Effect of structural and physico-chemical features of cellulosic substrates on the efficiency of enzymatic hydrolysis

Production of Biomodified Bleached Kraft Pulp by Catalytic Conversion Using Penicillium verruculosum Enzymes: Composition, Properties, Structure, and Application

The global development of the bioeconomy is impossible without technologies for comprehensive processing of plant renewable resources. The use of proven pretreatment technologies raises the possibility of the industrial implementation of the enzymatic conversion of polysaccharides from lignocellulose considering the process’s complexity. For instance, a well-tuned kraft pulping produces a substrate easily degraded by cellulases and hemicelulases. Enzymatic hydrolysis of bleached hardwood kraft pulp was carried out using an enzyme complex of endoglucanases, cellobiohydrolases, β-glucosidases, and xylanases produced by recombinant strains of Penicillium verruculosum at a 10 FPU/g mixture rate and a 10% substrate concentration. As a result of biocatalysis, the following products were obtained: sugar solution, mainly glucose, xylobiose, xylose, as well as other minor reducing sugars; a modified complex based on cellulose and xylan. The composition of the biomodified kraft pulp was determined by HPLC. The method for determining the crystallinity on an X-ray diffractometer was used to characterize the properties. The article shows the possibility of producing biomodified cellulose cryogels by amorphization with concentrated 85% H3PO4 followed by precipitation with water and supercritical drying. The analysis of the enzymatic hydrolysate composition revealed the predominance of glucose (55–67%) among the reducing sugars with a maximum content in the solution up to 6% after 72 h. The properties and structure of the modified kraft pulp were shown to change during biocatalysis; in particular, the crystallinity increased by 5% after 3 h of enzymatic hydrolysis. We obtained cryogels based on the initial and biomodified kraft pulp with conversion rates of 35, 50, and 70%. The properties of these cryogels are not inferior to those of cryogels based on industrial microcrystalline cellulose, as confirmed by the specific surface area, degree of swelling, porosity, and SEM images. Thus, kraft pulp enzymatic hydrolysis offers prospects not only for producing sugar-rich hydrolysates for microbiological synthesis, but also cellulose powders and cryogels with specified properties.

Bioconversion of Renewable Plant Biomass. Second-Generation Biofuels: Raw Materials, Biomass Pretreatment, Enzymes, Processes, and Cost Analysis

The review discusses various aspects of renewable plant biomass conversion and production of the second-generation biofuels, including the types of plant biomass, its composition and reaction ability in the enzymatic hydrolysis, and various pretreatment methods for increasing the biomass reactivity. Conversion of plant biomass into sugars requires the use of a complex of enzymes, the composition of which should be adapted to the biomass type and the pretreatment method. The efficiency of enzymatic hydrolysis can be increased by optimizing the composition of the enzymatic complex and by increasing the catalytic activity and operational stability of its constituent enzymes. The availability of active enzyme producers also plays an important role. Examples of practical implementation and scaling of processes for the production of second-generation biofuels are presented together with the cost analysis of bioethanol production.

Enzymatic degradation of maize shoots: monitoring of chemical and physical changes reveals different saccharification behaviors

BACKGROUND

The recalcitrance of lignocellulosics to enzymatic saccharification has been related to many factors, including the tissue and molecular heterogeneity of the plant particles. The role of tissue heterogeneity generally assessed from plant sections is not easy to study on a large scale. In the present work, dry fractionation of ground maize shoot was performed to obtain particle fractions enriched in a specific tissue. The degradation profiles of the fractions were compared considering physical changes in addition to chemical conversion.

RESULTS

Coarse, medium and fine fractions were produced using a dry process followed by an electrostatic separation. The physical and chemical characteristics of the fractions varied, suggesting enrichment in tissue from leaves, pith or rind. The fractions were subjected to enzymatic hydrolysis in a torus reactor designed for real-time monitoring of the number and size of the particles. Saccharification efficiency was monitored by analyzing the sugar release at different times. The lowest and highest saccharification yields were measured in the coarse and fine fractions, respectively, and these yields paralleled the reduction in the size and number of particles. The behavior of the positively- and negatively-charged particles of medium-size fractions was contrasted. Although the amount of sugar release was similar, the changes in particle size and number differed during enzymatic degradation. The reduction in the number of particles proceeded faster than that of particle size, suggesting that degradable particles were degraded to the point of disappearance with no significant erosion or fragmentation. Considering all fractions, the saccharification yield was positively correlated with the amount of water associated with [5-15 nm] pore size range at 67% moisture content while the reduction in the number of particles was inversely correlated with the amount of lignin.

CONCLUSION

Real-time monitoring of sugar release and changes in the number and size of the particles clearly evidenced different degradation patterns for fractions of maize shoot that could be related to tissue heterogeneity in the plant. The biorefinery process could benefit from the addition of a sorting stage to optimise the flow of biomass materials and take better advantage of the heterogeneity of the biomass.

