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.     

Pretreatment of lignocellulosic municipal solid waste by ammonia fiber explosion (AFEX)

The Ammonia Fiber Explosion (AFEX) process treats lignocellulose with high-pressure liquid ammonia and then explosively releases the pressure. The combined chemical effect (cellulose decrystallization) and physical effect (increased accessible surface area) dramatically increase lignocellulose susceptibility to enzymatic attack. For example, bagasse digestibility is increased 5.5 times and that of kenaf core is increased 11 times using extracellular cellulases fromTrichoderma reesei. In this study, we applied the AFEX process to mixed municipal solid waste (MSW) and individual components (e.g., softwood newspaper, kenaf newspaper, copy paper, paper towels, cereal boxes, paper bags, corrugated boxes, magazines, and waxed paper). Softwood newspaper proved to be the most difficult component to digest because of its high lignin content. A combination of oxidative lignin cleavage and AFEX was required to increase softwood newspaper digestibility substantially, whereas AFEX alone was able to make kenaf newspaper digestible. Because most MSW components have been substantially delignified in the paper-making process, AFEX only marginally increased their digestibility.

     

Cellulose and microcrystalline cellulose from rice straw and banana plant waste: preparation and characterization

As part of continuing efforts to prepare cellulose and microcrystalline cellulose (MCC) from renewable biomass resources, rice straw and banana plant waste were used as the available agricultural biomass wastes in Egypt. The cellulose materials were obtained in the first step from rice straw and banana plant waste after chemical treatment, mainly applying alkaline-acid or acid-alkaline pulping which was followed by hypochlorite bleaching method. The results indicate a higher α-cellulose content, 66.2 %, in case of acid-alkaline treatment for rice straw compared to 64.7 % in case of alkaline-acid treatment. A low degree of polymerization, 17, was obtained for the cellulose resulting from acid–alkaline treatment for banana plant waste indicating an oligomer and not a polymer, while it reached 178 in case of the cellulose resulting from alkaline–acid treatment for the rice straw. MCC was then obtained by enzymatic treatment of the resulting cellulose. The resulting MCC show an average diameter ranging from 7.6 to 3.6 μm compared to 25.8 μm for the Avicel PH101. On the other hand, the morphological structure was investigated by scanning electron microscopy indicating a smooth surface for the resulting cellulose, while it indicates that the length and the diameter appeared to be affected by the duration of enzyme treatment for the preparation of MCC. Moreover, the morphological shape of the enzyme treated fibers starts to be the same as the Avicel PH101 which means different shapes of MCC can be reached by the enzyme treatment. Furthermore, Fourier transform infrared spectroscopy was used to indicate characteristic absorption bands of the constituents and the crystallinity was evaluated by X-ray diffraction measurements and by iodine absorption technique. The reported crystallinity values were between 34.8 and 82.4 %, for the resulting cellulose and MCC, and the degree of crystallinity ranged between 88.8 and 96.3 % dependent on the X-ray methods and experimental iodine absorption method.     

Fundamental factors affecting biomass enzymatic reactivity

Poplar wood was treated with peracetic acid, KOH, and ball milling to produce 147 modellignocelluloses with a broad spectrum of lignin contents, acetyl contents, and crystallinity indices (CrIs), respectively. An empirical model was identified that describes the roles of these three properties in enzymatic hydrolysis. Lignin content and CrI have the greatest impact on biomass digestibility, whereas acetyl content has a minor impact. The digestibility of several lime-treated biomass samples agreed with the empirical model. Lime treatment removesallacetyl groups and a moderate amount of lignin and increases CrIslightly; lignin removal is the dominant benefit from lime treatment.     

