Impact of hydrothermal pre-treatment to chemical composition, enzymatic digestibility and spatial distribution of cell wall polymers

Antibacterial Effect of Phenolic Acids Derived from Rice Straw and in Combination with Antibiotics Against Escherichia coli

Many studies have demonstrated that natural plant extracts have inhibitory effects on microorganisms. The purpose of this study was to investigate the inhibitory effect of phenolic acids from rice straw (PAs) on Escherichia coli and their synergistic effect in combination with antibiotics. PAs can inhibit the growth of E. coli effectively by inducing the formation of H₂O₂; PA-treated cells had a tenfold greater intracellular H₂O₂ concentration than the control group. The synergistic effect caused by the interaction of PAs and antibiotics on inhibiting the growth of E. coli was significant. This effect may be caused by a PA-induced change in the permeability of E. coli cell membrane. The treatment with PAs made the extracellular K⁺ concentration reached 15 mg/L within 30 min, while the K⁺ concentration in the control group was very low and did not change significantly over time. Similarly to the extracellular K⁺, the extracellular protein concentration exceeded 150 mg/L in the PA treatment group, while it remained very low in the control group. Due to the increased cell permeability, more antibiotics can enter the cell. Hence, this study may provide a novel method of improving the safe use of antibiotics.

Organosolv and ionosolv processes for autohydrolyzed poplar fractionation: Lignin recovery and characterization

Biomass fractionation plays a major role in the search for competitive biorefineries, where the isolation and recovery of the three woody fractions is key. In this sense, we have used autohydrolyzed hemicellulose-free poplar as feedstock to compare two fractionation processes, organosolv and ionosolv, oriented to lignin recovery. The recovered lignins were then characterize by different techniques (NMR, GPC, TGA). Both treatments were tested at different temperatures to analyze temperature influence on lignin recovery and properties. The highest lignin recovery was obtained with the ionosolv process at 135 °C, reaching a solid yield of ~70%. Lignin characterization showed differences between both treatments. Lignins enriched in C-O linkages and G units were recovered with the organosolv process, where increasing temperature led to highly depolymerized lignins. However, lignins with higher C-C linkages and S units contents were obtained with the ionosolv process, producing more thermically stable lignins. In addition, increasing temperature caused lignin repolymerization when employing ionic liquids as solvents. Therefore, this work outlines the most important differences between ionosolv and organosolv processes for biomass fractionation, focusing on lignin recovery and its properties, which is the first step in order to valorize all biomass fractions.

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.

Ultrastructural change in lignocellulosic biomass during hydrothermal pretreatment

In recent years, visualization and characterization of lignocellulose at different scales elucidate the modifications of its ultrastructural and chemical features during hydrothermal pretreatment which include degradation and dissolving of hemicelluloses, swelling and partial hydrolysis of cellulose, melting and redepositing a part of lignin in the surface. As a result, cell walls are swollen, deformed and de-laminated from the adjacent layer, lead to a range of revealed droplets that appear on and within cell walls. Moreover, the certain extent morphological changes significantly promote the downstream processing steps, especially for enzymatic hydrolysis and anaerobic fermentation to bioethanol by increasing the contact area with enzymes. However, the formation of pseudo-lignin hinders the accessibility of cellulase to cellulose, which decreases the efficiency of enzymatic hydrolysis. This review is intended to bridge the gap between the microstructure studies and value-added applications of lignocellulose while inspiring more research prospects to enhance the hydrothermal pretreatment process.

Engineering aspects of hydrothermal pretreatment: From batch to continuous operation, scale-up and pilot reactor under biorefinery concept

Different pretreatments strategies have been developed over the years mainly to enhance enzymatic cellulose degradation. In the new biorefinery era, a more holistic view on pretreatment is required to secure optimal use of the whole biomass. Hydrothermal pretreatment technology is regarded as very promising for lignocellulose biomass fractionation biorefinery and to be implemented at the industrial scale for biorefineries of second generation and circular bioeconomy, since it does not require no chemical inputs other than liquid water or steam and heat. This review focuses on the fundamentals of hydrothermal pretreatment, structure changes of biomass during this pretreatment, multiproduct strategies in terms of biorefinery, reactor technology and engineering aspects from batch to continuous operation. The treatise includes a case study of hydrothermal biomass pretreatment at pilot plant scale and integrated process design.