Study on the Anti-Biodegradation Property of Tunicate Cellulose

Tunicate is a kind of marine animal, and its outer sheath consists of almost pure Iβ crystalline cellulose. Due to its high aspect ratio, tunicate cellulose has excellent physical properties. It draws extensive attention in the construction of robust functional materials. However, there is little research on its biological activity. In this study, cellulose enzymatic hydrolysis was conducted on tunicate cellulose. During the hydrolysis, the crystalline behaviors, i.e., crystallinity index (CrI), crystalline size and degree of polymerization (DP), were analyzed on the tunicate cellulose. As comparisons, similar hydrolyses were performed on cellulose samples with relatively low CrI, namely α-cellulose and amorphous cellulose. The results showed that the CrI of tunicate cellulose and α-cellulose was 93.9% and 70.9%, respectively; and after 96 h of hydrolysis, the crystallinity, crystalline size and DP remained constant on the tunicate cellulose, and the cellulose conversion rate was below 7.8%. While the crystalline structure of α-cellulose was significantly damaged and the cellulose conversion rate exceeded 83.8% at the end of 72 h hydrolysis, the amorphous cellulose was completely converted to glucose after 7 h hydrolysis, and the DP decreased about 27.9%. In addition, tunicate cellulose has high anti-mold abilities, owing to its highly crystalized Iβ lattice. It can be concluded that tunicate cellulose has significant resistance to enzymatic hydrolysis and could be potentially applied as anti-biodegradation materials.

Sorption of Methylene Blue for Studying the Specific Surface Properties of Biomass Carbohydrates

The surface area is an important parameter in setting any biorefining technology. The aim of this study was to investigate the applicability of sorption of methylene blue to characterize the surface of the main biomass carbohydrates: α-cellulose, sigmacell cellulose, natural gum, β-glucan, and starch. The morphology of particles of the model objects was studied by scanning electron microscopy. Nitrogen adsorption isotherms demonstrate that the selected carbohydrates are macroporous adsorbents. The monolayer capacities, the energy constants of the Brunauer–Emmett–Teller (BET) equation, and specific surface areas were calculated using the BET theory, the comparative method proposed by Gregg and Sing, and the Harkins–Jura method. The method of methylene blue sorption onto biomass carbohydrates was adapted and mastered. It was demonstrated that sorption of methylene blue proceeds successfully in ethanol, thus facilitating surface characterization for carbohydrates that are either soluble in water or regain water. It was found that the methylene blue sorption values correlate with specific surface area determined by nitrogen adsorption/desorption and calculated from the granulometric data. As a result of electrostatic attraction, the presence of ion-exchanged groups on the analyte surface has a stronger effect on binding of methylene blue than the surface area does. Sorption of methylene blue can be used in addition to gas adsorption/desorption to assess the accessibility of carbohydrate surface for binding large molecules.

Biocatalysis of Industrial Kraft Pulps: Similarities and Differences between Hardwood and Softwood Pulps in Hydrolysis by Enzyme Complex of Penicillium verruculosum

Kraft pulp enzymatic hydrolysis is a promising method of woody biomass bioconversion. The influence of composition and structure of kraft fibers on their hydrolysis efficiency was evaluated while using four substrates, unbleached hardwood pulp (UHP), unbleached softwood pulp (USP), bleached hardwood pulp (BHP), and bleached softwood pulp (BSP). Hydrolysis was carried out with Penicillium verruculosum enzyme complex at a dosage of 10 filter paper units (FPU)/g pulp. The changes in fiber morphology and structure were visualized while using optical and electron microscopy. Fiber cutting and swelling and quick xylan destruction were the main processes at the beginning of hydrolysis. The negative effect of lignin content was more pronounced for USP. Drying decreased the sugar yield of dissolved hydrolysis products for all kraft pulps. Fiber morphology, different xylan and mannan content, and hemicelluloses localization in kraft fibers deeply affected the hydrolyzability of bleached pulps. The introduction of additional xylobiase, mannanase, and cellobiohydrolase activities to enzyme mixture will further improve the hydrolysis of bleached pulps. A high efficiency of never-dried bleached pulp bioconversion was shown. At 10% substrate concentration, hydrolysates with more than 50 g/L sugar concentration were obtained. The bioconversion of never-dried BHP and BSP could be integrated into working kraft pulp mills.

Lignocellulosic Biomass: Understanding Recalcitrance and Predicting Hydrolysis

Lignocellulosic biomass (LB) is an abundant and renewable resource from plants mainly composed of polysaccharides (cellulose and hemicelluloses) and an aromatic polymer (lignin). LB has a high potential as an alternative to fossil resources to produce second-generation biofuels and biosourced chemicals and materials without compromising global food security. One of the major limitations to LB valorisation is its recalcitrance to enzymatic hydrolysis caused by the heterogeneous multi-scale structure of plant cell walls. Factors affecting LB recalcitrance are strongly interconnected and difficult to dissociate. They can be divided into structural factors (cellulose specific surface area, cellulose crystallinity, degree of polymerization, pore size and volume) and chemical factors (composition and content in lignin, hemicelluloses, acetyl groups). Goal of this review is to propose an up-to-date survey of the relative impact of chemical and structural factors on biomass recalcitrance and of the most advanced techniques to evaluate these factors. Also, recent spectral and water-related measurements accurately predicting hydrolysis are presented. Overall, combination of relevant factors and specific measurements gathering simultaneously structural and chemical information should help to develop robust and efficient LB conversion processes into bioproducts.

