Lignin from hydrothermally pretreated grass biomass retards enzymatic cellulose degradation by acting as a physical barrier rather than by inducing nonproductive adsorption of enzymes

Positive role of non-catalytic proteins on mitigating inhibitory effects of lignin and enhancing cellulase activity in enzymatic hydrolysis: Application, mechanism, and prospective

Fermentable sugar production from lignocellulosic biomass has received considerable attention and has been dramatic progress recently. However, due to low enzymatic hydrolysis (EH) yields and rates, a high dosage of the costly enzyme is required, which is a bottleneck for commercial applications. Over the last decades, various strategies have been developed to reduce cellulase enzyme costs. The progress of the non-catalytic additive proteins in mitigating inhibition in EH is discussed in detail in this review. The low efficiency of EH is mostly due to soluble lignin compounds, insoluble lignin, and harsh thermal and mechanical conditions of the EH process. Adding non-catalytic proteins into the EH is considered a simple and efficient approach to boost hydrolysis yield. This review discussed the multiple mechanical steps involved in the EH process. The effect of physicochemical properties of modified lignin on EH and its interaction with cellulase and cellulose are identified and discussed, which include hydrogen bonding, hydrophobic, electrostatic, and cation-π interactions, as well as physical barriers. Moreover, the effects of different conditions of EH that lead to cellulase deactivation by thermal and mechanical mechanisms are also explained. Finally, recent advances in the development, potential mechanisms, and economic feasibility of non-catalytic proteins on EH are evaluated and perspectives are presented.

Surface display of enzyme complex on Corynebacterium glutamicum as a whole cell biocatalyst and its consolidated bioprocessing using fungal-pretreated lignocellulosic biomass

A novel whole cell biocatalyst using fungal-pretreated lignocellulosic biomass was developed by displaying the enzyme complex consisting of N-acetylglucosaminidase (cNAG) and endoglucanse E (cCelE) on Corynebacterium glutamicum, hereafter called mNC. mNC showed a maximum 4.43-fold cNAG and 2.40-fold cCelE activity compared to single enzyme-secreting C. glutamicum. mNC also showed the highest efficiency of sugar production in various types of cellulose and fungal-pretreated biomass. The growth of mNC was 5.06-fold higher than that of the control. Then, the ability of mNC to produce a valuable chemical was confirmed. mNC overexpressing isopropanol biosynthesis genes showed a maximum titer of 218.9 ± 11.73 mg/L isopropanol and maintained high efficiency for isopropanol production in the recycling test, which was 90.07 ± 4.12 % during 4 cycles. This strategy can be applied to the direct saccharification of fungal-pretreated lignocellulosic biomass efficiently leading to the production of valuable products in various industrial fields.

Upstream processes of citrus fruit waste biorefinery for complete valorization

Citrus fruit waste (CW) is a useful biomass and its valorization into fuels and biochemicals has received much attention. For economic feasibility, increased efficiency of the preceding extraction and enzyme saccharification processes is necessary. However, at present, there is a lack of systematic reviews addressing these two integral upstream processes in concert for CW biorefinery. Here, the state-of-the-art advancements in enzyme extraction and saccharification processes-using which relevant essential oils, flavonoids, and sugars can be obtained-are reviewed. Specifically, the extraction options for two commercially available CW-derived products, essential oils and pectin, are discussed. With respect to enzyme saccharification, the use of an undefined commercial mixture routinely results in suboptimal sugar production. In this respect, applicable strategies for enzyme mixture customization are suggested for maximizing the hydrolytic efficiency of CW. The enzyme degradation system for CW-derived carbohydrates and its extensive application for sugar production are also discussed.

Qualitative and Quantitative Correlation of Microstructural Properties and In Vitro Glucose Adsorption and Diffusion Behaviors of Pea Insoluble Dietary Fiber Induced by Ultrafine Grinding

Ultrafine grinding is an important pretreatment to achieve the physical modification of dietary fiber. In this study, ultrafine grinding treatments were performed for different times to give pea insoluble dietary fiber (PIDF) samples with varied particle sizes (D50). The correlations and quantitative relationships between the microstructures of multi-scales PIDF and its in vitro glucose adsorption and diffusion behaviors were comprehensively evaluated. The results indicated that the specific surface area (SSA), pore volume (PV) and oxygen-to-carbon surface ratio (O/C) of PIDF were significantly increased by ultrafine grinding at the cellular scale, while D50 and cellulose crystallinity (CrI) were significantly decreased. These changes significantly improved the glucose adsorption capacity (GAC) of PIDF. The order of importance of microstructural changes on GAC was O/C > PV > SSA > CrI > D50. GAC showed positive exponential relationships with SSA, PV, and O/C and showed a negative linear relationship with CrI. The ability to retard glucose diffusion increased significantly with decreased fiber particle size because of improved adsorption and interception of glucose and the dense physical barrier effect of PIDF. The quantitative equation of maximum glucose dialysis retardation index was GDRImax = −1.65 ln(D50) + 16.82 ln(GAC) − 68.22 (R2 = 0.99). The results could provide theoretical support for quantitative and targeted intervention of dietary fiber structure for blood glucose control.