Physicochemical characteristics and pyrolysis performance of corn stalk torrefied in aqueous ammonia by microwave heating

The physicochemical characteristics and pyrolysis performance of corn stalk (CS) torrefied in water and aqueous ammonia by microwave heating were investigated. Physicochemical characterization revealed that both microwave water torrefied CS (MCS) and microwave ammonia torrefied CS (MACS) showed low hemicellulose content, disrupted macrostructure, improved porous properties, and low ash content. MACS exhibited a significantly lower crystallinity degree of 44.34% than CS (79.55%) and MCS (89.50%). MACS also showed increased methyl/methylene groups intensity, and complete acetyl groups disrupture. Pyrolysis coupled with gas chromatography/mass spectrometry (Py-GC/MS) revealed that compared with CS and MCS, MACS exhibited higher peak areas for ketones, aldehydes, furans and esters, and significantly lower peak areas for acids and phenols. A possible mechanism was proposed for the effects of wet torrefaction with aqueous ammonia on changes in physicochemical structure and pyrolysis behavior of corn stalk.

Pretreatment of wheat straw using basic ethanolamine-based deep eutectic solvents for improving enzymatic hydrolysis

A series of ethanolamine based deep eutectic solvents (DESs), which have strong basicity, were firstly applied in wheat straw pretreatment. Typically, choline chloride: monoethanolamine (C:M) as the best solvent among these DESs can remove 71.4% lignin and reserve 93.7% cellulose (70 °C, L/S mass ratio of 20:1, 9 h), and improve the enzymatic hydrolysis of residue, i.e., 89.8% cellulose and 62.0% xylan conversion. The pretreatment capacity of C:M is comparable to other solvents while C:M has several advantages, e.g., lower cost with cheap materials and simpler preparation process, mild conditions and lower polysaccharide loss. The XRD, SEM and FT-IR results verified that the polysaccharide conversion and sugars yield were enhanced by the removal of lignin in the pretreatment process. The basic ethanolamine based DESs are promising solvents for industrial application of wheat straw pretreatment.

Variation across a wheat genetic diversity panel for saccharification of hydrothermally pretreated straw

BACKGROUND

Wheat straw forms an important, reliable source of lignocellulosic biomass for use in second-generation ethanol production. However, there is limited understanding of the variation in quality of straw from current breeding cultivars, and studies on such variation have generally employed suboptimal pretreatments. There is also a degree of confusion regarding phenotypic characteristics relevant to optimising the enzymatic saccharification of cellulose after suitable pretreatments for biorefining compared with those which determine good ruminant digestibility. The aim of this study has been to (a) evaluate and compare the levels of glucose enzymatically released from straw obtained from 89 cultivars of winter wheat after optimised hydrothermal pretreatments and (b) identify the underlying phenotypic characteristics relevant to enhanced glucose production with special reference to the ratios of constituent tissue types.

RESULTS

Optimised pretreatment involved hydrothermal extraction at 210 °C for 10 min. Using excess cellulases, quantitative saccharification was achieved within 24 h. The amount of glucose released ranged from 192 to 275 mg/g. The extent of glucose release was correlated with (a) the level of internode tissue (R = 0.498; p = 6.84 × 10^-7), (b) stem height (R = 0.491; p = 1.03 × 10^-6), and (c) chemical characteristics particular to stem tissues including higher levels of cellulose (R = 0.552; p = 2.06 × 10^-8) and higher levels of lignin R = 0.494; p = 8.67 × 10^-7.

CONCLUSIONS

In order to achieve maximum yields of cellulosic glucose for second-generation ethanol production, a predisposition for wheat to produce cellulose-enriched internode stem tissue, particularly of longer length, would be beneficial. This contrasts with the ideotype for ruminant nutrition, in which an increased proportion of leaf tissue is preferable.

