Sustainable films and coatings from hemicelluloses: a review

This review summarizes the results of past research on films and coatings from hemicelluloses, biopolymers that are as yet relatively unexploited commercially. The targeted uses of hemicelluloses have primarily been packaging films and coatings for foodstuffs as well as biomedical applications. Oxygen permeability of hemicellulose films, an important characteristic for food packaging, was typically comparable to values found for other biopolymer films such as amylose and amylopectin. As expected, the modification of hemicelluloses to create more hydrophobic films reduced the water vapor permeability. However, modified hemicellulose coatings intended for food still exhibited water vapor permeabilities several magnitudes higher than those of other polymers currently used for this purpose. Research on hemicelluloses for biomedical applications has included biocompatible hydrogels and coatings and material surfaces with enhanced cell affinity. Numerous possibilities exist for chemically modifying hemicelluloses, and fundamental studies of films from modified hemicelluloses have identified other potential applications, including selective membranes.     

Effect of alkaline and autohydrolysis processes on the purity of obtained hemicelluloses from corn stalks

A study of the potential of autohydrolysis and alkaline extraction processes from corn stalks was performed for high purity hemicellulose extraction. The influence of process parameters on the purity of obtained hemicelluloses was analyzed. An experimental design was developed for the autohydrolysis treatments to determine the optimal conditions to solubilize the hemicelluloses with lowest content in contaminants. On the other hand, alkaline extraction, including raw material pretreatment (dewaxing and delignification step) was carried out analyzing the effectiveness of this processes for maximum pure hemicellulose recovery. The maximum yield (54% of the raw material hemicelluloses) and the best physicochemical properties (highest hemicellulose content free of lignin) were obtained with these pretreatments in alkaline extraction. Moreover, the effect of lignin removal by sulfuric acid from the autohydrolysis liquors before hemicellulose precipitation was studied. This purification step has allowed to obtain lignin-free autohydrolysis hemicellulose but with the presence of sulfur as predominant contaminant.     

Poroelastic Mechanical Effects of Hemicelluloses on Cellulosic Hydrogels under Compression

Hemicelluloses exhibit a range of interactions with cellulose, the mechanical consequences of which in plant cell walls are incompletely understood. We report the mechanical properties of cell wall analogues based on cellulose hydrogels to elucidate the contribution of xyloglucan or arabinoxylan as examples of two hemicelluloses displaying different interactions with cellulose. We subjected the hydrogels to mechanical pressures to emulate the compressive stresses experienced by cell walls in planta. Our results revealed that the presence of either hemicellulose increased the resistance to compression at fast strain rates. However, at slow strain rates, only xyloglucan increased composite strength. This behaviour could be explained considering the microstructure and the flow of water through the composites confirming their poroelastic nature. In contrast, small deformation oscillatory rheology showed that only xyloglucan decreased the elastic moduli. These results provide evidence for contrasting roles of different hemicelluloses in plant cell wall mechanics and man-made cellulose-based composite materials.     

N-(Carboxyacyl)chitosans

Hemicelluloses were isolated from pineapple-leaf fibers under different conditions. Study of the properties of these hemicelluloses gave direct evidence of some ester linkages between the hemicellulose and the lignin in this fiber. An aldobiouronic acid was isolated from this fiber hemicellulose, and characterized as 2-O-(4-O-methyl-α-d-glucopyranosyluronic acid)-d-xylose. This indicates that the general nature of the hemicellulose is similar to those of jute and other fiber hemicelluloses.     

Biodegradable polymers from renewable sources: rheological characterization of hemicellulose-based hydrogels

Hemicellulose-based hydrogels were prepared by radical polymerization of 2-hydroxyethyl methacrylate or poly(ethylene glycol) dimethacrylate with oligomeric hydrosoluble hemicellulose modified with well-defined amounts of methacrylic functions. The polymerization reaction was carried out in water at 40 degrees C using a redox initiator system. The hydrogels were in general elastic, soft, and easily swellable in water. Their viscoelastic properties were determined by oscillatory shear measurements on 2 mm thick hydrogels under a slight compression to avoid slip, over the frequency range 10(-1) to 10(2). The rheological characterization indicated that the elastic response of the hydrogels was stronger than the viscous response, leading to the conclusion that the hydrogel systems displayed a predominantly solid-like behavior. The curves showed an increase in shear storage modulus with increasing cross-linking density. The nature of the synthetic comonomer in the hemicellulose-based hydrogels also influenced the shear storage modulus. Comparison of hemicellulose-based hydrogels with pure poly(2-hydroxyethyl methacrylate) hydrogels showed that their behaviors were rather similar, demonstrating that the synthetic procedure made it possible to prepare hemicellulose-based hydrogels with properties similar to those of pure poly(2-hydroxyethyl methacrylate) hydrogels.     