Effect of Solvent Quality on Structure and Dynamics of Lignin in Solution

Due to its significant aromatic content, lignin is an attractive source of valuable organic chemicals. As most of the proposed lignin depolymerization processes are expected to be liquid-phase, it is necessary to understand the effect of solvent quality on the structure and dynamics of lignin. Here we use all-atom molecular dynamics simulations to understand the evolution of lignin structure as a function of methanol concentration in methanol/water solution at different temperatures. We utilize two different lignin models: softwood consisting of guaiacyl (G) monomer and hardwood consisting of heteropolymer containing guaiacyl/syringyl (S) with a 1.35:1 ratio. The presence of additional methoxy groups in the hardwood lignin leads to a more extended configuration than softwood lignin with increasing methanol concentration. Structural features (radius of gyration and solvent accessible surface area) of lignin correlate with the strength of intermolecular forces quantified using cohesive energy density. We find that methanol preferentially solvates the nonpolar segments of the lignin polymer while water molecules solvate the polar functional groups. Thus, as the methanol concentration increases, methanol can better solvate lignin polymer, leading to a more extended configuration suitable for catalytic transformation to value-added chemicals.

Flexible and digestible wood caused by viral-induced alteration of cell wall composition

Woody plant material represents a vast renewable resource that has the potential to produce biofuels and other bio-based products with favorable net CO₂ emissions.^1,2 Its potential has been demonstrated in a recent study that generated novel structural materials from flexible moldable wood.³ Apple rubbery wood (ARW) disease is the result of a viral infection that causes woody stems to exhibit increased flexibility.⁴ Although ARW disease is associated with the presence of an RNA virus⁵ known as apple rubbery wood virus (ARWV), how the unique symptoms develop is unknown. We demonstrate that the symptoms of ARWV infections arise from reduced lignification within the secondary cell wall of xylem fibers and result in increased wood digestibility. In contrast, the mid-lamellae region and xylem ray cells are largely unaffected by the infection. Gene expression and proteomic data from symptomatic xylem clearly show the downregulation of phenylalanine ammonia lyase (PAL), the enzyme catalyzing the first committed step in the phenylpropanoid pathway leading to lignin biosynthesis. A large increase in soluble phenolics in symptomatic xylem, including the lignin precursor phenylalanine, is also consistent with PAL downregulation. ARWV infection results in the accumulation of many host-derived virus-activated small interfering RNAs (vasiRNAs). PAL-derived vasiRNAs are among the most abundant vasiRNAs in symptomatic xylem and are likely the cause of reduced PAL activity. Apparently, the mechanism used by the virus to alter lignin exhibits similarities to the RNAi strategy used to alter lignin in genetically modified trees to generate comparable improvements in wood properties.6, 7, 8

Video abstract

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Apple rubbery wood (ARW) symptoms are caused by decreased lignin in woody tissue

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RNA-seq, proteomics, and metabolomics suggest phenylalanine levels decrease

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Virus-activated small interfering RNAs (vasiRNAs) are generated in response to ARWV infection

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VasiRNAs cause siRNA-based downregulation of phenylalanine ammonia

Effect of lignin on short-chain fatty acids production from anaerobic fermentation of waste activated sludge

Lignin, a biological resource with great potential, can be as high as ∼16% of the total organics in the waste activated sludge (WAS). This work therefore aims to fill the knowledge gap about the effect of lignin on short-chain fatty acids (SCFAs) production from anaerobic fermentation of sludge. Experimental results showed that lignin promoted rather than inhibited SCFAs production. Specifically, the presence of 15% lignin promoted the SCFAs production from 129.1 ± 6.5 to 223.14 ± 7.8 mg COD/g VSS compared with the control, and the proportion of acetic increased by 61.8%, while that of propionic decreased by 44.9%. Mechanism exploration revealed that lignin improved the solubilization of biodegradable substrates due to its hydrophobic characteristics. In addition, lignin enhanced the acidogenesis process, possibly by perfecting the electron transfer chain in the fermentation system, and the quinone structure in lignin may compete electrons with methanogens to inhibit the consumption of SCFAs. Microbiological analysis showed that the abundance of microorganisms related to acidogenesi, especially the acetogenesis, including Proteiniclasticum sp., Acetoanaerobium sp., in the fermenter with lignin increased, which caused the community to shift towards specialized and diverse SCFAs production.