Fractionation of rapeseed straw by hydrothermal/dilute acid pretreatment combined with alkali post-treatment for improving its enzymatic hydrolysis

The aim of the research was to evaluate the effect of combined treatments on fermentable sugar production from rapeseed straw. An optimum condition was found to be the combination of hydrothermal pretreatment at 180°C for 45min and post-treatment by 2% NaOH at 100°C for 2h, which was based on the quantity of monosaccharides released during enzymatic hydrolysis. As compared with the raw material without treatment, the combination of hydrothermal pretreatment and alkali post-treatment resulted in a significant increase of the saccharification rate by 5.9times. This process potentially turned rapeseed straw into value added products in accordance with the biorefinery concept.

Tissue specific response of Miscanthus×giganteus to dilute acid pretreatment for enhancing cellulose digestibility

The recalcitrance in grasses varies according to cell type and tissue. In this study, dilute acid pretreatment was performed on Miscanthus×giganteus internodes that include rind and pith regions which showing heterogeneous structural and chemical changes. Pretreatment on pith effectively hydrolyzed 73.33% hemicelluloses and separated cohesive cell walls from the compound middle lamella due to lignin migration. Lignin droplets with an average diameter of 49.5±29.3nm were concurrently coalesced on wall surface, that in turn exposed more microfibrils deep in walls to be enzymatically hydrolyzed reaching 82.55%. By contrast, the rind with a relatively intergrated cell structure was covered by larger lignin droplets (101.2±44.1nm) and filled with inaccessible microfibrils limiting enzymatic sacchrification (31.50%). Taken together, the cellulose digestibility of biomass was not majorly influenced by cellulose crystallinity, while it was strongly correlated with the positive effects of hemicelluloses degradation, lignin redistribution, cellulose exposure and loosening cell wall structure.

Effect of pretreatment severity in continuous steam explosion on enzymatic conversion of wheat straw: Evidence from kinetic analysis of hydrolysis time courses

Focusing on continuous steam explosion, the influence of pretreatment severity due to varied acid loading on hydrolysis of wheat straw by Trichoderma reesei cellulases was investigated based on kinetic evaluation of the saccharification of each pretreated substrate. Using semi-empirical descriptors of the hydrolysis time course, key characteristics of saccharification efficiency were captured in a quantifiable fashion. Not only hydrolysis rates per se, but also the transition point of their bi-phasic decline was crucial for high saccharification degree. After 48h the highest saccharification was achieved for substrate pretreated at relatively low severity (1.2% acid). Higher severity increased enzyme binding to wheat straw, but reduced the specific hydrolysis rates. Higher affinity of the lignocellulosic material for cellulases does not necessarily result in increased saccharification, probably because of lignin modifications occurring at high pretreatment severities. At comparable severity, continuous pretreatment produced a substrate more susceptible to enzymatic hydrolysis than the batch process.

The mechanism of xylans removal during hydrothermal pretreatment of poplar fibers investigated by immunogold labeling

Hydrothermal pretreatment initially removed the lignin-free xylan from the middle layer of secondary wall, followed by the lignin-bound xylan, but the cellulose-bound xylan was seldom removed by this pretreatment. An in-depth understanding of the mechanism of xylan removal during hydrothermal pretreatment (HTP) of wood is critical for cost-effective conversion of lignocellulosic biomass to biofuels. Several studies demonstrated the kinetics and mechanism of xylan removal during HTP on molecular scale, but the dissolution mechanism of xylan during HTP remains unclear at ultra-structural level. Our study investigated changes in the micro-distribution of xylan in poplar fiber cell walls during HTP by transmission electron microscopy (TEM) in combination with immunogold labeling. The study revealed that HTP caused greater decline in the density of xylan labeling in the S2 layer of fiber wall than in the S1 layer. There was a greater loss in the density of xylan labeling during HTP in the delignified and enzymatically treated fibers compared to untreated fibers. We propose that in the initial stages of HTP lignin-free xylan in the S2 layer was more readily hydrolyzed than in the S1 layer by hydronium ions. With increasing pretreatment time, the xylan covalently bound to lignin was also removed from the S2 layer due to the dissolution of lignin. The xylan tightly bound to cellulose was seldom removed during HTP, but was hydrolyzed in subsequent enzymatic treatment. This TEM-immunolabeling investigation reveals the manner in which different xylan fractions are removed from fiber cell wall during HTP, and we expect the information to be helpful in developing processes tailored for more effective conversion of cellulosic biomass into fermentable sugars.

