Unlocking the secret of lignin-enzyme interactions: Recent advances in developing state-of-the-art analytical techniques

How Resilient is Wood Xylan to Enzymatic Degradation in a Matrix with Kraft Lignin?

Despite the potential of lignocellulose in manufacturing value-added chemicals and biofuels, its efficient biotechnological conversion by enzymatic hydrolysis still poses major challenges. The complex interplay between xylan, cellulose, and lignin in fibrous materials makes it difficult to assess underlying physico- and biochemical mechanisms. Here, we reduce the complexity of the system by creating matrices of cellulose, xylan, and lignin, which consists of a cellulose base layer and xylan/lignin domains. We follow enzymatic degradation using an endoxylanase by high-speed atomic force microscopy and surface plasmon resonance spectroscopy to obtain morphological and kinetic data. Fastest reaction kinetics were observed at low lignin contents, which were related to the different swelling capacities of xylan. We demonstrate that the complex processes taking place at the interfaces of lignin and xylan in the presence of enzymes can be monitored in real time, providing a future platform for observing phenomena relevant to fiber-based systems.

Exploring NAD+ metabolism and NNAT: Insights from structure, function, and computational modeling

Nicotinamide Adenine Dinucleotide (NAD+), a coenzyme, is ubiquitously distributed and serves crucial functions in diverse biological processes, encompassing redox reactions, energy metabolism, and cellular signalling. This review article explores the intricate realm of NAD + metabolism, with a particular emphasis on the complex relationship between its structure, function, and the pivotal enzyme, Nicotinate Nucleotide Adenylyltransferase (NNAT), also known as nicotinate mononucleotide adenylyltransferase (NaMNAT), in the process of its biosynthesis. Our findings indicate that NAD + biosynthesis in humans and bacteria occurs via the same de novo synthesis route and the pyridine ring salvage pathway. Maintaining NAD homeostasis in bacteria is imperative, as most bacterial species cannot get NAD+ from their surroundings. However, due to lower sequence identity and structurally distant relationship of bacteria, including E. faecium and K. pneumonia, to its human counterpart, inhibiting NNAT, an indispensable enzyme implicated in NAD + biosynthesis, is a viable alternative in curtailing infections orchestrated by E. faecium and K. pneumonia. By merging empirical and computational discoveries and connecting the intricate NAD + metabolism network with NNAT's crucial role, it becomes clear that the synergistic effect of these insights may lead to a more profound understanding of the coenzyme's function and its potential applications in the fields of therapeutics and biotechnology.

Role of lignin in synergistic digestibility improvement of wheat straw by novel alkaline deep eutectic solvent and tetrahydrofuran pretreatment

A novel efficient pretreatment system containing alkaline deep eutectic solvent (DES) and tetrahydrofuran (THF) was developed in the present study. Under pretreatment conditions of 160 ℃ and 1 h, DES-THF pretreatment was more efficient (81.61%) in cellulose digestibility improvement than DES (choline chloride/monoethanolamine, 67.54%). To further explore lignin structural transformation and lignin-cellulase interaction after pretreatment, milled wood lignin (MWL) was extracted and characterized. Compared with DES-MWL, DES-THF-MWL showed an increased carboxyl group content (24.0%) and decreased condensed phenolic hydroxyl content (9.1%). In DES-MWL, β-O-4 content was 21.79%, while in DES-THF-MWL, β-O-4 accounted for 45.45%, indicating that the addition of THF alleviated cleavage of β-O-4 alkyl ether bonds. Fluorescence emission spectroscopy results showed that quenching mechanism of DES-THF-MWL and cellulase was dynamic, which was different from other lignin. Compared with DES-MWL, decreased Ka between DES-THF-MWL and cellulase indicated decreasing interaction between them. DES-THF pretreatment provides a novel pretreatment method for bioenergy.

