option for the large-scale sustainable GA production from biomass. Overall, approximately 105 g XOS and 340 g GA were co-produced from 1 kg of dried sugarcane bagasse as feedstock; this integrated process might be a cost-effective option for the comprehensive utilization of sugarcane bagasse.

Synthesis of Carbon Quantum Dot Nanoparticles Derived from Byproducts in Bio-Refinery Process for Cell Imaging and In Vivo Bioimaging

The carbon quantum dot (CQD), a fluorescent carbon nanoparticle, has attracted considerable interest due to its photoluminescent property and promising applications in cell imaging and bioimaging. In this work, biocompatible, photostable, and sustainably sourced CQDs were synthesized from byproducts derived from a biorefinery process using one-pot hydrothermal treatment. The main components of byproducts were the degradation products (autohydrolyzate) of biomass pretreated by autohydrolysis. The as-synthesized CQDs had a size distribution from 2.0⁻6.0 nm and had high percentage of sp² and sp³ carbon groups. The CQDs showed blue-green fluorescence with a quantum yield of ~13%, and the fluorescence behaviors were found to be stable with strong resistance to photobleaching and temperature change. In addition, it is found that the as-synthesized CQDs could be used for imaging of cells and tumors, which show potential applications in bioimaging and related fields such as phototherapy and imaging.

Stepwise enzymatic hydrolysis of alkaline oxidation treated sugarcane bagasse for the co-production of functional xylo-oligosaccharides and fermentable sugars

High chemical input is required for enzymatic production of xylo-oligosaccharides (XOS) using xylan extracted from lignocellulosic biomass. In this study, enzymatic hydrolysis of alkaline oxidation (AO) treated sugarcane bagasse (SCB) directly for the production of XOS was conducted. The effect of AO pretreatment on the chemical compositions and hydrolytic properties of SCB was investigated. The AO pretreatment conditions with low chemical input for the production of XOS were optimized by orthogonal design. Stepwise enzymatic hydrolysis of AO pretreated SCB with xylanase and cellulase produced XOS (1.78 g/L), meanwhile, the cellulose conversion increased from 84.97% to 91.51% compared with directly enzymatic hydrolysis using cellulase. HPLC-UV and MALDI-TOF-MS analysis indicated that the obtained XOS products were mainly composed of xylobiose and xylose with a small amount of arabinose/4-O-methylglucuronic acid substituted xylotriose and xylotetraose. The proposed strategy for the co-production of functional XOS and fermentable sugars from SCB showed potential of industrial application.

Hydrophobic resin treatment of hydrothermal autohydrolysate for prebiotic applications

∼30% of xylooligosaccharides (XOS) in autohydrolysate are likely bonded to lignin “tied,” contributing to loss during resin purification. Loss of “free” XOS depends on DP.

Production of xylooligosaccharides and monosaccharides from poplar by a two-step acetic acid and peroxide/acetic acid pretreatment

BACKGROUND

Populus (poplar) tree species including hybrid varieties are considered as promising biomass feedstock for biofuels and biochemicals production due to their fast growing, short vegetative cycle, and widely distribution. In this work, poplar was pretreated with acetic acid (AC) to produce xylooligosaccharides (XOS), and hydrogen peroxide-acetic acid (HPAC) was used to remove residual lignin in AC-pretreated poplar for enzymatic hydrolysis. The aim of this work is to produce XOS and monosaccharides from poplar by a two-step pretreatment method.

RESULTS

The optimal conditions for the AC pretreatment were 170 °C, 5% AC, and 30 min, giving a XOS yield of 55.8%. The optimal HPAC pretreatment conditions were 60 °C, 2 h, and 80% HPAC, resulting in 92.7% delignification and 87.8% cellulose retention in the AC-pretreated poplar. The two step-treated poplar presented 86.6% glucose yield and 89.0% xylose yield by enzymatic hydrolysis with a cellulases loading of 7.2 m/g dry mass. Very high glucose (93.8%) and xylose (94.6%) yields were obtained with 14.3 mg cellulases/g dry mass. Both Tween 80 and β-glucosidase enhanced glucose yield of HPAC-pretreated poplar by alleviating the accumulation of cellobiose. Under the optimal conditions, 6.9 g XOS, 40.3 g glucose, and 8.9 g xylose were produced from 100 g poplar.