Autohydrolysis of Tropical Agricultural Residues by Compressed Liquid Hot Water Pretreatment

Pretreatment is an essential step in biorefineries for improving digestibility of recalcitrant agricultural feedstocks prior to enzymatic hydrolysis to composite sugars, which can be further converted to fuels and chemicals. In this study, autohydrolysis by compressed liquid hot water (LHW) pretreatment of various tropical agricultural residues including sugarcane bagasse (BG), rice straw (RS), corn stover (CS), and empty palm fruit bunch (EPFB) was investigated. It was found that LHW pretreatment at 200 °C for 5–20 min resulted in high levels of hemicellulose solubilization into the liquid phase and marked improvement on enzymatic digestibility of the solid cellulose-enriched residues. The maximal yields of glucose and pentose were 409.8–482.7 mg/g and 81.1–174.0 mg/g of pretreated substrates, respectively. Comparative analysis based on severity factor showed varying susceptibility of biomass to LHW in the order of BG> RS> CS> EPFB. Structural analysis revealed surface modification of the pretreated biomass along with an increase in crystallinity index. Overall, 75.7–82.3 % yield of glucose and 27.4–42.4 % yield of pentose from the dried native biomass was recovered in the pretreated solid residues, while 18.3–29.7 % of pentoses were recovered in the liquid phase with dehydration by-product concentration under the threshold for ethanologens. The results suggest the potential of LHW as an efficient pretreatment strategy for implementation in biorefineries operated using various seasonal agricultural feedstocks.     

Ammonia fiber expansion pretreatment and enzymatic hydrolysis on two different growth stages of reed canarygrass

Plant materials from the vegetative growth stage of reed canarygrass and the seed stage of reed canarygrass are pretreated by ammonia fiber expansion (AFEX) and enzymatically hydrolyzed using 15 filter paper units (FPU) cellulase/g glucan to evaluate glucose and xylose yields. Percent conversions of glucose and xylose, effects of temperature and ammonia loading, and hydrolysis profiles are analyzed to determine the most effective AFEX treatment condition for each of the selected materials. The controls used in this study were untreated samples of each biomass material. All pretreatment conditions tested enhanced enzyme digestibility and improved sugar conversions for reed canarygrass compared with their untreated counterparts. Based on 168 h hydrolysis results using 15 FPU Spezyme CP cellulase/g glucan the most effective AFEX treatment conditions were determined as: vegetative growth stage of reed canarygrass--100 degrees C, 60% moisture content, 1.2:1 kg ammonia/kg of dry matter (86% glucose and 78% xylose) and seed stage of reed canarygrass--100 degrees C, 60% moisture content, 0.8:1 kg ammonia/kg of dry matter (89% glucose and 81% xylose). Supplementation by commercial Multifect 720 xylanase along with cellulase further increased both glucose and xylose yields by 10-12% at the most effective AFEX conditions.     

Evaluation of Selected White-Rot Fungal Isolates for Improving the Sugar Yield from Wheat Straw

Biological pretreatment of lignocellulosic biomass by fungi can represent a low-cost and eco-friendly alternative to physicochemical methods to facilitate enzymatic hydrolysis. However, fungal metabolism can cause cellulose loss and it is therefore necessary to use the appropriate fungal strain-biomass type combination. In this work, the effects of biological pretreatments carried out by five different fungi on enzymatic hydrolysis of wheat straw were investigated. The best results were obtained with a Ceriporiopsis subvermispora strain, which minimized weight and cellulose losses and gave the highest net sugar yield (calculated with respect to the holocellulose content of the untreated straw), up to 44 % after a 10-week pretreatment, more than doubling the yields obtained with the other isolates. Moreover, prolonging the pretreatment from 4 up to 10 weeks produced a 2-fold increase, up to 60 %, in digestibility (sugar yield, calculated considering the holocellulose content of the pretreated material). The hemicellulose content of the pretreated material resulted inversely correlated with digestibility, and it could thus be utilized as an index of the pretreatment efficacy. Finally, a correlation was also found between digestibility and the difference between the absorbance values at 290 and 320 nm of pretreated wheat straw extracts.     