Enzymatic hydrolysis of cellulosic materials using synthetic mixtures of purified cellulases bioengineered at N-glycosylation sites

Mutant forms of recombinant endoglucanase II (EG II, N194A), cellobiohydrolase I (CBH I, N45A) and cellobiohydrolase II (CBH II, N219A) from Penicillium verruculosum with enhanced cellulase activities, achieved by engineering of enzyme N-glycosylation sites in our previous studies, were used as components of the binary and ternary mixtures of cellulases in hydrolysis of Avicel and milled aspen wood. Using the engineered forms of the enzymes at a dosage of 10 mg/g substrate resulted in significant boosting of the glucose release from cellulose in the presence of excess β-glucosidase relative to the performance of the corresponding wild-type mixtures at the same loading. The boosting effects reached 11-40% depending on the reaction time and substrate type. In hydrolysis of both cellulosic substrates by the binary mixtures of cellulases, all the enzyme pairs exhibited synergism. The magnitude of the synergistic effects (K_s) did not depend notably upon the induced mutations in the enzymes, and they were in the range of 1.3-1.8 for the combinations of EG II with CBH I (or CBH II), and 2.3-2.9 for the CBH I-CBH II pair. The results of this study should provide a basis for the development of a more effective fungal strain capable of producing cellulase cocktails with enhanced hydrolytic performance against lignocellulosic materials.

Characterization of Cellulase-Treated Fibers and Resulting Cellulose Nanocrystals Generated through Acid Hydrolysis

Integrating enzymatic treatment and acid hydrolysis potentially improves the economics of cellulose nanocrystal (CNC) production and demonstrates a sustainable cellulosic ethanol co-generation strategy. In this study, the effect of enzymatic treatment on filter paper and wood pulp fibers, and CNCs generated via subsequent acid hydrolysis were assessed. Characterization was performed using a pulp quality monitoring system, scanning and transmission electron microscopies, dynamic light scattering, X-ray diffraction, and thermogravimetric analysis. Enzymatic treatment partially reduced fiber length, but caused swelling, indicating simultaneous fragmentation and layer erosion. Preferential hydrolysis of less ordered cellulose by cellulases slightly improved the crystallinity index of filter paper fiber from 86% to 88%, though no change was observed for wood pulp fibre. All CNC colloids were stable with zeta potential values below -39 mV and hydrodynamic diameters ranging from 205 to 294 nm. Furthermore, the temperature for the peak rate of CNC thermal degradation was generally not affected by enzymatic treatment. These findings demonstrate that CNCs of comparable quality can be produced from an enzymatically-mediated acid hydrolysis biorefining strategy that co-generates fermentable sugars for biofuel production.

Effect of Different Sugar Beet Pulp Pretreatments on Biogas Production Efficiency

The objective of this study was to determine the effect of different sugar beet pulp (SBP) pretreatments on biogas yield from anaerobic digestion. SBP was subjected to grinding, thermal-pressure processing, enzymatic hydrolysis, or combination of these pretreatments. It was observed that grinding of SBP to 2.5-mm particles resulted in the cumulative biogas productivity of 617.2 mL/g volatile solids (VS), which was 20.2 % higher compared to the biogas yield from the not pretreated SBP, and comparable to that from not ground, enzymatically hydrolyzed SBP. The highest cumulative biogas productivity, 898.7 mL/g VS, was obtained from the ground, thermal-pressure pretreated and enzymatically hydrolyzed SBP. The latter pretreatment variant enabled to achieve the highest glucose concentration (24.765 mg/mL) in the enzymatic hydrolysates. The analysis of energy balance showed that the increase in the number of SBP pretreatment operations significantly reduced the gain of electric energy.

An In-Depth Understanding of Biomass Recalcitrance Using Natural Poplar Variants as the Feedstock

In an effort to better understand the biomass recalcitrance, six natural poplar variants were selected as feedstocks based on previous sugar release analysis. Compositional analysis and physicochemical characterizations of these poplars were performed and the correlations between these physicochemical properties and enzymatic hydrolysis yield were investigated. Gel permeation chromatography (GPC) and ¹³ C  solid state NMR were used to determine the degree of polymerization (DP) and crystallinity index (CrI) of cellulose, and the results along with the sugar release study indicated that cellulose DP likely played a more important role in enzymatic hydrolysis. Simons' stain revealed that the accessible surface area of substrate significantly varied among these variants from 17.3 to 33.2 mg g-1biomass as reflected by dye adsorption, and cellulose accessibility was shown as one of the major factors governing substrates digestibility. HSQC and ³¹ P NMR analysis detailed the structural features of poplar lignin variants. Overall, cellulose relevant factors appeared to have a stronger correlation with glucose release, if any, than lignin structural features. Lignin structural features, such as a phenolic hydroxyl group and the ratio of syringyl and guaiacyl (S/G), were found to have a more convincing impact on xylose release. Low lignin content, low cellulose DP, and high cellulose accessibility generally favor enzymatic hydrolysis; however, recalcitrance cannot be simply judged on any single substrate factor.