Condensed lignin structures and re-localization achieved at high severities in autohydrolysis of Eucalyptus globulus wood and their relationship with cellulose accessibility

Eucalyptus globulus wood was subjected to autohydrolysis pretreatment at different severity factors. The pretreated materials were enzymatically saccharified at a substrate load of 10% (w/v) using a cellulase enzyme complex. Around 82-95% of original glucans were retained in the pretreated material, and the enzymatic hydrolysis yields ranged from 58% to 90%. The chemical and structural changes in the pretreated materials were investigated by microscopic (SEM, LSCM) and spectroscopic (2D-HSQC NMR and FT-IR) techniques. 2D-NMR results showed a reduction in the amounts of β-O-4 aryl-ether linkages and suggested the presence of newly condensed structures of lignin in the biomass pretreated at the more severe conditions. Furthermore, the microscopic analysis showed that lignin migrates out of the cell wall and re-deposits in certain regions of the fibers at the more severe conditions to form droplet-like structures and expose the cellulose surface. These changes improved the glucose yield up to 69%, on dry wood basis.

Visualization of Miscanthus × giganteus cell wall deconstruction subjected to dilute acid pretreatment for enhanced enzymatic digestibility

BACKGROUND

The natural recalcitrance of lignocellulosic plant cell walls resulting from complex arrangement and distribution of heterogeneous components impedes deconstruction of such cell walls. Dilute acid pretreatment (DAP) is an attractive method to overcome the recalcitrant barriers for rendering enzymatic conversion of polysaccharides. In this study, the internodes of Miscanthus × giganteus, a model bioenergy crop, were subjected to DAP to yield a range of samples with altered cell wall structure and chemistry. The consequent morphological and compositional changes and their possible impact on saccharification efficiency were comprehensively investigated. The use of a series of microscopic and microspectroscopic techniques including fluorescence microscopy (FM), transmission electron microscopy (TEM) and confocal Raman microscopy (CRM)) enabled correlative cell wall structural and chemical information to be obtained.

RESULTS

DAP of M. × giganteus resulted in solubilization of arabinoxylan and cross-linking hydroxycinnamic acids in a temperature-dependent manner. The optimized pretreatment (1% H2SO4, 170°C for 30 min) resulted in significant enhancement in the saccharification efficiency (51.20%) of treated samples in 72 h, which amounted to 4.4-fold increase in sugar yield over untreated samples (11.80%). The remarkable improvement could be correlated to a sequence of changes occurring in plant cell walls due to their pretreatment-induced deconstruction, namely, loss in the matrix between neighboring cell walls, selective removal of hemicelluloses, redistribution of phenolic polymers and increased exposure of cellulose. The consequently occurred changes in inner cell wall structure including damaging, increase of porosity and loss of mechanical resistance were also found to enhance enzyme access to cellulose and further sugar yield.

CONCLUSIONS

DAP is a highly effective process for improving bioconversion of cellulose to glucose by breaking down the rigidity and resistance of cell walls. The combination of the most relevant microscopic and microanalytical techniques employed in this work provided information crucial for evaluating the influence of anatomical and compositional changes on enhanced enzymatic digestibility.

Comparison of the effect of wet and dry torrefaction on chemical structure and pyrolysis behavior of corncobs

Wet and dry torrefaction of corncobs was conducted in high-pressure reactor and tube-type reactor, respectively. Effect of wet and dry torrefaction on chemical structure and pyrolysis behavior of corncobs was compared. The results showed that hemicellulose could be effectively removed from corncobs by torrefaction. However, dry torrefaction caused severe degradation of cellulose and the cross-linking and charring of corncobs. X-ray diffraction analysis revealed that crystallinity degree of corncobs was evidently enhanced during wet torrefaction, but reduced during dry torrefaction as raising treatment temperature. In thermogravimetric analysis, wet torrefied corncobs produced less carbonaceous residues than raw corncobs, while dry torrefied corncobs gave much more residues owing to increased content of acid insoluble lignin. Pyrolysis-gas chromatography/mass spectroscopy analysis indicated that wet torrefaction significantly promoted levoglucosan yield owing to the removal of alkali metals. Therefore, wet torrefaction can be considered as a more effective pretreatment method for fast pyrolysis of biomass.