Extrusion processing for ammonia fiber explosion (AFEX)

The ammonia fiber explosion (AFEX) process, previously run only in a batch reactor, has been adapted to run on a twin-screw extruder. The sugar yield of AFEX material after enzymatic hydrolysis has been increased up to 3.5 times over that of completely untreated material. The ruminant digestibility of corn fodder has been increased up to 32% (from 54–71%) over completely untreated material, and 23% (from 63–77%) over material extruded with no ammonia. Extrusion-treated material proved more digestible by the ruminant at 48 h than material treated in the batch reactor.     

Pretreatment of Rice Straw Using a Butanone or an Acetaldehyde Dilute Solution Explosion for Producing Ethanol

Ethanol conversion from rice straw using butanone and acetaldehyde dilute solution explosions was evaluated based on the optimization of pure water explosion. To decrease residual inhibitor content, the exploded slurry was dried and investigated at different temperature. Using a 0.9-mol/L butanone solution explosion, with the explosion pressure set at 3.1 MPa, the residence time at 7 min, the dried rice straw-to-water ratio at 1:3 (w/w), and the exploded slurry drying temperuture at 90 °C for 8 h, the yields of total sugar, glucose, and xylose were 85%, 88%, 82% (w/w), respectively, and the ethanol productivity was 26.0 g/100 g rice straw dry matter. Moreover, 0.5-mol/L acetaldehyde dilute solution explosion improved the efficiency of enzymatic hydrolysis (EH) and simultaneous saccharification and co-fermentation (SSCF), and the residual inhibitors had negligible effects on EH and SSCF after detoxification by drying. The results suggested that compared with pure water explosions, the use of butanone and of acetaldehyde dilute solution explosions lowered the explosive temperature and improved the sugar yield, although relative crystallinity of the rice straw dry matter was increased after the explosion.     

Supramolecular Structure - A Key Parameter for Cellulose Biodegradation

Summary: Three different cellulosic substrata, like microcrystalline cellulose, cotton cellulose and spruce dissolving pulp, were chosen for biodegradation. The kinetics of the enzymatic hydrolysis of these celluloses by Trichoderma reesei, has been investigated. The experiments proved the fact that both the morphological structure and the crystalline one are crucial to the process and the ratio of the reactions. In addition, in order to obtain the most accessible cellulose substratum it was studied the biodegradation of cellulose allomorphs of spruce dissolving pulp. The insoluble cellulose fraction remaining after enzymatic hydrolysis was examined by X-ray diffraction method and it was established the degree of crystallinity and the average crystallite size. The enzymatic degradation is also proved by the decrease in the degree of polymerization of hydrolyzed samples.     

Understanding factors that limit enzymatic hydrolysis of biomass

Spectroscopic characterization of both untreated and treated material is being performed in order to determine changes in the biomass and the effects of pretreatment on crystallinity, lignin content, selected chemical bonds, and depolymerization of hemicellulose and lignin. The methods used are X-ray diffraction for determination of cellulose crystallinity (CrI); diffusive reflectance infrared (DRIFT) for changes in C-C and C-O bonds; and fluorescence to determine lignin content. Changes in spectral characteristics and crystallinity are statistically correlated with enzymatic hydrolysis results to identify and better understand the fundamental features of biomass that govern its enzymatic conversion to monomeric sugars. Models of the hydrolysis initial rate and 72 h extent of conversion were developed and evaluated. Results show that the hydrolysis initial rate is most influenced by the cellulose crystallinity, while lignin content most influences the extent of hydrolysis at 72 h. However, it should be noted that in this study only crystallinity, lignin, and selected chemical bonds were used as inputs to the models. The incorporation of additional parameters that affect the hydrolysis, like pore volume and size and surface area accessibility, would improve the predictive capability of the models.     