Quantification and monosaccharide composition of hemicelluloses from different plant functional types

Hemicelluloses are the second most abundant polysaccharide in nature after cellulose. So far, the chemical heterogeneity of cell-wall hemicelluloses and the relatively large sample-volume required in existing methods represent major obstacles for large-scale, cross-species analyses of this important plant compound. Here, we apply a new micro-extraction method to analyse hemicelluloses and the ratio of ‘cellulose and lignin’ to hemicelluloses in different tissues of 28 plant species comprising four plant functional types (broad-leaved trees, conifers, grasses and herbs). For this study, the fiber analysis after Van Soest was modified to enable the simultaneous quantitative and qualitative measurements of hemicelluloses in small sample volumes. Total hemicellulose concentrations differed markedly among functional types and tissues with highest concentration in sapwood of broad-leaved trees (31% d.m. in Fraxinus excelsior) and lowest concentration between 10 and 15% d.m. in leaves and bark of woody species as well as in roots of herbs. As for total hemicellulose concentrations, plant functional types and tissues exhibited characteristic ratios between the sum of cellulose plus lignin and hemicelluloses, with very high ratios (>4) in bark of trees and low ratios (<2) in all investigated leaves. Additional HPLC analyses of hydrolysed hemicelluloses showed xylose to be the dominant hemicellulose monosaccharide in tissues of broad-leaved trees, grasses and herbs while coniferous species showed higher amounts of arabinose, galactose and mannose. Overall, the micro-extraction method permitted for the simultaneous determination of hemicelluloses of various tissues and plant functional types which exhibited characteristic hemicellulose concentrations and monosaccharide patterns.     

The effect of hemicelluloses on wood pulp nanofibrillation and nanofiber network characteristics

Hemicelluloses as matrix substances showed an important role in nanofibrillation of wood pulp. Never-dried and once-dried pulps with different amounts of hemicelluloses were fibrillated using a grinding treatment. The degree of fibrillation was evaluated by scanning electron microscopy observation of the fibrillated pulps and light transmittance measurements of the fibrillated pulp/acrylic resin composites. With a one-pass grinding treatment, the once-dried pulp with higher hemicellulose content was fibrillated into 10-20 nm wide fibers as easily as the never-dried pulps, while the once-dried pulp with lower hemicellulose content was not fibrillated into uniform nanosized fibers. This result indicates that hemicelluloses act as inhibitors of the coalescence of microfibrils during drying and facilitate the nanofibrillation of once-dried pulp. Furthermore, hemicelluloses provide adhesion between nanofibers, contributing to reduction of thermal expansion and enhancement of mechanical properties in the composites.     

Properties and Antioxidant Action of Actives Cassava Starch Films Incorporated with Green Tea and Palm Oil Extracts

There is an interest in the development of an antioxidant packaging fully biodegradable to increase the shelf life of food products. An active film from cassava starch bio-based, incorporated with aqueous green tea extract and oil palm colorant was developed packaging. The effects of additives on the film properties were determined by measuring mechanical, barrier and thermal properties using a response surface methodology design experiment. The bio-based films were used to pack butter (maintained for 45 days) under accelerated oxidation conditions. The antioxidant action of the active films was evaluated by analyzing the peroxide index, total carotenoids, and total polyphenol. The same analysis also evaluated unpacked butter, packed in films without additives and butter packed in LDPE films, as controls. The results suggested that incorporation of the antioxidants extracts tensile strength and water vapor barrier properties (15 times lower) compared to control without additives. A lower peroxide index (231.57%), which was significantly different from that of the control (p<0.05), was detected in products packed in film formulations containing average concentration of green tea extracts and high concentration of colorant. However, it was found that the high content of polyphenols in green tea extract can be acted as a pro-oxidant agent, which suggests that the use of high concentration should be avoided as additives for films. These results support the applicability of a green tea extract and oil palm carotenoics colorant in starch films totally biodegradable and the use of these materials in active packaging of the fatty products.     