Spent mushroom substrates for ethanol production - Effect of chemical and structural factors on enzymatic saccharification and ethanolic fermentation of Lentinula edodes-pretreated hardwood

Spent mushroom substrates (SMS) from cultivation of shiitake (Lentinula edodes) on three hardwood species were investigated regarding their potential for cellulose saccharification and for ethanolic fermentation of the produced hydrolysates. High glucan digestibility was achieved during enzymatic saccharification of the SMSs, which was related to the low mass fractions of lignin and xylan, and it was neither affected by the relative content of lignin guaiacyl units nor the substrate crystallinity. The high nitrogen content in SMS hydrolysates, which was a consequence of the fungal pretreatment, was positive for the fermentation, and it ensured ethanol yields corresponding to 84-87% of the theoretical value in fermentations without nutrient supplementation. Phenolic compounds and acetic acid were detected in the SMS hydrolysates, but due to their low concentrations, the inhibitory effect was limited. The solid leftovers resulting from SMS hydrolysis and the fermentation residues were quantified and characterized for further valorisation.

The enzyme interactome concept in filamentous fungi linked to biomass valorization

Biomass represents an abundant and inexpensive source of sugars and aromatic compounds that can be used as raw materials for conversion into value-added bioproducts. Filamentous fungi are sources of plant cell wall degrading enzymes in nature. Understanding the interactions between enzymes is crucial for optimizing biomass degradation processes. Herein, the concept of the interactome is presented as a holistic approach that depicts the interactions among enzymes, substrates, metabolites, and inhibitors. The interactome encompasses several stages of biomass degradation, starting with the sensing of the substrate and the subsequent synthesis of hydrolytic and oxidative enzymes (fungus-substrate interaction). Enzyme-enzyme interactions are exemplified in the complex processes of lignocellulosic biomass degradation. The enzyme-substrate-metabolite-inhibitor interaction also provides a better understanding of biomass conversion, allowing bioproduct production from recalcitrant agro-industrial residues, thus bringing greater value to residual biomass. Finally, technological applications are presented for optimizing the interactome at various levels.

Bacterial cellulose/lignin nanoparticles composite films with retarded biodegradability

In practical applications, the full biodegradability of all-biomass-based bacterial cellulose (BC) composites enhances their environmentally friendliness but results in the poor durability especially at humid conditions. This work prepared BC/lignin nanoparticles (LNPs) composite films with retarded biodegradability, which could broaden their application area. Three LNPs were fabricated using technical lignins extracted by deep eutectic solvent (DES), ethanol organosolv, soda/anthraquinone from poplar. LNPs involvement during BC fermentation showed limited influence on its productivity but significantly retarded the biodegradation of composite films. The potential inhibition mechanism was physical barrier and non-productive binding of LNPs. The BC/Soda LNPs showed much higher retarded degradation property (~58 wt% degradation) compared to BC/Organosolv LNPs and BC/DES LNPs (~85 wt% and ~ 97 wt% degradation respectively) at high enzyme loadings of 5 mg g^-1 BCE. While at low enzyme loadings of 1 mg g^-1 BCE, all these three composite films showed comparable retarded degradation property of ~60 wt%.

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.

Selective removal of lignin with sodium chlorite to improve the quality and antioxidant activity of xylo-oligosaccharides from lignocellulosic biomass

As a key anti-degradation barrier that restricts the biotransformation of lignocellulose, the presence of lignin usually severely affects the quality of the extracted xylo-oligosaccharides (XOS). Herein, this study proposed a practical route to improve the quality and antioxidant activity of XOS extracted from lignocellulosic biomass via selective removal of lignin. The highest delignification of 92.6% was successfully achieved with 8% sodium chlorite at 75°C for 2 h. An ideal hemicellulose sample with a purity of 86.1% was obtained by selective removal of lignin. A high-quality XOS sample with a purity of 96.3%, a yield of 77.4%, and a color value of 814 was obtained by separating and purifying the enzymatic hydrolysate. Antioxidant activity assay showed that the highest radical scavenging activity of XOS was 87.3%. Importantly, this study provide a feasible and effective route for the lignocellulosic biomass utilization strategy based on the selective removal of lignin.

Evaluation of lignin inhibition in anaerobic digestion from the perspective of reducing the hydrolysis rate of holocellulose

In this study, Anaerobic Digestion Model No. 1 (ADM1) were modified to simulate anaerobic digestion (AD) process of microcrystalline cellulose (MCC) and five lignocellulosic substrates, with the goal of predicting the hydrolysis rates of holocellulose fractions in environments with and without lignin inhibition. After model verification, the hydrolysis rate constant of MCC, i.e., the hydrolyzability of cellulose without lignin inhibition, was 3.227 d^-1, while those of the holocellulose fractions of five lignocellulosic substrates (I_k_hyd) were in the range of 1.270 d^-1 to 3.364 d^-1 (average of 2.242 d^-1), which demonstrated remarkable suppression of holocellulose hydrolysis by lignin. Lignin inhibition index (LII) was proposed as an indicator to intuitively quantify and characterize the lignin inhibitory strength in a specific substrate. A series of factors with the potential to affect the LII were analyzed sequentially. This study provides an advanced understanding of the participation and behavior of lignin in the AD process.