CONCLUSIONS

The AC and HPAC pretreatment of poplar represented an efficient strategy to produce XOS and fermentable sugars with high yields. This two-step pretreatment was a recyclable benign and advantageous scheme for biorefinery of the poplar into XOS and monosaccharides.

Reactivity improvement by phenolation of wheat straw lignin isolated from a biorefinery process

This work describes an effective phenolation process to improve wheat straw biorefinery lignin reactivity.

Simultaneous consumption of cellobiose and xylose by Bacillus coagulans to circumvent glucose repression and identification of its cellobiose-assimilating operons

BACKGROUND

The use of inedible lignocellulosic biomasses for biomanufacturing provides important environmental and economic benefits for society. Efficient co-utilization of lignocellulosic biomass-derived sugars, primarily glucose and xylose, is critical for the viability of lignocellulosic biorefineries. However, the phenomenon of glucose repression prevents co-utilization of both glucose and xylose in cellulosic hydrolysates.

RESULTS

To circumvent glucose repression, co-utilization of cellobiose and xylose by Bacillus coagulans NL01 was investigated. During co-fermentation of cellobiose and xylose, B. coagulans NL01 simultaneously consumed the sugar mixtures and exhibited an improved lactic acid yield compared with co-fermentation of glucose and xylose. Moreover, the cellobiose metabolism of B. coagulans NL01 was investigated for the first time. Based on comparative genomic analysis, two gene clusters that encode two different operons of the cellobiose-specific phosphoenolpyruvate-dependent phosphotransferase system (assigned as CELO1 and CELO2) were identified. For CELO1, five genes were arranged as celA (encoding EIIA^cel), celB (encoding EIIB^cel), celC (encoding EIIC^cel), pbgl (encoding 6-phospho-β-glucosidase), and celR (encoding a transcriptional regulator), and these genes were found to be ubiquitous in different B. coagulans strains. Based on gene knockout results, CELO1 was confirmed to be responsible for the transport and assimilation of cellobiose. For CELO2, the five genes were arranged as celR, celB, celA, celX (encoding DUF871 domain-containing protein), and celC, and these genes were only found in some B. coagulans strains. However, through a comparison of cellobiose fermentation by NL01 and DSM1 that only possess CELO1, it was observed that CELO2 might also play an important role in the utilization of cellobiose in vivo despite the fact that no pbgl gene was found. When CELO1 or CELO2 was expressed in Escherichia coli, the recombinant strain exhibited distinct cellobiose uptake and consumption.

CONCLUSIONS

This study demonstrated the cellobiose-assimilating pathway of B. coagulans and provided a new co-utilization strategy of cellobiose and xylose to overcome the obstacles that result from glucose repression in a biorefinery system.

Pretreatment with lower feed moisture and lower extrusion temperatures aids in the increase in the fermentable sugar yields from fine-milled Douglas-fir

The impact of independent variables of extrusion on dependent variables of pre-milled Douglas-fir forest residuals was studied to enhance the enzymatic hydrolysis for production of fermentable sugar without catalysts. Co-rotating twin screw extruder was operated with three different feedstock moisture contents (30, 40, and 50%) at four different barrel temperatures (25, 50, 100, and 150 °C) as a pretreatment. The specific mechanical energy input ranged from 0.07 and 0.30 kWh/kg and had a very strong positive correlation with torque (r = 0.96, p < 0.01), glucose (r = 0.92, p < 0.01) and xylose/mannose yields with (r = 0.84, p < 0.01). Douglas-fir residuals extruded at lowest moisture content (30%) and temperature (25 °C) had the highest sugar yield, requiring the highest SME. Higher barrel temperature increased the median particle size and had lower glucose and xylose/mannose yields. Recrystallization and agglomeration were observed under higher temperature conditions.