Enzymatic degradation of (ligno)cellulose

Glycoside-degrading enzymes play a dominant role in the biochemical conversion of cellulosic biomass into low-price biofuels and high-value-added chemicals. New insight into protein functions and substrate structures, the kinetics of recognition, and degradation events has resulted in a substantial improvement of our understanding of cellulose degradation.

Effect of enzyme-aided cell wall disintegration on protein extractability from intact and dehulled rapeseed (Brassica rapa L. and Brassica napus L.) press cakes

Cell-wall- and pectin-degrading enzyme preparations were used to enhance extractability of proteins from rapeseed press cake. Rapeseed press cakes from cold pressing of intact Brassica rapa and partially dehulled Brassica napus seeds, containing 36-40% protein and 35% carbohydrates, were treated with pectinolytic (Pectinex Ultra SP-L), xylanolytic (Depol 740L), and cellulolytic (Celluclast 1.5L) enzyme preparations. Pectinex caused effective disintegration of embryonic cell walls through hydrolysis of pectic polysaccharides and glucans and increased protein extraction by up to 1.7-fold in comparison to treatment without enzyme addition. Accordingly, 56% and 74% of the total protein in the intact and dehulled press cakes was extracted. Light microscopy of the press cakes suggested the presence of pectins colocalized with proteins inside the embryo cells. Hydrolysis of these intracellular pectins and deconstruction of embryonic cell walls during Pectinex treatment were concluded to relate with enhanced protein release.

Lactic acid and thermal treatments trigger the hydrolysis of myo-inositol hexakisphosphate and modify the abundance of lower myo-inositol phosphates in barley (Hordeum vulgare L.)

Barley is an important source of dietary minerals, but it also contains myo-inositol hexakisphosphate (InsP₆) that lowers their absorption. This study evaluated the effects of increasing concentrations (0.5, 1, and 5%, vol/vol) of lactic acid (LA), without or with an additional thermal treatment at 55°C (LA-H), on InsP₆ hydrolysis, formation of lower phosphorylated myo-inositol phosphates, and changes in chemical composition of barley grain. Increasing LA concentrations and thermal treatment linearly reduced (P<0.001) InsP₆-phosphate (InsP₆-P) by 0.5 to 1 g compared to the native barley. In particular, treating barley with 5% LA-H was the most efficient treatment to reduce the concentrations of InsP₆-P, and stimulate the formation of lower phosphorylated myo-inositol phosphates such as myo-inositol tetraphosphate (InsP₄) and myo-inositol pentaphosphates (InsP₅). Also, LA and thermal treatment changed the abundance of InsP₄ and InsP₅ isomers with Ins(1,2,5,6)P₄ and Ins(1,2,3,4,5)P₅ as the dominating isomers with 5% LA, 1% LA-H and 5% LA-H treatment of barley, resembling to profiles found when microbial 6-phytase is applied. Treating barley with LA at room temperature (22°C) increased the concentration of resistant starch and dietary fiber but lowered those of total starch and crude ash. Interestingly, total phosphorus (P) was only reduced (P<0.05) in barley treated with LA-H but not after processing of barley with LA at room temperature. In conclusion, LA and LA-H treatment may be effective processing techniques to reduce InsP₆ in cereals used in animal feeding with the highest degradation of InsP₆ at 5% LA-H. Further in vivo studies are warranted to determine the actual intestinal P availability and to assess the impact of changes in nutrient composition of LA treated barley on animal performance.