Construction of catalase@hollow silica nanosphere: Catalase with immobilized but not rigid state for improving catalytic performances

Enzyme immobilization usually make use of nanomaterials to hold up biocatalysis stability in various unamiable reaction conditions, but also lead large discount on enzyme activity. Thus, there are abundant researches focus on how to deal with the relation of enzyme molecules and supports. In this work, a new state of highly active enzymes has been established through facile and novel in situ immobilization and soft template removal method to construct enzyme contained hollow silica nanosphere (catalase@HSN) biocatalysts where enzymes in the cavity exhibit "immobilized but not rigid state". The obtained catalase@HSN was characterized by transmission electron microscopy, scanning electron microscopy and confocal laser scanning microscopy et al. Catalase@HSN exhibits excellent activity (about 80 % activity recovery rate) and stability suffers from extreme pH, temperature, and organic solvents. Moreover, the reusability and storage stability of catalase@HSN also are satisfactory. This proposed strategy provides a facile method for preparing biocatalysts under mild conditions, facilitating the applications of immobilized enzyme in the fields of real biocatalytic industry with high apparent activity and passable stability.

Molecular insights into inhibiting effects of lignin on cellulase investigated by molecular dynamics simulation

The hydrolysis of lignocellulose into fermentable monosaccharides using cellulases represents a critical stage in lignocellulosic bioconversion. However, the inactivation of cellulase in the presence of lignin is attributed to the high cost of biofinery. To address this challenge, a comprehensive investigation into the structure-function relationship underlying lignin-driven cellulase inactivation is essential. In this study, molecular docking and molecular dynamics (MD) simulations were employed to explore the impacts of lignin fragments on the catalytic efficiency of cellulase at the atomic level. The findings revealed that soluble lignin fragments and cellulose could spontaneously form stable complexes with cellulase, indicating a competitive binding scenario. The enzyme's structure remained unchanged upon binding to lignin. Furthermore, specific amino acid residues have been identified as involved in interactions with lignin and cellulose. Hydrophobic interactions were found to dominate the binding of lignin to cellulase. Based on the mechanisms underlying the interactions between lignin fragments and cellulase, decreased hydrophobicity and change in the charge of lignin may mitigate the inhibition of cellulase. Furthermore, site mutations and chemical modification are also feasible to improve the efficiency of cellulase. This study may contribute valuable insights into the design of more lignin-resistant enzymes and the optimization of lignocellulosic pretreatment technologies.Communicated by Ramaswamy H. Sarma.

Addressing two major limitations in high-solids enzymatic hydrolysis by an ordered polyethylene glycol pre-incubated strategy: Rheological properties and lignin adsorption for enzyme

High-solids enzymatic hydrolysis for biomass has currently received considerable interest. However, the solid effect during the process limits its economic feasibility. This work presented an ordered polyethylene glycol (PEG) pre-incubated strategy for enhancing the auxiliary effect of PEG in a high-solids enzymatic hydrolysis system. The substrate and enzyme were separately pre-incubated with PEG in this strategy. The ordered PEG pre-incubated strategies yielded a maximum glucose concentration of 166.6 g/L from 32 % (w/v) pretreated corncob with an enzymatic yield of 94.1 % by 72 h hydrolysis. Using this method, PEG not only lessened the lignin adsorption to cellulase but also altered particle rheological characteristics in the high-solids enzymatic hydrolysis system as a viscosity modifier. This study offered a new insight into the mechanism behind the PEG synergistic effect and would make it possible to achieve efficient high-solids loading hydrolysis in the commercial manufacture of cellulosic ethanol.

Improving Enzymatic Saccharification of Peach Palm (Bactris gasipaes) Wastes via Biological Pretreatment with Pleurotus ostreatus

The white-rot fungus Pleurotus ostreatus was used for biological pretreatment of peach palm (Bactris gasipaes) lignocellulosic wastes. Non-treated and treated B. gasipaes inner sheaths and peel were submitted to hydrolysis using a commercial cellulase preparation from T. reesei. The amounts of total reducing sugars and glucose obtained from the 30 d-pretreated inner sheaths were seven and five times higher, respectively, than those obtained from the inner sheaths without pretreatment. No such improvement was found, however, in the pretreated B. gasipaes peels. Scanning electronic microscopy of the lignocellulosic fibers was performed to verify the structural changes caused by the biological pretreatments. Upon the biological pretreatment, the lignocellulosic structures of the inner sheaths were substantially modified, making them less ordered. The main features of the modifications were the detachment of the fibers, cell wall collapse and, in several cases, the formation of pores in the cell wall surfaces. The peel lignocellulosic fibers showed more ordered fibrils and no modification was observed after pre-treatment. In conclusion, a seven-fold increase in the enzymatic saccharification of the Bactris gasipaes inner sheath was observed after pre-treatment, while no improvement in enzymatic saccharification was observed in the B. gasipaes peel.