Overexpression of a Domain of Unknown Function 266-containing protein results in high cellulose content, reduced recalcitrance, and enhanced plant growth in the bioenergy crop Populus

BACKGROUND

Domain of Unknown Function 266 (DUF266) is a plant-specific domain. DUF266-containing proteins (DUF266 proteins) have been categorized as 'not classified glycosyltransferases (GTnc)' due to amino acid similarity with GTs. However, little is known about the function of DUF266 proteins.

RESULTS

Phylogenetic analysis revealed that DUF266 proteins are only present in the land plants including moss and lycophyte. We report the functional characterization of one member of DUF266 proteins in Populus, PdDUF266A. PdDUF266A was ubiquitously expressed with high abundance in the xylem. In Populus transgenic plants overexpressing PdDUF266A (OXPdDUF266A), the glucose and cellulose contents were significantly higher, while the lignin content was lower than that in the wild type. Degree of polymerization of cellulose in OXPdDUF266A transgenic plants was also higher, whereas cellulose crystallinity index remained unchanged. Gene expression analysis indicated that cellulose biosynthesis-related genes such as CESA and SUSY were upregulated in mature leaf and xylem of OXPdDUF266A transgenic plants. Moreover, PdDUF266A overexpression resulted in an increase of biomass production. Their glucose contents and biomass phenotypes were further validated via heterologous expression of PdDUF266A in Arabidopsis. Results from saccharification treatment demonstrated that the rate of sugar release was increased by approximately 38% in the OXPdDUF266A transgenic plants.

CONCLUSIONS

These results suggest that the overexpression of PdDUF266A can increase cellulose content, reduce recalcitrance, and enhance biomass production, and that PdDUF266A is a promising target for genetic manipulation for biofuel production.

Effect of hydrothermal pretreatment on Miscanthus anaerobic digestion

Miscanthus is a promising source of bioenergy with high lignocellulose content. This paper studied the effect of hydrothermal pretreatment on Miscanthus biogas production. Different pretreatment temperature from 125 to 200°C was tested. After pretreatment, hemicellulose was partially removed and this led to a change in cellulose accessibility. Enzymatic hydrolysis was used to examine the digestibility of different samples. There was no obvious enhancement in low temperature (125 and 150°C) conditions. According to the results, 200°C hydrothermal pretreatment was the optimal condition saving 50% on the digestion time and increasing glucose production 13.2 times compared with the raw material. Although the cellulose crystallinity increased after the pretreatment, its effect on biogas production and enzymatic hydrolysis was limited.

Necessity of enzymatic hydrolysis for production and functionalization of nanocelluloses

Nanocellulose (NC) from cellulosic biomass has recently gained attention owing to their biodegradable nature, low density, high mechanical properties, economic value and renewability. They still suffer, however, some drawbacks. The challenges are the exploration of raw materials, scaling, recovery of chemicals utilized for the production or functionalization and most important is toxic behavior that hinders them from implementing in medical/pharmaceutical field. This review emphasizes the structural behavior of cellulosic biomass and biological barriers for enzyme interactions, which are pertinent to understand the enzymatic hydrolysis of cellulose for the production of NCs. Additionally, the enzymatic catalysis for the modification of solid and NC is discussed. The utility of various classes of enzymes for introducing desired functional groups on the surface of NC has been further examined. Thereafter, a green mechanistic approach is applied for understanding at molecular level.

A critical review on analysis in pretreatment of lignocelluloses: Degree of polymerization, adsorption/desorption, and accessibility

The pretreatment of lignocelluloses results in changes in the different properties of these materials. In a recent review (Karimi and Taherzadeh, 2016), the details of compositional, imaging, and crystallinity analyses of lignocelluloses were reviewed and critically discussed. Changes in the cellulose degree of polymerization, accessibility, and enzyme adsorption/desorption by pretreatments are also among the effective parameters. This paper deals with the measurement techniques, modifications, and relation to bioconversions, as well as the challenges of these three properties. These analyses are very helpful to investigate the pretreatment processes; however, the pretreatments are very complicated and challenging processes. It is not easily possible to study the effects of only one of these parameters and even to find which one is the dominant one. Moreover, it is not possible to accurately predict the changes in the bioconversion yield using these methods.

Saccharification of pretreated sawdust by Aspergillus niger cellulase

The efficiency of two methods of pretreatment (NaOH and H₂O₂) on lignocelluloses—saw dust, wheat straw, sugarcane bagasse and rice bran—was compared in the present study. Alkali treatment of lignocelluloses relatively removed more hemicelluloses and lignin leaving behind cellulose content in the residues than peroxide treatment. Crude cellulase of Aspergillus niger, produced on the pretreated sawdust with highest cellulose content, was further tested for the release of soluble and reducing sugars during the saccharification process of same pretreated saw dust. The saccharification process of the pretreated sawdust with enzyme was optimized for pH, temperature, and substrate concentration and proceeded optimally at pH of 5.0, 50 °C and 0.5 % pretreated sawdust. The rate of saccharification with crude enzyme of A. niger on alkali-treated sawdust was found to be maximum (23 %) as against 5.4 % on native sawdust under optimal conditions after 48 h. The present study indicates NaOH-treated sawdust as a potential raw material for both production of cellulase and saccharification in a large scale.