Bending properties and cell wall structure of alkali-treated wood

Bending tests and X-ray diffraction studies were conducted on oven-dried wood samples (Picea jezoensis Carr.) treated with various concentrations of aqueous NaOH solution to investigate the influence of alkali treatment on the longitudinal contraction, bending properties, and cellulose structure. The length of the wood samples decreased and the density increased at NaOH concentrations greater than 10%. The Young’s modulus and the specific Young’s modulus decreased and the strain at yield increased for the same concentration range. However, the stress at yield was almost constant for all concentration ranges. X-ray diffraction analysis showed that lattice transformation from cellulose I to cellulose II did not occur during alkali treatment and the crystallinity index decreased at NaOH concentrations greater than 10%. The crystallinity index was linearly correlated with the changes in longitudinal contraction and the bending properties, which indicates that the increase in the proportion of amorphous components of the cellulose influences the longitudinal contraction and the bending properties of wood samples during alkali treatment.     

The enzymatic hydrolysis of pretreated agricultural residues and the fermentation of the liberated sugars to ethanol

Agricultural residues were pretreated by steam explosion and the cellulosic component of these substrates were converted to ethanol using a combined enzymatic hydrolysis and fermentation (CHF) process. The enzymatic hydrolysis was carried out using culture filtrates ofTrichoderma harzianum E58 while the liberated sugars were fermented to ethanol byS. cerevisiae. Initially, pretreatment conditions were optimized to ensure that the substrates were readily hydrolyzed and fermented. The agricultural residues were steamed for various times between 30 and 120 s at approximately 240‡C prior to rapid decompression (explosion) in a small masonite-type gun. The various substrates were selectively extracted by water and alkali to see whether the enzymatic hydrolysis and fermentability of the substrates were enhanced. A comparison between the overall conversion of wheat and barley straw was made since these are the two most readily available agricultural residues in Canada. Steam explosion did not affect the hexosan content of the residues, although the pentosan content of the substrates decreased with increasing duration of steaming. The hexosan (cellulose) content of wheat straw was 50.7% of the total substrate while a slightly higher 52.9% cellulose content was detected in the barley straw. Wheat straw was more efficiently hydrolyzed after it had been steamed for 90 s while optimum hydrolysis of the barley straw was detected after 60 s. Steam exploded wheat and barley straw that was subsequently extracted with water was readily hydrolyzed to their component sugars.S. cerevisiae could almost quantitatively convert these sugars to ethanol. This indicated that water washing not only enhanced the enzymatic hydrolysis of the steam exploded substrates, it also removed inhibitory material that restricted the growth of S.cerevisiae. Maximum hydrolysis (78.5%) and ethanol yields (10 mg/mL) were obtained when wheat straw was steamed for 90 s. Slightly lower hydrolysis (76.0%) and ethanol yields (9.5 mg/mL) were obtained with barley straw that had been steamed for 120 s.     

Illumination of Cellulose with Linearly Polarized Visible Light

Summary: Aqueous suspensions of cellulose of long polysaccharide chains, were illuminated with visible polarized light (VPL) for 20 and 50 hrs. Crystal structure, thermal properties with Differential Scanning Calorimetry (DSC) and degree of polymerization (DP) of the samples were determined. Additionally, kinetic of enzymatic as well as acid-catalyzed hydrolysis of cellulose was estimated. Illumination of cellulose with VPL for 50 hrs increased its DP by 15%. X-ray diffraction patterns revealed that the illumination resulted in an increase in the amount of cellulose crystalline phase. The DSC measurements indicated differences in the water molecules distribution depending on the sample treatment confirming an increase in the crystallinity of the illuminated cellulose. After prolonged illumination, cellulose was resistant to oxidation and had lower susceptibility to enzymatic and acid-catalyzed hydrolysis.     