Chemistry of lignin and hemicellulose structures interacts with hydrothermal pretreatment severity and affects cellulose conversion

Understanding of how the plant cell walls of different plant species respond to pretreatment can help improve saccharification in bioconversion processes. Here, we studied the chemical and structural modifications in lignin and hemicellulose in hydrothermally pretreated poplar and wheat straw using wet chemistry and 2D heteronuclear single quantum coherence nuclear magnetic resonance (NMR) and their effects on cellulose conversion. Increased pretreatment severity reduced the levels of β─O─4 linkages with concomitant relatively increased levels of β─5 and β─β structures in the NMR spectra. β─5 structures appeared at medium and high severities for wheat straw while only β─β structures were observed at all pretreatment severities for poplar. These structural differences accounted for the differences in cellulose conversion for these biomasses at different severities. Changes in the hemicellulose component include a complete removal of arabinosyl and 4-O-methyl glucuronosyl substituents at low and medium pretreatment severities while acetyl groups were found to be relatively resistant toward hydrothermal pretreatment. This illustrates the importance of these groups, rather than xylan content, in the detrimental role of xylan in cellulose saccharification and helps explain the higher poplar recalcitrance compared to wheat straw. The results point toward the need for both enzyme preparation development and pretreatment technologies to target specific plant species.

Immobilization of cellulase enzymes on nano and micro-materials for breakdown of cellulose for biofuel production-a narrative review

Cellulose is a very abundant polymer that is found in nature. Cellulose has been used as a raw material for production of biofuels for many years. However, there are multiple processing steps that are required so that cellulose can be used as a raw material for biofuel production. One of the most important steps is the breakdown of cellulose into intermediate sugars which can then be a viable substrate for biofuel production. Cellulases are enzymes which play a role in the catalysis of the breakdown of cellulose into glucose. Nanomaterials and micromaterials have been gaining a lot of attention over the past few years for its potential in immobilizing enzymes for industrial procedures. Immobilization of enzymes on these nanomaterials has been observed to be of great value due to the improvement in thermal stability, pH stability, regenerative capacity, increase in activity and the reusability of enzymes. Similarly, there have been multiple reports of cellulase enzymes being immobilized on various nanoparticles. The immobilization of these cellulase enzymes have resulted in very efficient processing and provide a great and economic solution for the processing of cellulose for biofuel production. Hence in this paper, we review and discuss the various advantages and disadvantages of enzymes on various available nanomaterials.

Production of xylo-oligosaccharides from poplar by acetic acid pretreatment and its impact on inhibitory effect of poplar lignin

Recently, efficient production of xylo-oligosaccharides (XOS) from poplar by acetic acid (AA) pretreatment was developed; but the effect of residual lignin on subsequent cellulase hydrolysis was unclear. Herein, XOS was produced from poplar by AA pretreatment and the effect of AA pretreatment on lignin inhibition to cellulase hydrolysis was investigated. The results indicated that a high XOS yield of 55.8% was obtained, and the inhibition degree of lignin in poplar increased from 1.0% to 6.8% after AA pretreatment. Lignin was acetylated and its molecular weight decreased from 12,211 to 2871 g/mol after AA pretreatment. The increase of S/G ratio, phenolic hydroxyl, and condensed units of lignin after AA pretreatment might be reasons for this intensified inhibition. The results advanced our understanding of the structural and inhibitory properties of lignin after production of XOS from poplar with AA pretreatment, and provided references for efficient cellulase hydrolysis of poplar after AA pretreatment.

Breeding Targets to Improve Biomass Quality in Miscanthus

Lignocellulosic crops are attractive bioresources for energy and chemicals production within a sustainable, carbon circular society. Miscanthus is one of the perennial grasses that exhibits great potential as a dedicated feedstock for conversion to biobased products in integrated biorefineries. The current biorefinery strategies are primarily focused on polysaccharide valorization and require severe pretreatments to overcome the lignin barrier. The need for such pretreatments represents an economic burden and impacts the overall sustainability of the biorefinery. Hence, increasing its efficiency has been a topic of great interest. Inversely, though pretreatment will remain an essential step, there is room to reduce its severity by optimizing the biomass composition rendering it more exploitable. Extensive studies have examined the miscanthus cell wall structures in great detail, and pinpointed those components that affect biomass digestibility under various pretreatments. Although lignin content has been identified as the most important factor limiting cell wall deconstruction, the effect of polysaccharides and interaction between the different constituents play an important role as well. The natural variation that is available within different miscanthus species and increased understanding of biosynthetic cell wall pathways have specified the potential to create novel accessions with improved digestibility through breeding or genetic modification. This review discusses the contribution of the main cell wall components on biomass degradation in relation to hydrothermal, dilute acid and alkaline pretreatments. Furthermore, traits worth advancing through breeding will be discussed in light of past, present and future breeding efforts.