Use of chemical fractionation and proton nuclear magnetic resonance to probe the physical structure of the primary plant cell wall

Proton magnetic resonance has been used to monitor the microscopic physical properties of etiolated hypocotyl cell walls from Phaseolus vulgaris L. at all stages in a series of chemical fractionations with ammonium oxalate and potassium hydroxide. Solid echo measurements indicate that 75% of the polymers in the intact cell wall, including the cellulose and most of the hemicelluloses, are arranged such that there is almost complete restraint of molecular motion. The chemical fractionations generally altered the physical structures of the remaining cell wall components. Digestion with 0.25% ammonium oxalate/oxalic acid solubilized the pectin and increased the mobility of the hemicellulose I component. Extraction with 4% potassium hydroxide removed the hemicellulose I component and loosened the hemicellulose II. Further extraction with 24% potassium hydroxide removed the hemicellulose II and loosened some of the cellulose. The cellulose crystallinity, as monitored by Jeener echo measurements decreased from 83% to 63% during these fractionations. We conclude that, while hemicellulose I is firmly attached to hemicellulose II, it is not in a closely packed structure. Hemicellulose II is strongly bound to cellulose and has a much more closely packed structure.     

Digestion of polysaccharide constituents of tropical pasture herbage in the bovine rumen. IV. The hydrolysis of hemicelluloses from spear grass by cell-free enzyme systems from rumen fluid

Hemicelluloses have been isolated from spear grass (Heteropogon contortus), before and after digestion in the rumen, and separated into linear and branched fractions. Similar fractions have also been obtained from a pasture sample and from the faeces fibre of a steer fed on the same pasture. The rates of hydrolysis of all of these hemicellulose fractions have been determined in the presence of extracellular enzymes from rumen fluid and of enzymes liberated by disruption of rumen microorganisms. The oligosaccharide products of such enzymic degradations have been partially identified. The results confirm that the incomplete digestion of hemicelluloses in the rumen is due to physical protection (e.g. by lignin), rather than to structural differences between different components of the hemicelluloses. There is no difference between rates of digestion of branched and linear hemicelluloses, and previous results which indicated such differences were probably caused by presence of a readily digested glucan in linear hemicellulose fractions.     

Hemicellulose concentration and composition in plant cell walls under extreme carbon source–sink imbalances

Hemicelluloses account for one‐quarter of the global dry plant biomass and therefore are the second most abundant biomass fraction after cellulose. Despite their quantitative significance, the responsiveness of hemicelluloses to atmospheric carbon oversupply is still largely unknown, although hemicelluloses could serve as carbon sinks with increasing CO₂ concentrations. This study aimed at clarifying the role hemicelluloses play as carbon sinks, analogous to non‐structural carbohydrates (NSC), by experimentally manipulating the plants' carbon supply. Sixteen plant species from four different plant functional types (grasses, herbs, seedlings of broad‐leaved trees and conifers) were grown for 2 months in greenhouses at either extremely low (140 ppm), medium (280 ppm) or high (560 ppm) atmospheric CO₂ concentrations, thus inducing situations of massive C‐limitation or ‐oversupply. Above and belowground biomass as well as NSC significantly increased in all species and tissues with increasing CO₂ concentrations. Increasing CO₂ concentrations had no significant effect on total hemicellulose concentrations in leaves and woody tissues in all species, except for two out of four grass species, where hemicellulose concentrations increased with atmospheric CO₂ supply. Despite the overall stable total hemicellulose concentrations, the monosaccharide spectra of hemicelluloses showed a significant increase in glucose monomers in leaves of woody species as C‐supply increased. In summary, total hemicellulose concentrations in de novo built biomass seem to be largely unaffected by changed atmospheric CO₂ concentrations, while significant increases of hemicellulose‐derived glucose with increasing CO₂ concentrations in leaves of broad‐leaved and conifer tree seedlings showed differential responses among the different hemicellulose classes in response to varying CO₂ concentrations.     