Pretreatment of Rice Straw by Proton Beam Irradiation for Efficient Enzyme Digestibility

Biomass was pretreated with proton beam irradiation (PBI) in order to enhance enzyme digestibility. Rice straw and soaking in aqueous ammonia (SAA)-treated rice straw were treated with 1-25 kGy doses of PBI at a beam energy of 45 MeV. The optimal doses of PBI for efficient sugar recovery were 15 and 3 kGy for rice straw and SAA-treated rice straw, respectively. When PBI was applied to rice straw at 15 kGy, the glucose conversion reached 68% of the theoretical maximum at 72 h. When 3 kGy of PBI was applied to SAA-treated rice straw, approximately 90% of the theoretical glucose conversion was obtained at 12 h compared to a 89% conversion at 48 h. After 2 h of enzymatic saccharification, the initial reaction rates of raw rice straw pretreated with 15 kGy of PBI and SAA-treated rice straw pretreated with 3 kGy of PBI were 1.4?×?10?? and 9.7?×?10?? g L?1 s?1, respectively. Further, the results of X-ray diffractometry support the effect of PBI on sugar recovery, whereas scanning electron microscopy images revealed a more rugged rice straw surface.     

Exploration on the characteristics of cellulose microfibers from Palmyra palm fruits

To obtain cellulose microfibers from Palmyra palm fruit fibers, a new succession of specific chemical treatments including acidified chlorination, alkalization, and acid hydrolysis have been developed. Cellulose microfibers obtained were characterized by different techniques. The chemical analysis indicated an increase in α-cellulose content and decrease in lignin and hemicellulose for the cellulose microfibers over raw fibers. Fourier transform infrared and ¹³C NMR spectra confirmed the removal of non-cellulosic (lignin and hemicellulose) components after chemical treatments. The X-ray diffraction results revealed that the cellulose I was partly transformed into cellulose II by chemical treatments and the crystallinity index of cellulose microfibers was significantly increased as compared to raw fibers owing to removal of non-cellulosic components. Thermogravimetric analysis results demonstrated that the thermal stability was enhanced noticeably for cellulose microfibers than for the raw fibers. The scanning electron micrographs illustrated cleaner and rough surfaces for the cellulose microfibers when compared to those of raw fibers.     

On the determination of crystallinity and cellulose content in plant fibres

A comparative study of cellulose crystallinity based on the sample crystallinity and the cellulose content in plant fibres was performed for samples of different origin. Strong acid hydrolysis was found superior to agricultural fibre analysis and comprehensive plant fibre analysis for a consistent determination of the cellulose content. Crystallinity determinations were based on X-ray powder diffraction methods using side-loaded samples in reflection (Bragg-Brentano) mode. Rietveld refinements based on the recently published crystal structure of cellulose Iβ followed by integration of the crystalline and amorphous (background) parts were performed. This was shown to be straightforward to use and in many ways advantageous to traditional crystallinity determinations using the Segal or the Ruland–Vonk methods. The determined cellulose crystallinities were 90–100 g/100 g cellulose in plant-based fibres and 60–70 g/100 g cellulose in wood based fibres. These findings are significant in relation to strong fibre composites and bio-ethanol production.     

Enzymatic hydrolysis of high-moisture corn fiber pretreated by afex and recovery and recycling of the enzyme complex