The effects of solid lignin on the anaerobic digestion of microcrystalline cellulose and application of smoothing splines for extended data analysis of its inhibitory effects

Lignocellulose is an abundant substrate for biogas production; however, for efficient utilization, proper pre-treatment is required to enhance the biomethane yield and hydrolysis rate significantly. Phenolic compounds from dissolved lignin, produced during alkali pre-treatment, have inhibitory effects on the anaerobic digestion; however, the possible inhibitory effects of solid lignin have not gathered enough interest. Especially, the effect of solid lignin on methanogenesis remains a knowledge gap. In this study, kraft lignin was used as a model solid lignin substrate for its co-digestion with microcrystalline cellulose. A new approach of modelling biomethane production curves using smoothing splines was developed to describe the long-term inhibitory effects of solid lignin on hydrolysis and methanogenesis. The method gives possibility to describe long-term inhibitory effects by using batch instead of continuous test data. Results revealed that kraft lignin showed mild inhibitory effects on methanogens. However lignin impact combined with volatile fatty accumulation can prolong hydrolysis and reactor recovery start-up by 47.3% and 75.3%, respectively. For small dosages of solid lignin adaptation of methanogens is possible.

Derived high reducing sugar and lignin colloid particles from corn stover

Lignocellulosic biomass is considered as the largest potential candidate to develop alternative energy, such as biofuel, biomaterial. However, the efficient conversion of cellulose and practical utilization of lignin are great challenges for sustainable biorefinery. In this study, high reducing sugar yield and different size of lignin colloid particles (LCPs) were obtained via tetrahydrofuran-water (THF-H2O) pretreatment of corn stover (CS). THF-H2O as a co-solvent, could efficiently dissolve lignin and retain cellulose. After the pretreatment, 640.87 mg/g of reducing sugar was produced, that was 6.66-fold higher than that of the untreated CS. Meanwhile, the pretreatment liquor could form spherical LCPs with different sizes ranged from 202 to 732 nm through self-assembly. We studied the optimal pretreatment condition to simultaneously realize the high reducing sugar yield (588.4 mg/g) and excellent LCPs preparation with average size of 243 nm was under TH22 (THF-H2O pretreatment at 120 °C for 2 h). To further explore the formation of LCPs with different sizes. We studied the lignin structure changes of various conditions, concluded the size of LCPs was related to the lignin concentration and syringyl/guaiacyl (S/G) ratio. As the increase of the lignin concentration and S/G, the sizes of LCPs were increased. G-type lignin was easier to dissolve in the mild pretreatment supernatant, contributing to form smaller LCPs with a good dispersibility. In the severe condition, both of S and G-type lignin were dissolved due to the lignin depolymerization, formed the larger sphere particles. This work provides a novel perspective for the technical design of lignocellulosic biomass conversion.

A steady-state approach for inhibition of heterogeneous enzyme reactions

The kinetic theory of enzymes that modify insoluble substrates is still underdeveloped, despite the prevalence of this type of reaction both in vivo and industrial applications. Here, we present a steady-state kinetic approach to investigate inhibition occurring at the solid-liquid interface. We propose to conduct experiments under enzyme excess (E0 ≫ S0), i.e. the opposite limit compared with the conventional Michaelis-Menten framework. This inverse condition is practical for insoluble substrates and elucidates how the inhibitor reduces enzyme activity through binding to the substrate. We claim that this type of inhibition is common for interfacial enzyme reactions because substrate accessibility is low, and we show that it can be analyzed by experiments and rate equations that are analogous to the conventional approach, except that the roles of enzyme and substrate have been swapped. To illustrate the approach, we investigated the major cellulases from Trichoderma reesei (Cel6A and Cel7A) acting on insoluble cellulose. As model inhibitors, we used catalytically inactive variants of Cel6A and Cel7A. We made so-called inverse Michaelis-Menten curves at different concentrations of inhibitors and found that a new rate equation accounted well for the data. In most cases, we found a mixed type of surface-site inhibition mechanism, and this probably reflected that the inhibitor both competed with the enzyme for the productive binding-sites (competitive inhibition) and hampered the processive movement on the surface (uncompetitive inhibition). These results give new insights into the complex interplay of Cel7A and Cel6A on cellulose and the approach may be applicable to other heterogeneous enzyme reactions.