Unveiling the role of long-range and short-range forces in the non-productive adsorption between lignin and cellulases at different temperatures

Quantitatively understanding of interaction mechanism between lignin and cellulases is essential for the efficient improvement of lignocellulose enzymatic hydrolysis. However, the individual contribution of multiple forces between lignin and cellulases to the non-productive adsorption of enzymes still remains deeply ambiguous, especially in situations of near enzymatic hydrolysis temperatures. Herein, atomic force microscopy (AFM) and computational simulations were utilized to quantitatively analyze the intermolecular forces between lignin and enzyme at 25 °C and 40 °C. Our results unveiled that an increase in temperature obviously improved adsorption capacity and total intermolecular forces between lignin and cellulases. This positive relationship mainly comes from the increase in the decay length of hydrophobic forces for lignin-cellulases when temperature increases. Different from the hydrophobic interaction which provides long-range part of attractions, van der Waals forces dominate the intermolecular force only at approaches < 2 nm. On the other hand, electrostatic forces exhibited repulsive effects, and its intensity and distance were limited due to the low surface potential of cellulases. Short-range forces including hydrogen bonding (main) and π-π stacking (minor) stabilize the non-specific binding of enzymes to lignin, but increasing temperature reduces hydrogen bond number. Therefore, the relative contribution of long-range forces increased markedly at higher temperatures, which benefits protein capture and brings lignin and cellulase close together. Finally, the structure-activity relationships between lignin physicochemical properties and its inhibitory effect to enzymes indicated that hydrophobic interactions, hydrogen bonding, and steric effects drive the final adsorption capacity and glucose yields. This work provides quantitative and basic insights into the mechanism of lignin-cellulase interfacial interactions and guides design of saccharification enhancement approaches.

Enhancing oxidation ability of graphitic carbon nitride photocatalysts to promote lignin Cα-Cβ bond cleavage in micellar aqueous media

As the most abundant aromatic resource, lignin is an appreciated biomass material to obtain aromatic high-value chemicals. However, the selective cleavage of lignin C_α-C_β bonds under mild conditions constitutes a challenge. Herein, a photocatalyst having high oxidation capacity was successfully synthesized by codoping S and Cl atoms into graphite carbon nitride (g-C₃N₄). The resulting S,Cl/CN-1.5 photocatalyst exhibits enhanced photogenerated electron-hole separation ability and higher valence band potential than g-C₃N₄. S,Cl/CN-1.5 can efficiently break lignin C_α-C_β bonds in micellar aqueous medium to produce benzaldehyde and benzyl alcohol as the main products. Mechanism studies show that the photocatalytic cleavage of lignin C_α-C_β bonds proceeds via single-electron transfer and C_β radical mechanisms in which hydroxyl radicals and photogenerated holes play an important role. Isotopic experiments show that the O atoms required for the photocatalytic cleavage of lignin C_α-C_β bonds come from water in the micellar aqueous medium based on the full contact between water and substrate. Although O₂ atmosphere is beneficial for the photocatalytic efficiency, O₂ is not necessary for the photocatalytic cleavage of lignin C_α-C_β bonds. This research provides a useful guide for designing efficient photocatalysts to depolymerize lignin into high-value chemicals.

In-situ lignin sulfonation for enhancing enzymatic hydrolysis of poplar using mild organic solvent pretreatment

Biomass pretreatment is an essential strategy to overcome biomass recalcitrance and promote lignocellulosic bioconversion. Here, a reusable organic solvent system (formic acid-methanesulfonic acid) was explored to pretreat poplar under a mild temperature (below 100 °C). The results showed that the co-solvent system could extract basically complete hemicelluloses and part of lignin with original cellulose retained in the pretreated substrates. Meanwhile, sulfonic acid groups were introduced into lignin structure remained in the substrates. The glucose conversion yield of the substrates with a higher concentration of sulfonic acid groups (13.2 mmol/kg) reached 45.9 % by reducing the hydrophobic interaction between lignin and cellulase, showing 89.3 % improvement compared with that of the substrates treated with single formic acid. This progressive study aimed to develop a new strategy to realize sulfonation and promote enzymatic hydrolysis of substrates by using mild organic solvent pretreatment.