Corn fiber is a grain-processing residue containing significant amounts of cellulose, hemicellulose, and starch, which is collected in facilities where fuel ethanol is currently manufactured. Preliminary research has shown that corn fiber (30% moisture dry weight basis [dwb]) responds well to ammonia-fiber explosion (AFEX) pretreatment. However, an important AFEX pretreatment variable that has not been adequately explored for corn fiber is sample moisture. In the present investigation, we determined the best AFEX operating conditions for pretreatment of corn fiber at high moisture content (150% moisture dwb). The optimized AFEX treatment conditions are defined in terms of the moisture content, particle size, ammonia to biomass ratio, temperature, and residence time using the response of the pretreated biomass to enzymatic hydrolysis as an indicator. Approximate optimal-pretreatment conditions for unground corn fiber containing 150% (dwb) moisture were found to be: temperature, 90?C; ammonia: dry corn fiber mass ratio, 1:1; and residence time 30 min (average reactor pressure under these conditions was 200 pounds per square inch [psig]). Enzymatic hydrolysis of the treated corn fiber was performed with three different enzyme combinations. More than 80% of the theoretical sugar yield was obtained during enzymatic hydrolysis using the best enzyme combination after pretreatment of corn fiber under the optimized conditions previously described. A simple process for enzyme recovery and reuse to hydrolyze multiple portions of AFEX-treated corn fiber by one portion of enzyme preparation is demonstrated. Using this process, five batches of fresh substrate (at a concentration of 5% w/v) were successfully hydrolyzed by repeated recovery and reuse of one portion of enzyme preparation, with the addition of a small portion of fresh enzyme in each subsequent recycling step.     

Influence of ionic-liquid incubation temperature on changes in cellulose structure,biomass composition,and enzymatic digestibility

Varying ionic liquid, 1-ethyl 3-methyl imidazolium acetate, pretreatment incubation temperature on lignocellulosic biomass substrates, corn stover, switchgrass and poplar, can have dramatic effects on the enzymatic digestibility of the resultant regenerated biomass. In order to delineate the chemical and physical changes resulting from the pretreatment process and correlate changes with enzymatic digestibility, X-ray powder and fiber diffraction, ¹³C cross polarization/magic angle spinning nuclear magnetic resonance spectroscopy, and compositional analysis was completed on poplar, corn stover and switchgrass samples. Optimal pretreatment incubation temperatures were most closely associated with the retention of amorphous substrates upon drying of regenerated biomass. Maximal glucan to glucose conversion for 24 h enzyme hydrolysis was observed for corn stover, switchgrass and poplar at ionic liquid incubation temperatures of 100, 110 and 120 °C, respectively. We hypothesize that effective pretreatment temperatures must attain lignin redistribution and retention of xylan for optimal enzyme digestibility.     

Comparison of changes in cellulose ultrastructure during different pretreatments of poplar

One commonly cited factor that contributes to the recalcitrance of biomass is cellulose crystallinity. The present study aims to establish the effect of several pretreatment technologies on cellulose crystallinity, crystalline allomorph distribution, and cellulose ultrastructure. The observed changes in the cellulose ultrastructure of poplar were also related to changes in enzymatic hydrolysis, a measure of biomass recalcitrance. Hot-water, organo-solv, lime, lime-oxidant, dilute acid, and dilute acid-oxidant pretreatments were compared in terms of changes in enzymatic sugar release and then changes in cellulose ultrastructure measured by ¹³C cross polarization magic angle spinning nuclear magnetic resonance and wide-angle X-ray diffraction. Pretreatment severity and relative chemical depolymerization/degradation were assessed through compositional analysis and high-performance anion-exchange chromatography with pulsed amperometric detection. Results showed minimal cellulose ultrastructural changes occurred due to lime and lime-oxidant pretreatments, which at short residence time displayed relatively high enzymatic glucose yield. Hot water pretreatment moderately changed cellulose crystallinity and crystalline allomorph distribution, yet produced the lowest enzymatic glucose yield. Dilute acid and dilute acid-oxidant pretreatments resulted in the largest increase in cellulose crystallinity, para-crystalline, and cellulose-I_β allomorph content as well as the largest increase in cellulose microfibril or crystallite size. Perhaps related, compositional analysis and Klason lignin contents for samples that underwent dilute acid and dilute acid-oxidant pretreatments indicated the most significant polysaccharide depolymerization/degradation also ensued. Organo-solv pretreatment generated the highest glucose yield, which was accompanied by the most significant increase in cellulose microfibril or crystallite size and decrease in relatively lignin contents. Hot-water, dilute acid, dilute acid-oxidant, and organo-solv pretreatments all showed evidence of cellulose microfibril coalescence.