Structure and interactions of plant cell-wall polysaccharides by two- and three-dimensional magic-angle-spinning solid-state NMR

The polysaccharide-rich cell walls (CWs) of plants perform essential functions such as maintaining tensile strength and allowing plant growth. Using two- and three-dimensional magic-angle-spinning (MAS) solid-state NMR and uniformly (13)C-labeled Arabidopsis thaliana, we have assigned the resonances of the major polysaccharides in the intact and insoluble primary CW and determined the intermolecular contacts and dynamics of cellulose, hemicelluloses, and pectins. Cellulose microfibrils showed extensive interactions with pectins, while the main hemicellulose, xyloglucan, exhibited few cellulose cross-peaks, suggesting limited entrapment in the microfibrils rather than extensive surface coating. Site-resolved (13)C T(1) and (1)H T(1ρ) relaxation times indicate that the entrapped xyloglucan has motional properties that are intermediate between the rigid cellulose and the dynamic pectins. Xyloglucan absence in a triple knockout mutant caused the polysaccharides to undergo much faster motions than in the wild-type CW. These results suggest that load bearing in plant CWs is accomplished by a single network of all three types of polysaccharides instead of a cellulose-xyloglucan network, thus revising the existing paradigm of CW structure. The extensive pectin-cellulose interaction suggests a central role for pectins in maintaining the structure and function of plant CWs. This study demonstrates the power of multidimensional MAS NMR for molecular level investigation of the structure and dynamics of complex and energy-rich plant materials.     

Pretreatment and Lignocellulosic Chemistry

Lignocellulosic materials such as wood, grass, and agricultural and forest residues are promising alternative energy resources that can be utilized to produce ethanol. The yield of ethanol production from native lignocellulosic material is relatively low due to its native recalcitrance, which is attributed to, in part, lignin content/structure, hemicelluloses, cellulose crystallinity, and other factors. Pretreatment of lignocellulosic materials is required to overcome this recalcitrance. The goal of pretreatment is to alter the physical features and chemical composition/structure of lignocellulosic materials, thus making cellulose more accessible to enzymatic hydrolysis for sugar conversion. Various pretreatment technologies to reduce recalcitrance and to increase sugar yield have been developed during the past two decades. This review examines the changes in lignocellulosic structure primarily in cellulose and hemicellulose during the most commonly applied pretreatment technologies including dilute acid pretreatment, hydrothermal pretreatment, and alkaline pretreatment.     

Preparation and characterization of hemicellulosic derivatives containing carbamoylethyl and carboxyethyl groups

A series of novel water-soluble hemicellulosic derivatives, containing carbamoylethyl and carboxyethyl groups, were heterogeneously synthesized from wheat-straw hemicelluloses with acrylamide (AA) under alkaline conditions. The factors such as reaction temperature, reaction time, molar ratio of catalysis to xylose unit in hemicelluloses and molar ratio of acrylamide to xylose unit in hemicelluloses, were investigated. The average degree of substitution (DS) was calculated by ¹H NMR spectroscopy. DS values up to 0.23 in a one-step synthesis of hemicelluloses derivatives were obtained. Under optimum conditions (60 °C, NaOH to xylose unit in hemicelluloses molar ratio of 0.8, AA to xylose unit in hemicelluloses molar ratio of 8.0, reaction time of 1 h) an expected ratio of carbamoylethyl group to carboxyethyl group of 4.8 in the hemicellulosic derivatives was obtained. The structural features of the hemicellulosic derivatives were characterized by FTIR, NMR spectroscopy, and by elemental analysis. The current work provides a facile method for the synthesis of hemicellulose derivatives with bifunctional groups, which could be used as wet-end additives in the papermaking industry.     

Hemicellulose biosynthesis

One major component of plant cell walls is a diverse group of polysaccharides, the hemicelluloses. Hemicelluloses constitute roughly one-third of the wall biomass and encompass the heteromannans, xyloglucan, heteroxylans, and mixed-linkage glucan. The fine structure of these polysaccharides, particularly their substitution, varies depending on the plant species and tissue type. The hemicelluloses are used in numerous industrial applications such as food additives as well as in medicinal applications. Their abundance in lignocellulosic feedstocks should not be overlooked, if the utilization of this renewable resource for fuels and other commodity chemicals becomes a reality. Fortunately, our understanding of the biosynthesis of the various hemicelluloses in the plant has increased enormously in recent years mainly through genetic approaches. Taking advantage of this knowledge has led to plant mutants with altered hemicellulosic structures demonstrating the importance of the hemicelluloses in plant growth and development. However, while we are on a solid trajectory in identifying all necessary genes/proteins involved in hemicellulose biosynthesis, future research is required to combine these single components and assemble them to gain a holistic mechanistic understanding of the biosynthesis of this important class of plant cell wall polysaccharides.     