Enzymes and enzymatic mechanisms in enzymatic degradation of lignocellulosic biomass: A mini-review

Enzymatic hydrolysis is the key step limiting the efficiency of the biorefinery of lignocellulosic biomass. Enzymes involved in enzymatic hydrolysis and their interactions with biomass should be comprehended to form the basis for looking for strategies to improve process efficiency. This article updates the contemporary research on the properties of key enzymes in the lignocellulose biorefinery and their interactions with biomass, adsorption, and hydrolysis. The advanced analytical techniques to track the interactions for exploiting mechanisms are discussed. The challenges and prospects for future research are outlined.

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.

Revealing the mechanism of surfactant-promoted enzymatic hydrolysis of dilute acid pretreated bamboo

To improve the enzymatic digestibility of dilute acid pretreated bamboo residue (DABR), surfactants including PEG 4000 and Tween 80 were added to prevent the non-productive adsorption between residual lignin and enzyme. At the optimal loadings (e.g., 0.2 and 0.3 g surfactant/g lignin), the enzymatic digestibility of DABR improved from 29.4% to 64.6% and 61.6% for PEG 4000 and Tween 80, respectively. Furthermore, the promoting mechanism of these surfactants on enzymatic hydrolysis was investigated by real-time surface plasmon resonance (SPR) and fluorescence spectroscopy. Results from SPR analysis showed that Tween 80 outperformed PEG 4000 in terms of dissociating the irreversible cellulase adsorption onto lignin. Fluorescence quenching mechanism revealed that PEG 4000 and Tween 80 intervened the interaction between lignin and cellulase by hydrogen bonds/Van der Waals and hydrophobic action, respectively. This work provided an in-depth understanding of the mechanisms of PEG 4000 and Tween 80 on enhancing the enzymatic hydrolysis efficiency.

Lignin fractionation to realize the comprehensive elucidation of structure-inhibition relationship of lignins in enzymatic hydrolysis

A better understanding of the relationship between lignin structures and their inhibitory effects in enzymatic saccharification would facilitate the development of lignocellulose biorefinery process. However, the heterogeneity of lignins challenges the elucidation of lignin structure-inhibition correlation. In this study, two types of lignin fractions including ethanol soluble lignins and ethanol insoluble lignins were respectively isolated from the poplars pretreated with various severities. The impacts of pretreatment severities on the structural changes of lignin fractions were studied from the perspective of inter-units linkages, condensed aromatic substructure, and hydroxyl groups. Furthermore, it was observed that lignin addition strongly inhibited the enzymatic saccharification of pure cellulose by 13.3 ∼ 56.3%. Lignin inhibition extents were increased with the elevated pretreatment severity. The relationships between the lignin structural features and lignin inhibition were analyzed, which revealed that the contents of condensed aromatic units and phenolic hydroxyl were crucial factors determining the lignin inhibition.

Efficient fractionation of bamboo residue by autohydrolysis and deep eutectic solvents pretreatment

Bamboo processing residue, which is rich in parenchyma cells, was treated as huge waste in bamboo processing industry, such as reassemble bamboo and bamboo flooring. Herein, autohydrolysis and rapid different deep eutectic solvents (DES) delignification strategy were consecutively performed to remove hemicelluloses and lignin from bamboo processing residue. The xylooligosaccharides (XOS) with high yield (34.35%) was achieved in the autohydrolysis process. Results showed that alkaline DES pretreatment resulted in the highest glucose yield (88.22%) and relatively high delignification rate (83.75%) as well as well-preserved lignin structures. However, the lignin fractions obtained under acidic DES conditions were tending to assemble into lignin nanoparticles (LNPs) and having excellent antioxidant activity as compared to those obtained from alkaline DES system. In brief, the combination of autohydrolysis and rapid DES delignification can achieve orientated fractionation of the components from the industrialized bamboo.