Use of metal chlorides during waste wheat straw autohydrolysis to overcome the self-buffering effect

High ash content of waste wheat straw (WWS) is resistant to biorefinery autohydrolysis pretreatment due to its self-buffering effects. In this work, minor addition FeCl₃ and AlCl₃ were applied to overcome the self-buffering effects of WWS by cationic occupation of the negatively charged sites present on particulate ash's surface. The results showed that with the increasing concentrations (0-20 mM) of AlCl₃ and FeCl₃, the enzymatic efficiencies of autohydrolyzed WWS were enhanced from 49.7% to 62.1% and 66.6%, respectively. Acid buffer and cation exchange capacity of pretreated WWS were decreased by adding metal chlorides and the reducing results were mainly attributed to cation exchange. Meanwhile, a maximum monosaccharide production (185.3 mg/g-WWS) was achieved with 62.0 mg/g-WWS xylooligosaccharide by using 20 mM FeCl₃ during WWS autohydrolysis. The results demonstrated that the implications of FeCl₃ and AlCl₃ in WWS autohydrolysis were an effective strategy to enhance autohydrolysis efficiency by overcoming self-buffering effects.

Influence of sulfur dioxide-ethanol-water pretreatment on the physicochemical properties and enzymatic digestibility of bamboo residues

SO₂-ethanol-water (SEW) is a promising pretreatment for improving enzymatic digestibility of biomass through simultaneously removing hemicellulose and lignin. In this work, SEW pretreatment was performed at different cooking times (10 min-60 min) and different SO₂ concentrations (0.5%-2%) to produce pretreated bamboo residues for enzymatic hydrolysis. Meanwhile, physicochemical features of the residual cellulose and lignin were analyzed to better understand how SEW improves enzymatic digestibility. Under optimized SEW pretreatment condition (1% SO₂ concentration, 150 °C, 60 min), 81.7% of xylan and 80.3% of lignin were solubilized, along with 89.1% of cellulose preserved in pretreated solid. A good enzymatic digestibility (80.4%) was achieved at optimum SEW condition. Several compelling correlations (R² > 0.7) were observable between enzymatic digestibility and physicochemical features, demonstrating the importance of SEW pretreatment abilities of hemicellulose and lignin removal, reducing cellulose's degree of polymerization, and improving the amount of sulfonyl groups imparted to the original lignin structure.

Improving enzymatic hydrolysis efficiency of wheat straw through sequential autohydrolysis and alkaline post-extraction

In this work, a two-step pretreatment process of wheat straw was established by combining autohydrolysis pretreatment and alkaline post-extraction. The results showed that employing alkaline post-extraction to autohydrolyzed wheat straw could significantly improve its enzymatic hydrolysis efficiency from 36.0% to 83.7%. Alkaline post-extraction lead to the changes of the structure characteristics of autohydrolyzed wheat straw. Associations between enzymatic hydrolysis efficiency and structure characteristics were also studied. The results showed that the factors of structure characteristics such as delignification, xylan removal yield, crystallinity, accessibility and hydrophobicity are positively related to enzymatic hydrolysis efficiency within a certain range for alkaline post-extracted wheat straw. The results demonstrated that autohydrolysis coupled with alkaline post-extraction is an effective and promising method to gain fermentable sugars from biomass.

Furfural production from biomass pretreatment hydrolysate using vapor-releasing reactor system

Biomass hydrolysate from autohydrolysis pretreatment was used for furfural production considering it is in rich of xylose, xylo-oligomers, and other decomposition products from hemicellulose structure. By using the vapor-releasing reactor system, furfural was protected from degradation by separating it from the reaction media. The maximum furfural yield of 73% was achieved at 200 °C for biomass hydrolysate without the use of the catalyst. This is because the presence of organic acids such as acetic acid in hydrolysate functioned as a catalyst. According to the results in this study, biomass hydrolysate with a vapor-releasing system proves to be efficient for furfural production. The biorefinery process which allows the separation of xylose-rich autohydrolysate from other parts from biomass feedstock also improves the overall application of the biomass.