A novel hybrid organosolv-steam explosion pretreatment and fractionation method delivers solids with superior thermophilic digestibility to methane

Rising environmental concerns and the imminent depletion of fossil resources have sparked a strong interest towards the production of renewable energy such as biomethane. Inclusion of alternative feedstock's such as lignocellulosic biomass could further expand the production of biomethane. The present study evaluated the potential of a novel hybrid organosolv-steam explosion fractionation for delivering highly digestible pretreated solids from birch and spruce woodchips. The highest methane production yield was 176.5 mLCH₄ gVS^-1 for spruce and 327.2 mL CH₄ gVS^-1 for birch. High methane production rates of 1.0-6.3 mL min^-1 (spruce) and 6.0-35.5 mL min^-1 (birch) were obtained, leading to a rapid digestion, with 92% of total methane from spruce being generated in 80 h and 95% of that from birch in 120 h. These results demonstrate the elevated potential of the novel method to fractionate spruce and birch biomass and deliver cellulose-rich pretreated solids with superior digestibility.

Enzymatic processing of lignocellulosic biomass: principles, recent advances and perspectives

Efficient saccharification of lignocellulosic biomass requires concerted development of a pretreatment method, an enzyme cocktail and an enzymatic process, all of which are adapted to the feedstock. Recent years have shown great progress in most aspects of the overall process. In particular, increased insights into the contributions of a wide variety of cellulolytic and hemicellulolytic enzymes have improved the enzymatic processing step and brought down costs. Here, we review major pretreatment technologies and different enzyme process setups and present an in-depth discussion of the various enzyme types that are currently in use. We pay ample attention to the role of the recently discovered lytic polysaccharide monooxygenases (LPMOs), which have led to renewed interest in the role of redox enzyme systems in lignocellulose processing. Better understanding of the interplay between the various enzyme types, as they may occur in a commercial enzyme cocktail, is likely key to further process improvements.

Exploring the flexibility of cellulase cocktail obtained from mutant UV-8 of Talaromyces verruculosus IIPC 324 in depolymerising multiple agro-industrial lignocellulosic feedstocks

Effective management and the valorization of agro-industrial lignocellulosic feedstocks can only be realized if a versatile cellulase cocktail is developed that can release glucose at affordable cost irrespective of biomass type. In the present study the flexibility of using cellulase cocktail obtained from mutant UV-8 of Talaromyces verruculosus IIPC 324 in depolymerizing multiple agro-industrial lignocellulosic feedstocks was explored. Five different dilute acid pretreated biomasses were evaluated and cellulase loading was done at 25 mg protein/g cellulose content. After 72 h of hydrolysis at 55 °C and pH 4.5, corn cob and rice straw emerged as the easiest and toughest substrates with saccharification yield of 83.9 ± 1.17 and 35.5 ± 1.16% respectively from their cellulose fraction. Addition of PEG 6000 could retain >65% of all mono-component enzymes present in cellulase cocktail. Structural elucidation of biomasses gave an insight about key features responsible for variable recalcitrance in the different agro-industrial feedstock. Cellulose hydrolysis showed a significant negative correlation in the order of Cr I > S/G ratio > ash content. The chemical composition of lignin had a major impact on enzyme-lignin interactions. Higher H lignin content and lower S/G ratio promoted enzyme desorption, thereby increasing the likelihood of their recycling and reuse.

Interaction of enzymes with lignocellulosic materials: causes, mechanism and influencing factors

For the production of biofuel (bioethanol), enzymatic adsorption onto a lignocellulosic biomass surface is a prior condition for the enzymatic hydrolysis process to occur. Lignocellulosic substances are mainly composed of cellulose, hemicellulose and lignin. The polysaccharide matrix (cellulose and hemicellulose) is capable of producing bioethanol. Therefore, lignin is removed or its concentration is reduced from the adsorption substrates by pretreatments. Selected enzymes are used for the production of reducing sugars from cellulosic materials, which in turn are converted to bioethanol. Adsorption of enzymes onto the substrate surface is a complicated process. A large number of research have been performed on the adsorption process, but little has been done to understand the mechanism of adsorption process. This article reviews the mechanisms of adsorption of enzymes onto the biomass surfaces. A conceptual adsorption mechanism is presented which will fill the gaps in literature and help researchers and industry to use adsorption more efficiently. The process of enzymatic adsorption starts with the reciprocal interplay of enzymes and substrates and ends with the establishment of molecular and cellular binding. The kinetics of an enzymatic reaction is almost the same as that of a characteristic chemical catalytic reaction. The influencing factors discussed in detail are: surface characteristics of the participating materials, the environmental factors, such as the associated flow conditions, temperature, concentration, etc. Pretreatment of lignocellulosic materials and optimum range of shear force and temperature for getting better results of adsorption are recommended.