Fluorescent Imaging of Extracellular Fungal Enzymes Bound onto Plant Cell Walls

Lignocelluloytic enzymes are industrially applied as biocatalysts for the deconstruction of recalcitrant plant biomass. To study their biocatalytic and physiological function, the assessment of their binding behavior and spatial distribution on lignocellulosic material is a crucial prerequisite. In this study, selected hydrolases and oxidoreductases from the white rot fungus Phanerochaete chrysosporium were localized on model substrates as well as poplar wood by confocal laser scanning microscopy. Two different detection approaches were investigated: direct tagging of the enzymes and tagging specific antibodies generated against the enzymes. Site-directed mutagenesis was employed to introduce a single surface-exposed cysteine residue for the maleimide site-specific conjugation. Specific polyclonal antibodies were produced against the enzymes and were labeled using N-hydroxysuccinimide (NHS) ester as a cross-linker. Both methods allowed the visualization of cell wall-bound enzymes but showed slightly different fluorescent yields. Using native poplar thin sections, we identified the innermost secondary cell wall layer as the preferential attack point for cellulose-degrading enzymes. Alkali pretreatment resulted in a partial delignification and promoted substrate accessibility and enzyme binding. The methods presented in this study are suitable for the visualization of enzymes during catalytic biomass degradation and can be further exploited for interaction studies of lignocellulolytic enzymes in biorefineries.

A structure-activity understanding of the interaction between lignin and various cellulase domains

To elucidate the structure-activity relationship between lignin and various cellulase domains, four lignin fractions with specific structures and molecular weight were prepared from bamboo kraft lignin (BKL) and used to investigate the adsorption mechanism between different cellulase domains by fluorescence spectroscopy and SDS-PAGE. Endo-cellulase 6B exhibited a higher affinity to BKL fractions than the carbohydrate-binding module (CBM4A) of cellulase, which is positively correlated to molecular weight. The thermodynamic mechanism showed that the adsorption between BKL fractions and endo-cellulase 6B was dominated by van der Waals and electrostatic forces, while hydrophobic force is the driver for BKL fractions to adsorb CBM4A. Structure-activity relationship between lignin fractions and cellulase domain revealed that thermodynamics and interaction forces were more easily affected by the structure of BKL, including S/G ratio, molecular weight and hydrophobicity. The aforementioned results demonstrated that lignin's structure plays a critical role in its adsorption with various cellulase domains.

Surface Gelatin-Coated β-Mannanase-Immobilized Lignin for Delayed Release of β-Mannanase to Remediate Guar-Based Fracturing Fluid Damage

Herein, we developed an efficient and convenient method to address the problem of thickener decomposition in the low- permeability oilfield production process. It is crucial to design breakers that reduce viscosity by delaying thickener decomposition in appropriate environments. By using lignin in biomass as a substrate for β-mannanase immobilization (MIL), we fabricated a gel breaker, surface gelatin-coated β-mannanase-immobilized lignin (Ge@MIL). Through experiments and performance tests, we confirmed that the prepared Ge@MIL can release enzymes at a specific temperature, meanwhile having temperature-sensitive phase change properties and biodegradability. The results also show the tight tuning over the surface coating of Ge@MIL by a water-in-oil emulsion. Therefore, the prepared Ge@MIL has a promising application in the field of oil extraction as a green and efficient temperature-sensitive sustained-release capsule.

Structural Changes of Alkali Lignin under Ozone Treatment and Effect of Ozone-Oxidized Alkali Lignin on Cellulose Digestibility

In this study, the structural changes of alkali lignin induced by ozonation were investigated, and the effect of ozone-treated alkali lignin and its mechanism on Avicel enzymatic hydrolysis was examined. The physicochemical properties of alkali lignin were analyzed by FTIR, 1H-13C HSQC NMR, and GPC. It was revealed that ozone pretreatment increased the content of carboxyl and/or aldehyde groups and the negative zeta potential of alkali lignin, which enhanced the electrostatic repulsion between alkali lignin and cellulase; The S/G ratio was reduced, indicating the hydrophobic interaction was diminished. The Langmuir adsorption isotherm showed that the cellulase binding strength of ozone pretreated alkali lignin (OL-pH3, OL-pH7, and OL-pH12 were 16.67, 13.87, and 44.05 mL/g, respectively) was significantly lower than that of alkali lignin (161.29 mL/g). The 72 h hydrolysis yields of Avicel added with OL-pH3, OL-pH7, and OL-pH12 were 55.4%, 58.6%, and 54.9% respectively, which were 2.6–6.3% higher than that of Avicel added with AL (52.3%). This research aimed to reduce the non-productive adsorption between cellulase and lignin by investigating the structural changes of lignin caused by ozone treatment. For the first time, we discovered that ozone-treated alkali lignin has a further promotion effect on the enzymatic digestion of cellulose, providing a green and feasible pretreatment process for the enzymatic hydrolysis of lignocellulose and aiding in the more efficient utilization of biomass.