Simultaneously separation of xylo-oligosaccharide and lignosulfonate from wheat straw magnesium bisulfite pretreatment spent liquor using ion exchange resin

For wheat straw, an ideal bio-refinery process is that all three major components of biomass could be efficiently utilized to make high value chemicals, MBSP could directly convert the hemicelluloses and lignin into xylo-oligosaccharides and lignosulfonate. However, these value-added compounds still present in spent liquor and thus should be isolated as an individual product. In present work, a simple and efficient ion exchange process was developed for separating xylo-oligosaccharides and lignosulfonate simultaneously from spent liquor. D354 resin was selected for its high adsorption capacity of magnesium lignosulfonate and remarkable selectivity. 93.09% of XOS and 98.03% of lignosulfonate were recovered from the treated spent liquor in a fixed bed column with D354 resin. Overall, 1 L of MBSP spent liquor could coproduce 9.5 g XOS and 74 g lignosulfonate. These results offer an opportunity for complete and effective utilization of biomass by a novel integrated process coupling of MBSP and ion-exchange process.

Xylooligosaccharides production by crude microbial enzymes from agricultural waste without prior treatment and their potential application as nutraceuticals

Aspergillus fumigatus R1, on submerged fermentation using agricultural residues as carbon source produced extracellular xylanase (152IU/ml after 96h of incubation at 37°C with constant shaking at 100rpm). A maximum yield of 1gm% Xylooligosaccharides (XOS) mixture was obtained after 12h by enzymatic hydrolysis of xylan rich wheat husk without any prior pretreatment using the crude enzyme without any purification. HP-TLC data confirmed the presence of an array of XOS for its prebiotic properties by carrying out studies on ten standard probiotic cultures. Six of ten probiotic cultures were able to utilize XOS produced from agricultural wastes and showed remarkable growth in the media containing XOS as the sole source of carbon. XOS mixture also exhibited concentration dependent anti-oxidant activity. Thus, the results showed that XOS produced from agricultural residues have great prebiotic potential and good antioxidant activity; therefore, it can be used in food-related applications.

Lignocentric analysis of a carbohydrate-producing lignocellulosic biorefinery process

A biologically-based lignocellulosic biorefinery process for obtaining carbohydrates from raw biomass was investigated across six diverse biomasses (three hardwoods & three nonwoods) for the purpose of decoding lignin's influence on sugar production. Acknowledging that lignin could positively alter the economics of an entire process if valorized appropriately, we sought to correlate the chemical properties of lignin within the process to the traditional metrics associated with carbohydrate production-cellulolytic digestibility and total sugar recovery. Based on raw carbohydrate, enzymatic recovery ranged from 40 to 64% w/w and total recovery ranged from 70 to 87% w/w. Using nitrobenzene oxidation to quantify non-condensed lignin structures, it was found that raw hardwoods bearing increasing non-condensed S/V ratios (2.5-5.1) render increasing total carbohydrate recovery from hardwood biomasses. This finding indicates that the chemical structure of hardwood lignin influences the investigated biorefinery process' ability to generate carbohydrates from a given raw hardwood feedstock.

Co-production of functional xylooligosaccharides and fermentable sugars from corncob with effective acetic acid prehydrolysis

A novel and green approach for the coproduction of xylooligosaccharides (XOS), in terms of a series of oligosaccharide components from xylobiose to xylohexose, and fermentable sugars was developed using the prehydrolysis of acetic acid that was fully recyclable and environmentally friendly, followed by enzymatic hydrolysis. Compared to hydrochloric acid and sulfuric acid, acetic acid hydrolysis provided the highest XOS yield of 45.91% and the highest enzymatic hydrolysis yield. More than 91% conversion of cellulose was achieved in a batch-hydrolysis using only a cellulase loading of 20FPU/g cellulose and even a high solid loading of 20% without any special strategies. The acetic acid pretreated corncob should be washed adequately before saccharification to achieve complete hydrolysis. Consequently, a mass balance analysis showed that 139.8g XOS, 328.1g glucose, 25.1g cellobiose, and 147.8g xylose were produced from 1000g oven dried raw corncob.