Progressive structural changes of Avicel, bleached softwood, and bacterial cellulose during enzymatic hydrolysis

A comprehensive picture of structural changes of cellulosic biomass during enzymatic hydrolysis is essential for a better understanding of enzymatic actions and development of more efficient enzymes. In this study, a suite of analytical techniques including sum frequency generation (SFG) spectroscopy, infrared (IR) spectroscopy, x-ray diffraction (XRD), and x-ray photoelectron spectroscopy (XPS) were employed for lignin-free model biomass samples—Avicel, bleached softwood, and bacterial cellulose—to find correlations between the decrease in hydrolysis rate over time and the structural or chemical changes of biomass during the hydrolysis reaction. The results showed that the decrease in hydrolysis rate over time appears to correlate with the irreversible deposition of non-cellulosic species (either reaction side products or denatured enzymes, or both) on the cellulosic substrate surface. The crystallinity, degree of polymerization, and meso-scale packing of cellulose do not seem to positively correlate with the decrease in hydrolysis rate observed for all three substrates tested in this study. It was also found that the cellulose Iα component of the bacterial cellulose is preferentially hydrolyzed by the enzyme than the cellulose Iβ component.

Relationship between physicochemical properties and enzymatic hydrolysis of sugarcane bagasse varieties for bioethanol production

The structural and physicochemical characteristics are associated with resistance of plant cell walls to saccharification by enzymes. The effect of physicochemical properties on glucose yield of bagasse from different varieties of sugarcane at low and high enzyme dosages was investigated. The result showed that glucose yield at low enzyme dosage was positively linear correlated with the yield at high enzyme dosage, for both the untreated and pretreated materials. The pretreatment significantly increased the accessibility of substrates by enzyme due to the increase of internal and external surface area. Glucose yield also showed a linear correlation with dye adsorption. However, the increase in glucose yield as a result of pretreatment did not correlate with the increases in crystallinity index and decreases in degree of polymerization. The Principal Component Analysis of infrared data indicated that lignin was the main component that differentiated the varieties before and after pretreatment. These results suggested that the key differences in pretreatment responses among varieties could be mainly attributed to their differences in the internal and external surface area after pretreatment.

Revisiting size-exclusion chromatography for measuring structural changes in raw and pretreated mixed hardwoods and switchgrass

The study of the biomass porous structure and its role in defining the accessibility of cell-wall-degrading enzymes (CWDEs) to the substrate is very important for understanding the cellulase-cellulose reaction system. Specific pore volume and specific surface area are two important measures of accessibility and a variety of methods have been used to make these measurements. For this study a size exclusion chromatography system was developed to measure specific pore volume and specific surface areas for raw and pretreated mixed-hardwood and switchgrass. Polyethylene glycol (PEG) probes of known molecular diameter (1.8-13 nm) were allowed to diffuse into the pore structure of the various biomass substrate packed in the column and subsequently eluted to generate high resolution concentration measurements with excellent reproducibility. Replicate measurements of probe concentrations from this system consistently yielded coefficient of variance of less than 1.5%. Our results showed that particle size reduction had a smaller influence on the specific pore volume distribution of raw mixed-hardwoods, whereas for switchgrass the larger particles yielded a significantly lower estimate for the pore volume distribution compared to the smaller particles. Our results also clearly showed that our bi-phasic pretreatment yielded the largest increase in pore volume accessibility for mixed-hardwoods relative to switchgrass. From these results a pore size change mechanism was proposed that could explain the influence of size reduction and pretreatment on pore volume measurements.

Structural insights into rice straw pretreated by hot-compressed water in relation to enzymatic hydrolysis

Pretreatment-induced structural alteration is critical in influencing the rate and extent of enzymatic saccharification of lignocellulosic biomass. The present work has investigated structural features of rice straw pretreated by hot-compressed water (HCW) from 140 to 240 °C for 10 or 30 min and enzymatic hydrolysis profiles of pretreated rice straw. Compositional profiles of pretreated rice straw were examined to offer the basis for structural changes. The wide-angle X-ray diffraction analysis revealed possible modification in crystalline microstructure of cellulose and the severity-dependent variation of crystallinity. The specific surface area (SSA) of pretreated samples was able to achieve more than 10-fold of that of the raw material and was in linear relationship with the removal of acetyl groups and xylan. The glucose yield by enzymatic hydrolysis of pretreated materials correlated linearly with the SSA increase and the dissolution of acetyl and xylan. A quantitatively intrinsic relationship was suggested to exist between enzymatic hydrolysis and the extraction of hemicellulose components in hydrothermally treated rice straw, and SSA was considered one important structural parameter signaling the efficiency of enzymatic digestibility in HCW-treated materials in which hemicellulose removal and lignin redistribution happened.

Factors affecting cellulose hydrolysis based on inactivation of adsorbed enzymes

The rate of enzymatic hydrolysis of cellulose reaction is known to decrease significantly as the reaction proceeds. Factors such as reaction temperature, time, and surface area of substrate that affect cellulose conversion were analyzed relative to their role in a mechanistic model based on first order inactivation of adsorbed cellulases. The activation energies for the hydrolytic step and inactivation step were very close in magnitude: 16.3 kcal mol(-1) for hydrolysis and 18.0 kcal mol(-1) for inactivation, respectively. Therefore, increasing reaction temperature would cause a significant increase in the inactivation rate in addition to the catalytic reaction rate. Vmax,app was only 20% or less of the value at 72 h compared to at 2h as a result of inactivation of adsorbed cellulases, suggesting prolonged hydrolysis is not an efficient way to improve cellulose hydrolysis. Hydrolysis rate increased with corresponding increases in available substrate surface binding area.