Degradation and Utilization of Hemicellulose from Intact Forages by Pure Cultures of Rumen Bacteria

Several pure strains of rumen bacteria have previously been shown to degrade isolated hemicelluloses from a form insoluble in 80% acidified ethanol to a soluble form, regardless of the eventual ability of the organism to utilize the end products as energy sources. This study was undertaken to determine whether similar hemicellulose degradation or utilization, or both, occurs from intact forages. Fermentations by pure cultures were run to completion by using three maturity stages of alfalfa and two maturity stages of bromegrass as individual substrates. Organisms capable of utilizing xylan or isolated hemicelluloses could degrade and utilize intact forage hemicellulose, with the exception of two strains of Bacteroides ruminicola which were unable to degrade or utilize hemicellulose from grass hays. Intact forage hemicelluloses were extensively degraded by three cellulolytic strains that were unable to use the end products; in general, these strains degraded a considerably greater amount of hemicelluloses than the hemicellulolytic organisms. Hemicellulose degradation or utilization, or both, varied markedly with the different species and strains of bacteria, as well as with the type and maturity stage of the forage. Definite synergism was observed when a degrading nonutilizer was combined with either one of two hemicellulolytic strains on the bromegrass substrates. One hemicellulolytic strain, which could not degrade or utilize any of the intact bromegrass hemicellulose alone, almost completely utilized the end products solubilized by the nonutilizer. Similar synergism, although of lesser magnitude, was observed when alfalfa was used as a substrate.     

生物质生物预处理研究进展与展望

木质纤维素生物质分布广、产量大、可再生,用于制备生物基能源、生物基材料和生物基化学品。木质纤维素生物质组成复杂,包含纤维素、半纤维素和木质素等,木质素与半纤维素通过共价键、氢键交联形成独特的“包裹结构”,纤维素含有复杂的分子内与分子间氢键,上述因素制约着其资源化利用。生物预处理以其独特优越性成为生物质研究的重要方面。系统阐述了生物预处理过程中木质素降解和基团修饰对纤维素酶解的影响,纤维素含量及结晶区变化,半纤维素五碳糖利用,微观物理结构的改变。进一步提出了以生物预处理为核心的组合预处理、基于不同功能的多酶协同催化体系、木质纤维素组分分级利用和新型高效细菌预处理工艺是生物预处理未来发展的重要趋势。     

Hemicellulosic Polysaccharides from the Xylopodium of Ocimum nudicaule: Changes in Composition in Dormancy and Sprouting

Changes in the yield and composition of hemicelluloses fromthe underground organs (xylopodia) of Ocimum nudicaule wereinvestigated. Hemicelluloses constituted about 12% of the delipidizedpowder in sprouting and about 30 % in dormant phases. Xyloseis the major component of hemicelluloses A and B (and is alsopresent in C), followed by arabinose, galactose, glucose, rhamnoseand mannose. The amounts of hemicellulose B decreased by sixtimes between dormancy and sprouting, whereas the yields ofhemicelluloses A and C remained constant. This, together withthe higher solubility of hemicellulose B and its higher susceptibilityto hemicellulase in sprouting indicates that this fraction constitutesa cell-wall bound storage polysaccharide, which may play a rolein the onset of xylopodia bud sprouting. Ocimum nudicaule, hemicelluloses, cell-wall storage polysaccharide     

Routes to Potential Bioproducts from Lignocellulosic Biomass Lignin and Hemicelluloses

An essential feature of proposed fermentation-based lignocellulose to biofuel conversion processes will be the co-production of higher value chemicals from lignin and hemicellulose components. Over the years, many routes for chemical conversion of lignin and hemicelluloses have been developed by the pulp and paper industry and we propose that some of these can be applied for bioproducts manufacturing. For lignin products, thermochemical, chemical pulping, and bleaching methods for production of polymeric and monomeric chemicals are reviewed. We conclude that peroxyacid chemistry for phenol and ring-opened products looks most interesting. For hemicellulose products, preextraction of hemicelluloses from woody biomass is important and influences the mixture of solubilized material obtained. Furfural, xylitol, acetic acid, and lactic acid are possible targets for commercialization, and the latter can be further converted to acrylic acid. Pre-extraction of hemicelluloses can be integrated into most biomass-to-biofuel conversion processes.