The critical role of lignin in lignocellulosic biomass conversion and recent pretreatment strategies: A comprehensive review

Heterogeneity and rigidity of lignocellulose causing resistance to its deconstruction have provided technical and economic challenges in the current biomass conversion processes. Lignin has been considered as a crucial recalcitrance component in biomass utilization. An in-depth understanding of lignin properties and their influences on biomass conversion can provide clues to improve biomass utilization. Also, utilization of lignin can significantly increase the economic viability of biorefinery. Recent lignin-targeting pretreatments have aimed not only to overcome recalcitrance for biomass conversion but also to selectively fractionate lignin for lignin valorization. Numerous studies have been conducted in biomass characteristics and conversion technologies, and the role of lignin is critical for lignin valorization and biomass pretreatment development. This review provides a comprehensive review of lignin-related biomass characteristics, the impact of lignin on the biological conversion of biomass, and recent lignin-targeting pretreatment strategies. The desired lignin properties in biorefinery and future pretreatment directions are also discussed.

Constraints and advances in high-solids enzymatic hydrolysis of lignocellulosic biomass: a critical review

The industrial production of sugar syrups from lignocellulosic materials requires the conduction of the enzymatic hydrolysis step at high-solids loadings (i.e., with over 15% solids [w/w] in the reaction mixture). Such conditions result in sugar syrups with increased concentrations and in improvements in both capital and operational costs, making the process more economically feasible. However, this approach still poses several technical hindrances that impact the process efficiency, known as the "high-solids effect" (i.e., the decrease in glucan conversion yields as solids load increases). The purpose of this review was to present the findings on the main limitations and advances in high-solids enzymatic hydrolysis in an updated and comprehensive manner. The causes for the rheological limitations at the onset of the high-solids operation as well as those influencing the "high-solids effect" will be discussed. The subject of water constraint, which results in a highly viscous system and impairs mixing, and by extension, mass and heat transfer, will be analyzed under the perspective of the limitations imposed to the action of the cellulolytic enzymes. The "high-solids effect" will be further discussed vis-à-vis enzymes end-product inhibition and the inhibitory effect of compounds formed during the biomass pretreatment as well as the enzymes' unproductive adsorption to lignin. This review also presents the scientific and technological advances being introduced to lessen high-solids hydrolysis hindrances, such as the development of more efficient enzyme formulations, biomass and enzyme feeding strategies, reactor and impeller designs as well as process strategies to alleviate the end-product inhibition. We surveyed the academic literature in the form of scientific papers as well as patents to showcase the efforts on technological development and industrial implementation of the use of lignocellulosic materials as renewable feedstocks. Using a critical approach, we expect that this review will aid in the identification of areas with higher demand for scientific and technological efforts.

A comparative study on enzyme adsorption and hydrolytic performance of different scale corn stover by two-step kinetics

To investigate the effect of two-step kinetics on enzyme adsorption and hydrolytic properties of different structural substrates at low enzyme doses. The two-step kinetic experiments of ultrafine grinding (UGCS) and sieve-based grinding corn stover (SGCS) were performed respectively with enzyme loading of 2.5 + 2.5 FPU/g and 5 + 5 FPU/g. The different performance of these two samples were illustrated by characterizing the particle size distribution, SEM and XPS. The results showed that ultrafine grinding can promote the structural properties which is beneficial to adsorption and hydrolysis. The main factors influencing adsorption kinetics are enzyme concentration and the surface cellulose amount. Pre-adsorbed enzyme has no effects on the subsequent enzyme adsorption quantity but produces some small competitive and impeditive effects. The hydrolysis kinetics mainly depend on the structure of the substrate and its complexity of hydrolysis. The two-step hydrolysis didn't promote the total sugar yield under the same enzyme concentration, but the first step contributed more to the total sugar yield.

Novel xylanolytic triple domain enzyme targeted at feruloylated arabinoxylan degradation