The inhibition of p-hydroxyphenyl hydroxyl group in residual lignin on enzymatic hydrolysis of cellulose and its underlying mechanism

The controlling factors of the inhibition on enzymatic hydrolysis caused by residual lignin were identified with molecular level understanding of the mechanism. Residual lignin samples with different properties were isolated, characterized and added into the enzymatic hydrolysis of Avicel. It was found that the phenolic hydroxyl group (OH) was the main inhibitor in residual lignin, and the p-hydroxyphenyl OH was the crucial sub-structure that exhibited the highest inhibition and non-productive adsorption, ascribing to its higher electrophilicity and lower steric hindrance. The H-bond interaction and π-π stacking between phenolic OH of lignin and phenolic OH of tyrosine on the planar face of carbohydrate binding module of cellulase were probably responsible for the non-productive adsorption. The binding sites of H-bonds may be the H in phenolic OH of lignin and the O in phenolic OH of tyrosine, respectively, and that of the π-π stacking may be the benzene rings of them.

Changes in the structure and composition of pineapple leaf fiber after alkali and ionic surfactant pretreatments and their impact on enzymatic hydrolysis

The current study investigated the effects of two different pretreatments (NaOH and alkaline surfactant assisted) on the chemical, morphological and enzymatic saccharification of pineapple leaf fiber (PALF). Results showed that both pretreatments significantly reduced lignin content of the biomass, achieving a 69.6 and a 76.3% reduction for NaOH and surfactant pretreated materials, respectively. SEM, CLSM and FTIR-ATR techniques were used to evaluate morphological changes in the fibers after pretreatments. Images obtained revealed cellulose exposure and lignin redistribution in the pretreated fibers. Surfactant pretreated material provided the best results after enzymatic hydrolysis compared to NaOH and untreated PALF. A final enzymatic hydrolysis yield of 81.8% was obtained after a 24 h process using surfactant pretreated fibers, in comparison to 75.9 and 45.1% yields for NaOH pretreated material and raw fibers, respectively. Nowadays, the use of agricultural residues for high added value products is of great importance for sustainable development. This work specifically studied an effective and green approach for lignin removal and enzymatic hydrolysis from pineapple leaf fiber that is an abundant waste in Costa Rica and an interesting feedstock for biorefinery processes design.

Microwave Assisted Pretreatment of Szarvasi (Agropyron elongatum) Biomass to Enhance Enzymatic Saccharification and Direct Glucose Production

Biomass from perennial plants can be considered a carbon-neutral renewable resource. The tall wheatgrass hybrid Szarvasi-1 (Agropyron elongatum, hereafter referred to as "Szarvasi") belongs to the perennial Poaceae representing a species, which can grow on marginal soils and produce large amounts of biomass. Several conventional and advanced pretreatment methods have been developed to enhance the saccharification efficiency of plant biomass. Advanced pretreatment methods, such as microwave-assisted pretreatment methods are faster and use less energy compared to conventional pretreatment methods. In this study, we investigated the potential of Szarvasi biomass as a biorefinery feedstock. For this purpose, the lignocellulosic structure of Szarvasi biomass was investigated in detail. In addition, microwave-assisted pretreatments were applied to Szarvasi biomass using different reagents including weak acids and alkali. The produced pulp, hydrolysates, and extracted lignin were quantitatively characterized. In particular, the alkali pretreatment significantly enhanced the saccharification efficiency of the pulp 16-fold compared to untreated biomass of Szarvasi. The acid pretreatment directly converted 25% of the cellulose into glucose without the need of enzymatic digestion. In addition, based on lignin compositional and lignin linkage analysis a lignin chemical model structure present in Szarvasi biomass could be established.

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.