Relative extents of activity loss between enzyme-substrate interactions and combined environmental mechanisms

Enzymatic hydrolysis of biomass undergoes a significant decrease in rate, which is often attributed to activity loss of enzyme during the incubation. Activity loss due to both interaction with substrate (for example inactivation of adsorbed enzyme) and all combined environmental mechanisms in a substrate free buffer solution were compared in this study. Enzyme-substrate interactions contributed more towards the overall activity loss than did the combined environmental sources as evidenced from three independent metrics. (1) Relative extents of inactivation were higher for enzyme-substrate interactions than for environmental mechanisms. (2) Apparent half-lives (1.37-11.01 h) following interaction with substrate were relatively small compared to environmental inactivation, which was 21.5h. (3) The inactivation rate constant for enzyme-substrate interactions (0.56 h(-1)) was 46 times higher than that of environmental inactivation (0.0123 h(-1)). These results suggest enzyme-substrate interaction is the main cause of cellulase activity loss and contributes significantly to the slow rate of hydrolysis.

Microbial ecology of anaerobic digesters: the key players of anaerobiosis

Anaerobic digestion is the method of wastes treatment aimed at a reduction of their hazardous effects on the biosphere. The mutualistic behavior of various anaerobic microorganisms results in the decomposition of complex organic substances into simple, chemically stabilized compounds, mainly methane and CO₂. The conversions of complex organic compounds to CH₄ and CO₂ are possible due to the cooperation of four different groups of microorganisms, that is, fermentative, syntrophic, acetogenic, and methanogenic bacteria. Microbes adopt various pathways to evade from the unfavorable conditions in the anaerobic digester like competition between sulfate reducing bacteria (SRB) and methane forming bacteria for the same substrate. Methanosarcina are able to use both acetoclastic and hydrogenotrophic pathways for methane production. This review highlights the cellulosic microorganisms, structure of cellulose, inoculum to substrate ratio, and source of inoculum and its effect on methanogenesis. The molecular techniques such as DGGE (denaturing gradient gel electrophoresis) utilized for dynamic changes in microbial communities and FISH (fluorescent in situ hybridization) that deal with taxonomy and interaction and distribution of tropic groups used are also discussed.

Mechanical pretreatments of lignocellulosic biomass: towards facile and environmentally sound technologies for biofuels production

The transformation of lignocellulosic biomass into biofuels represents an interesting and sustainable alternative to fossil fuel for the near future.

Adsorption of proteins involved in hydrolysis of lignocellulose on lignins and hemicelluloses

Protein adsorption onto eight lignocellulosic substances (six lignin preparations and two hemicelluloses) was investigated at pH 4.8 and at two different temperatures (4°C and 45°C). The kinetics of the adsorption of cellulase, xylanase, and β-glucosidase were determined by enzyme activity measurements. The maximum adsorption capacities, the affinity constants and the binding strengths varied widely and were typically higher for the lignins than for the carbohydrates. As indicated by BET and gel permeation chromatography, different substances had widely different surface area, pore size, weight average molecular weight, and polydispersity index, but these properties were difficult to relate to protein binding. In most cases, an increase in temperature from 4°C to 45°C and a low content of carboxylic acid groups, as indicated by Fourier-Transform Infra-Red (FTIR) spectroscopy, resulted in increased protein adsorption capacity, which suggests that hydrophobic interactions play an important role.

Glycerol carbonate as green solvent for pretreatment of sugarcane bagasse

BACKGROUND

Pretreatment of lignocellulosic biomass is a prerequisite for effective saccharification to produce fermentable sugars. In this study, "green" solvent systems based on acidified mixtures of glycerol carbonate (GC) and glycerol were used to treat sugarcane bagasse and the roles of each solvent in deconstructing biomass were determined.

RESULTS

Pretreatment of sugarcane bagasse at 90°C for only 30 min with acidified GC produced a solid residue having a glucan digestibility of 90% and a glucose yield of 80%, which were significantly higher than a glucan digestibility of 16% and a glucose yield of 15% obtained for bagasse pretreated with acidified ethylene carbonate (EC). Biomass compositional analyses showed that GC pretreatment removed more lignin than EC pretreatment (84% vs 54%). Scanning electron microscopy (SEM) showed that fluffy and size-reduced fibres were produced from GC pretreatment whereas EC pretreatment produced compact particles of reduced size. The maximal glucan digestibility and glucose yield of GC/glycerol systems were about 7% lower than those of EC/ethylene glycol (EG) systems. Replacing up to 50 wt% of GC with glycerol did not negatively affect glucan digestibility and glucose yield. The results from pretreatment of microcrystalline cellulose (MCC) showed that (1) pretreatment with acidified alkylene glycol (AG) alone increased enzymatic digestibility compared to pretreatments with acidified alkylene carbonate (AC) alone and acidified mixtures of AC and AG, (2) pretreatment with acidified GC alone slightly increased, but with acidified EC alone significantly decreased, enzymatic digestibility compared to untreated MCC, and (3) there was a good positive linear correlation of enzymatic digestibility of treated and untreated MCC samples with congo red (CR) adsorption capacity.