A three catalytic domain multi-enzyme; a CE1 ferulic acid esterase, a GH62 α-l-arabinofuranosidase and a GH10 β-d-1,4-xylanase was identified in a metagenome obtained from wastewater treatment sludge. The capability of the CE1-GH62-GH10 multi-enzyme to degrade arabinoxylan was investigated to examine the hypothesis that CE1-GH62-GH10 would degrade arabinoxylan more efficiently than the corresponding equimolar mix of the individual enzymes. CE1-GH62-GH10 efficiently catalyzed the production of xylopyranose, xylobiose, xylotriose, arabinofuranose and ferulic acid (FA) when incubated with insoluble wheat arabinoxylan (WAX-I) (k_cat = 20.8 ± 2.6 s^-1). Surprisingly, in an equimolar mix of the individual enzymes a similar k_cat towards WAX-I was observed (k_cat = 17.3 ± 3.8 s^-1). Similarly, when assayed on complex plant biomass the activity was comparable between CE1-GH62-GH10 and an equimolar mix of the individual enzymes. This suggests that from a hydrolytic point of view a CE1-GH62-GH10 multi-enzyme is not an advantage. Determination of the melting temperatures for CE1-GH62-GH10 (71.0 ± 0.05 °C) and CE1 (69.9 ± 0.02), GH62 (65.7 ± 0.06) and GH10 (71 ± 0.05 °C) indicates that CE1 and GH62 are less stable as single domain enzymes. This conclusion was corroborated by the findings that CE1 lost ˜50% activity within 2 h, while GH62 retained ˜50% activity after 24 h, whereas CE1-GH62-GH10 and GH10 retained ˜50% activity for 72 h. GH62-GH10, when appended to each other, displayed a higher specificity constant (k_cat/K_m = 0.3 s^-1 mg^-1 ml) than the individual GH10 (k_cat/K_m = 0.12 s^-1 ± 0.02 mg^-1 ml) indicating a synergistic action between the two. Surprisingly, CE1-GH62, displayed a 2-fold lower k_cat towards WAX-I than GH62, which might be due to the presence of a putative carbohydrate binding module appended to CE1 at the N-terminal. Both CE1 and CE1-GH62 released insignificant amounts of FA from WAX-I, but FA was released from WAX-I when both CE1 and GH10 were present, which might be due to GH10 releasing soluble oligosaccharides that CE1 can utilize as substrate. CE1 also displayed activity towards solubilized 5-O-trans-feruloyl-α-l-Araf (k_cat = 36.35 s^-1). This suggests that CE1 preferably acts on soluble oligosaccharides.

Enzymatic hydrolysis is limited by biomass-water interactions at high-solids: improved performance through substrate modifications

BACKGROUND

To improve process economics for production of fuels and chemicals from lignocellulosic biomass, high solids concentrations are applied in enzymatic hydrolysis, to increase product concentration and reduce energy input. However, increasing solids concentrations decrease cellulose conversion yields, the so called 'high-solids effect.' Previous work suggests that product inhibition and mixing contribute, but an understanding of how biomass properties influence the high-solids effect, is lacking.

RESULTS

Cellulose hydrolysis yields with an industrial cellulase (Ctec2) were measured on pretreated wheat straw and spruce from 5 to 30% dry matter (DM), and compared to yields of an older industrial cellulase mixture (Celluclast 1.5L/Novozym188). For Ctec2, yield was independent of DM below 15-18% DM, while yields decreased with increasing DM above this range, but at different rates for each biomass. For Celluclast 1.5L/Novozym188, yields decreased already from the lowest DM, suggesting that the high-solids effect was more a function of product inhibition, while the yields of the newer Ctec2 mixture were driven more by biomass-water interactions. LF-NMR relaxometry showed that the onset of the high-solids effect for Ctec2 corresponded to the disappearance of free water from the system, and a decrease in water self-diffusion rates. While the spruce had higher yields at low-solids, the wheat straw had higher yields at high-solids conditions, exhibiting that relative yields at low and high-solids are not related. Higher yields corresponded to increased water constraint by the biomass at high-solids conditions. Modifications to the pretreated wheat straw resulted in improved yields, and changes to the inflection point and intensity of the high-solids effect, showing that this effect can be reduced.

CONCLUSIONS

The high-solids effect is both enzyme and substrate dependent, and can be reduced by modifying the pretreated biomass, suggesting that pretreatment processes can be designed to achieve similar effects. Yields at low and high-solids concentrations do not correlate for a given biomass, and thus industrial evaluation of biomass recalcitrance should be carried out at high-solids conditions.

Cellulases adsorb reversibly on biomass lignin

Adsorption of cellulases onto lignin is considered a major factor in retarding enzymatic cellulose degradation of lignocellulosic biomass. However, the adsorption mechanisms and kinetics are not well understood for individual types of cellulases. This study examines the binding affinity, kinetics of adsorption, and competition of four monocomponent cellulases of Trichoderma reesei during adsorption onto lignin. TrCel7A, TrCel6A, TrCel7B, and TrCel5A were radiolabeled for adsorption experiments on lignin‐rich residues (LRRs) isolated from hydrothermally pretreated spruce (L‐HPS) and wheat straw (L‐HPWS), respectively. On the basis of adsorption isotherms fitted to the Langmuir model, the ranking of binding affinities was TrCel5A >  TrCel6A >  TrCel7B >  TrCel7A on both types of LRRs. The enzymes had a higher affinity to the L‐HPS than to the L‐HPWS. Adsorption experiments with dilution after 1 and 24 hr and kinetic modeling were performed to quantify any irreversible binding over time. Models with reversible binding parameters fitted well and can explain the results obtained. The adsorption constants obtained from the reversible models agreed with the fitted Langmuir isotherms and suggested that reversible adsorption–desorption existed at equilibrium. Competitive binding experiments showed that individual types of cellulases competed for binding sites on the lignin and the adsorption data fitted the Langmuir adsorption model. Overall, the data strongly indicate that the adsorption of cellulases onto lignin is reversible and the findings have implications for the development of more efficient cellulose degrading enzymes.