Prediction of phenolic compounds and glucose content from dilute inorganic acid pretreatment of lignocellulosic biomass using artificial neural network modeling

Dilute inorganic acids hydrolysis is one of the most promising pretreatment strategies with high recovery of fermentable sugars and low cost for sustainable production of biofuels and chemicals from lignocellulosic biomass. The diverse phenolics derived from lignin degradation during pretreatment are the main inhibitors for enzymatic hydrolysis and fermentation. However, the content features of derived phenolics and produced glucose under different conditions are still unclear due to the highly non-linear characteristic of biomass pretreatment. Here, an artificial neural network (ANN) model was developed for simultaneous prediction of the derived phenolic contents (CPhe) and glucose yield (CGlc) in corn stover hydrolysate before microbial fermentation by integrating dilute acid pretreatment and enzymatic hydrolysis. Six processing parameters including inorganic acid concentration (CIA), pretreatment temperature (T), residence time (t), solid-to-liquid ratio (RSL), kinds of inorganic acids (kIA), and enzyme loading dosage (E) were used as input variables. The CPhe and CGlc were set as the two output variables. An optimized topology structure of 6-12-2 in the ANN model was determined by comparing root means square errors, which has a better prediction efficiency for CPhe (R2 = 0.904) and CGlc (R2 = 0.906). Additionally, the relative importance of six input variables on CPhe and CGlc was firstly calculated by the Garson equation with net weight matrixes. The results indicated that CIA had strong effects (22%-23%) on CPhe or CGlc, then followed by E and T. In conclusion, the findings provide new insights into the sustainable development and inverse optimization of biorefinery process from ANN modeling perspectives.

The Interplay of Autoclaving with Oxalate as Pretreatment Technique in the View of Bioethanol Production Based on Corn Stover

Bio-based treatment technologies are gaining great interest worldwide, and significant efforts are being afforded to develop technology for the use of lignocellulosic biomass. The potential of corn stover (CS) as a feedstock for bioethanol production was investigated by creating an optimal pretreatment condition to maximize glucose production. The current study undertook the impact of novel physico-chemical pretreatment methods of CS, i.e., autoclave-assisted oxalate (CSOA) and ultrasound-assisted oxalate (CSOU), on the chemical composition of CS and subsequent saccharification and fermentation for bioethanol production. The delignification was monitored by physicochemical characterizations such as SEM, XRD, FTIR, CHNs, and TGA. The results evidenced that delignification and enzymatic saccharification of the CS pretreated by CSOA was higher than CSOU. The optimum enzymatic saccharification operating conditions were 1:30 g solid substrate/mL sodium acetate buffer at 50 °C, shaking speed 100 rpm, and 0.4 g enzyme dosage. This condition was applied to produce glucose from CS, followed by bioethanol production by S. cerevisiae using an anaerobic fermentation process after 72 h. S. cerevisiae showed high conversion efficiency by producing a 360 mg/dL bioethanol yield, which is considered 94.11% of the theoretical ethanol yield. Furthermore, this research provides a potential path for waste material beneficiation, such as through utilizing CS.

Recent advances in understanding the effects of lignin structural characteristics on enzymatic hydrolysis

Enzymatic hydrolysis of lignocellulose for bioethanol production shows a great potential to remit the rapid consumption of fossil fuels, given the fact that lignocellulose feedstocks are abundant, cost-efficient, and renewable. Lignin results in low enzymatic saccharification by forming the steric hindrance, non-productive adsorption of cellulase onto lignin, and deactivating the cellulase. In general, the non-productive binding of cellulase on lignin is widely known as the major cause for inhibiting the enzymatic hydrolysis. Pretreatment is an effective way to remove lignin and improve the enzymatic digestibility of lignocellulose. Along with removing lignin, the pretreatment can modify the lignin structure, which significantly affects the non-productive adsorption of cellulase onto lignin. To relieve the inhibitory effect of lignin on enzymatic hydrolysis, enormous efforts have been made to elucidate the correlation of lignin structure with lignin-enzyme interactions but with different views. In addition, contrary to the traditional belief that lignin inhibits enzymatic hydrolysis, in recent years, the addition of water-soluble lignin such as lignosulfonate or low molecular-weight lignin exerts a positive effect on enzymatic hydrolysis, which gives a new insight into the lignin-enzyme interactions. For throwing light on their structure-interaction relationship during enzymatic hydrolysis, the effect of residual lignin in substrate and introduced lignin in hydrolysate on enzymatic hydrolysis are critically reviewed, aiming at realizing the targeted regulation of lignin structure for improving the saccharification of lignocellulose. The review is also focused on exploring the lignin-enzyme interactions to mitigate the negative impact of lignin and reducing the cost of enzymatic hydrolysis of lignocellulose.