CONCLUSIONS

Acidified GC alone was a more effective solvent for pretreatment of sugarcane bagasse than acidified EC alone. The higher glucose yield obtained with GC-pretreated bagasse is possibly due to the presence of one hydroxyl group in the GC molecular structure, resulting in more significant biomass delignification and defibrillation, though both solvent pretreatments reduced bagasse particles to a similar extent. The maximum glucan digestibility of GC/glycerol systems was less than that of EC/EG systems, which is likely attributed to glycerol being less effective than EG in biomass delignification and defibrillation. Acidified AC/AG solvent systems were more effective for pretreatment of lignin-containing biomass than MCC.

Advancing energy cane cell wall digestibility screening by near-infrared spectroscopy

Breeding energy cane for cellulosic biofuel production involves manipulating various traits. An important trait to optimize is cell wall degradability as defined by enzymatic hydrolysis. We investigated the feasibility of using near-infrared spectroscopy (NIRS) combined with multivariate calibration to predict energy cane cell wall digestibility based upon fiber samples from a range of sugarcane genotypes and related species. These samples produced digestibility values ranging between 6 and 31%. To preserve the practicality of the technique, spectra obtained from crudely prepared samples were used. Various spectral pre-processing methods were tested, with the best NIRS calibration obtained from second derivative, orthogonal signal-corrected spectra. Model performance was evaluated by cross-validation and independent validation. Large differences between the performance results from the two validation approaches indicated that the model was sensitive to the choice of test data. This may be remedied by using a larger calibration training set containing diverse sample types. The best result was obtained through independent validation which produced a R(2) value of 0.86, a root mean squared error of prediction (RMSEP) of 1.59, and a ratio of prediction to deviation (RPD) of 2.7. This study has demonstrated that it is feasible and practical to use NIRS to predict energy cane cell wall digestibility.

Specific effects of fiber size and fiber swelling on biomass substrate surface area and enzymatic digestibility

To clarify the specific effect of biomass substrate surface area on its enzymatic digestibility, factors of fiber size reduction and swelling changes were investigated by using poplar substrates with controlled morphological and chemical properties after modified chemical pulping. Results showed that fiber size changes had insignificant influence on enzymatic hydrolysis, although the external surface area increased up to 41% with the reduction of fiber size. Swelling changes caused by increased biomass fiber porosities after PFI refining showed a significant influence on the efficiency of enzymatic hydrolysis. It is also found that chemical properties such as xylan and lignin content can influence the swelling effect. Xylan is confirmed to facilitate substrate hydrolysability by swelling, while lignin restricts swelling effect and thus minimizes the enzyme accessibility to substrates.

Effect of the structural features of hydrochloric acid-deamidated wheat gluten on its susceptibility to enzymatic hydrolysis

The effect of the structural features of hydrochloric acid-deamidated wheat gluten with different degrees of deamidation (DDs) on the susceptibility to enzymatic hydrolysis by pancreatin was investigated. The wheat gluten deamidated by hydrochloric acid with a DD of 55% revealed the highest susceptibility to enzymatic hydrolysis as evaluated by the hydrolysis degree and nitrogen solubility index of the hydrolysates. An increase of peptides with MW below 3000 Da was observed as the DD increased. Raman spectra in the 1740-1800 cm⁻¹ and 521-530 cm⁻¹ range suggested that wheat gluten had taken off the deamidation with different DDs and that the disulfide bond had disrupted the sulfhydryl groups with different intensities, respectively. Results from the deconvolution of the amide I region of FTIR spectra in the 1600-1700 cm⁻¹ range showed that the content of the α-helix decreased and that the content of the β-turn and β-sheet increased with increasing DDs, which improved the molecular structure and flexibility of wheat gluten. A scanning electron microscope (SEM) revealed that the image of HDG-55% presented the smoothest surface and the least uniform pore, enabling the sample to be more susceptible to enzymatic hydrolysis. The above information will enable us to better understand the effect of structure on the susceptibility of deamidated wheat gluten.

Dry fractionation process as an important step in current and future lignocellulose biorefineries: a review

The use of lignocellulosic biomass is promising for biofuels and materials and new technologies for the conversion need to be developed. However, the inherent properties of native lignocellulosic materials make them resistant to enzymatic and chemical degradation. Lignocellulosic biomass requires being pretreated to change the physical and chemical properties of lignocellulosic matrix in order to increase cell wall polymers accessibility and bioavailability. Mechanical size reduction may be chemical free intensive operation thanks to decreasing particles size and cellulose crystallinity, and increasing accessible surface area. Changes in these parameters improve the digestibility and the bioconversion of lignocellulosic biomass. However, mechanical size reduction requires cost-effective approaches from an energy input point of view. Therefore, the energy consumption in relation to physicochemical properties of lignocellulosic biomass was discussed. Even more, chemical treatments combined with physicochemical size reduction approaches are proposed to reduce energy consumption in this review.