Chemical characterization and hydrothermal pretreatment of Salicornia bigelovii straw for enhanced enzymatic hydrolysis and bioethanol potential

Salicornia bigelovii straw was characterized and evaluated as a potential lignocellulosic bioethanol feedstock. S. bigelovii used in the study was grown in the United Arab Emirates using saltwater (40ppt) for irrigation. Salt removal was performed prior to pretreatment to protect the processing equipment and avoid inhibition of enzymes and yeast. Composition of the washed biomass was comparable to traditional lignocellulosic biomasses with relatively high glucan and xylan content (26 and 22g/100gDM, respectively) but with lower lignin content (7g/100gDM). The washed feedstock was subjected to hydrothermal pretreatment, producing highly digestible (up to 92% glucan-to-glucose conversion) and fermentable (up to 100% glucose-to-ethanol conversion) fiber fractions. Liquid fractions obtained in the pretreatment did not show inhibition towards Saccharomyces cerevisiae. No significant differences among the enzymatic convertibility and microbial fermentability of the fibers as well as low xylose recoveries suggest that lower severity pretreatment conditions could be exploited for S. bigelovii.

Effect of the combined physical and chemical treatments with microbial fermentation on corn straw degradation

In order to improve corn straw degradation, steam explosion, sodium hydroxide soaking and Aspergillus oryzae fermentation were used. The optimal sodium hydroxide pretreatment condition for lignin degradation was obtained. The degradation rates of hemicellulose, cellulose and lignin were 54.68%, 17.76% and 33.14% for the exploded straw (P<0.05); 67.92%, 2.44% (P>0.05) and 76.54% for the alkali-treated straw (P<0.05); 75.98%, 39.93% and 77.88% for the exploded and alkali-treated straw (P<0.05), respectively. The following microbial fermentation could degrade hemicellulose and cellulose further (P<0.05). Cellulase, amylase and protease activities produced during microbial fermentation in the pretreated corn straw were lower than that in the untreated one (P<0.05); however, glucose content was increased by microbial fermentation (P<0.05). It can be concluded that the combined treatments of steam explosion, sodium hydroxide and microbial fermentation will be a good method for straw degradation.

Strong cellulase inhibitors from the hydrothermal pretreatment of wheat straw

BACKGROUND

The use of the enzymatic hydrolysis of lignocellulose with subsequent fermentation to ethanol provides a green alternative for the production of transportation fuels. Because of its recalcitrant nature, the lignocellulosic biomass must be pretreated before enzymatic hydrolysis. However, the pretreatment often results in the formation of compounds that are inhibitory for the enzymes or fermenting organism. Although well recognized, little quantitative information on the inhibition of individual cellulase components by identified inhibitors is available.

RESULTS

Strong cellulase inhibitors were separated from the liquid fraction of the hydrothermal pretreatment of wheat straw. HPLC and mass-spectroscopy analyses confirmed that the inhibitors were oligosaccharides (inhibitory oligosaccharides, IOS) with a degree of polymerization from 7 to 16. The IOS are composed of a mixture of xylo- (XOS) and gluco-oligosaccharides (GOS). We propose that XOS and GOS are the fragments of the xylan backbone and mixed-linkage β-glucans, respectively. The IOS were approximately 100 times stronger inhibitors for Trichoderma reesei cellobiohydrolases (CBHs) than cellobiose, which is one of the strongest inhibitors of these enzymes reported to date. Inhibition of endoglucanases (EGs) by IOS was weaker than that of CBHs. Most of the tested cellulases and hemicellulases were able to slowly degrade IOS and reduce the inhibitory power of the liquid fraction to some extent. The most efficient single enzyme component here was T. reesei EG TrCel7B. Although reduced by the enzyme treatment, the residual inhibitory power of IOS and the liquid fraction was strong enough to silence the major component of the T. reesei cellulase system, CBH TrCel7A.

CONCLUSIONS

The cellulase inhibitors described here may be responsible for the poor yields from the enzymatic conversion of the whole slurries from lignocellulose pretreatment under conditions that do not favor complete degradation of hemicellulose. Identification of the inhibitory compounds helps to design better enzyme mixtures for their degradation and to optimize the pretreatment regimes to minimize their formation.