Three lignocellulose features that distinctively affect biomass enzymatic digestibility under NaOH and H2SO4 pretreatments in Miscanthus

In this study, total 80 typical Miscanthus accessions were examined with diverse lignocellulose features, including cellulose crystallinity (CrI), degree of polymerization (DP), and mole number (MN). Correlation analysis revealed that the crude cellulose CrI and MN, as well as crystalline cellulose DP, displayed significantly negative influence on biomass enzymatic digestibility under pretreatments with NaOH or H(2)SO(4) at three concentrations. By contrast, the comparative analysis of two Miscanthus samples with similar cellulose contents showed that crude cellulose DP and crystalline cellulose MN were positive factors on biomass saccharification, indicating cross effects among the cellulose levels and the three cellulose features. The results can provide insights into mechanism of the lignocellulose enzymatic digestion, and also suggest potential approaches for genetic engineering of bioenergy crops.

Bioenergy grass feedstock: current options and prospects for trait improvement using emerging genetic, genomic, and systems biology toolkits

For lignocellulosic bioenergy to become a viable alternative to traditional energy production methods, rapid increases in conversion efficiency and biomass yield must be achieved. Increased productivity in bioenergy production can be achieved through concomitant gains in processing efficiency as well as genetic improvement of feedstock that have the potential for bioenergy production at an industrial scale. The purpose of this review is to explore the genetic and genomic resource landscape for the improvement of a specific bioenergy feedstock group, the C4 bioenergy grasses. First, bioenergy grass feedstock traits relevant to biochemical conversion are examined. Then we outline genetic resources available bioenergy grasses for mapping bioenergy traits to DNA markers and genes. This is followed by a discussion of genomic tools and how they can be applied to understanding bioenergy grass feedstock trait genetic mechanisms leading to further improvement opportunities.

Investigation of the susceptibility of acid-deamidated wheat gluten to in vitro enzymatic hydrolysis using Raman spectra and free amino acid analysis

BACKGROUND

The number and surface nature of amino acids (AAs) in substrate proteins available to hydrolytic enzymes are critical. Among them, the micro-environmental properties of specific AAs in substrates before hydrolysis would probably dominate the susceptibility of substrates to enzymatic hydrolysis. Fundamental knowledge concerning this regard is lacking. The objective of this work was to investigate the relationship between the exposure level of AAs in acid-deamidated wheat gluten and their susceptibilities to in vitro enzymatic hydrolysis by pancreatin through both high-performance liquid chromatography and Raman spectra. Wheat gluten deamidated with HCl (HDWG), citric acid (CDWG), succinic acid (SDWG) and acetic acid (ADWG) at the same degree of deamidation under the same heat treatment were chosen as the substrates. Substrate characterisations including degree of hydrolysis, surface hydrophobicity and structural characteristics before hydrolysis, together with analysis of free AAs of the corresponding hydrolysates during hydrolysis, were investigated.

RESULTS

Hydrolysates from SDWG had the highest value for the degree of hydrolysis. The susceptibility of CDWG to pancreatin hydrolysis was the lowest, lower than native wheat gluten (CK) after the initial 36 h. Compared with free AAs, the mole increase profiles of CK, Arg production levelled off in HDWG after 12 h whereas it was inhibited in ADWG. For SDWG, Arg release was dramatically inhibited after 12 h and was replaced by Trp. Investigations using Raman spectra of the micro-environment of Cys, Trp, Tyr and His and the mole increase trend of them indicated that the exposure level of these amino acids in substrates was positively related to their susceptibilities to pancreatin hydrolysis especially after 24 h of hydrolysis.

CONCLUSION

Deamidation by four acids has a distinct influence on the structural characteristics of wheat gluten substrates. Although the substrates were selected at the same level of deamidation by the same heat treatment, their resultant conformational differences significantly influenced the exposure level of amino acids for binding to enzymes and the susceptibility of substrates to in vitro enzymatic hydrolysis. Therefore, it had an influence on changing enzyme cutting sites of pancreatin. This information will provide a better understanding of specific behaviour of AAs in wheat gluten during enzymatic hydrolysis from a new perspective.

Fibre size does not appear to influence the ease of enzymatic hydrolysis of organosolv-pretreated softwoods

To determine the effect of fibre size on enzymatic hydrolysis, organosolv-pretreated lodgepole pine was size-fractionated into six substrates ranging in average size from 0.20 to 3.4mm. Other than the fines fraction (<0.2mm) which contained most of the lignin, the fractionated substrates were more readily hydrolyzed than the original substrate with nearly complete hydrolysis after 72 h at 5 FPU g(-1) cellulose. Surprisingly, fibre size was found to have little influence on enzymatic hydrolysis likely due to similarities in the substrates' chemical composition, accessible surface area, cellulose crystallinity and degree of polymerization. To determine the influence of the fines on enzymatic hydrolysis, their content was artificially increased (from 8.9% to 55.4%) however; this did not have a noticeable effect. These results show that within the range of fibre sizes tested, other substrate characteristics likely play a more significant role in the ease of hydrolysis of pretreated substrates.