Lignin Complexes Research Articles

Cassava (Manihot esculenta) dual use for food and bioenergy: A review

AbstractCassava (Manihot esculenta. Crantz) is a starch‐rich, woody tuberous, root crop important for food, with little being done to investigate its potential as a bioenergy crop despite its enormous potential. The major bottleneck in the crop being able to serve this dual role is the competition of its storage roots for both purposes. The major cassava production regions primarily use the tuberous roots for food, and this has resulted in its neglect as a bioenergy crop. The use of non‐food cassava parts as a feedstock in cellulosic biofuel production is a promising strategy that can overcome this challenge. However, in non‐tuber parts, most of the sugars are highly sequestered in lignin complexes making them inaccessible to bacterial bioconversion. Additionally, cassava production in these major growing areas is not optimal owing to several production constraints. The challenges affecting cassava production as a food and bioenergy crop are interconnected and therefore need to be addressed together. Cassava improvement against biotic and abiotic stresses can enhance productivity and cater for the high demand of the roots for food and bioenergy production. Furthermore, increased production will enhance the usability of non‐food parts for bioenergy as the bigger goal. This review addresses efforts in cassava improvement against stresses that reduce its productivity as well as strategies that enhance biomass production, both important for food and bioenergy. Additionally, prospective strategies that could ease bioconversion of cassava for enhanced bioenergy production are explored.

Chitin and Lignin: Old Polymers and New Bio-Tissue- Carriers

Worldwide consumers are nowadays much more focusing on their wealth and appearance, having increased their worry caused from the pollution, plastic wastes and the earth' disasters further increased for the COVID-19 pandemic. This trend has created an heightened demand for products which, formulated with natural and functional ingredients and carried by sustainable delivery systems, should be produced and packed with biodegradable compounds. The paper suggests to formulate innovative cosmetic and medical products based on the use of carriers made by biodegradable polysaccharide-tissues embedded by micro-nano particles of chitin nano fibril-nano lignin complexes, encapsulating different active ingredients. Thus, data on chitin, lignin and their complexes are reported and discussed, focusing the attention on their possible use to make innovative products, characterized for their effectiveness, safeness, and biodegradability.

Potential of bagasse obtained using hydrothermal liquefaction pre‐treatment as a natural emulsifier

SummaryBagasse, a by‐product from raw sugar factories, is conventionally burned for energy production. In this study, bagasse extracts from hydrothermal liquefaction (HTL) treatment (160 °C, 1 MPa and 30 min) with a carbohydrate content of 510.3 mg g−1 and 0.5 mg g−1 of total phenols were applied as emulsifiers in oil‐in‐water (O/W) emulsions. Bagasse extracts from HTL (0.5–4 wt%) lowered the interfacial tension between oil–water interphase from 19.8 to 14.0 mN m−1, owing possibly to the surface‐active hydrophilic carbohydrate‐hydrophobic lignin complexes in the extracts (lignin content: 7.1% w/w). Emulsions stabilised by bagasse extracts from HTL with average droplet size, dav of 0.79 μm were comparable with gum arabic (GA), dav of 2.24 μm after 11 days at 25 °C. Bagasse extracts containing biopolymers have the potential for industrial applications involving emulsion systems; therefore, HTL treatment of bagasse without any solvents can be regarded as an effective tool for producing natural emulsifiers.

Exploring why sodium lignosulfonate influenced enzymatic hydrolysis efficiency of cellulose from the perspective of substrate\u2013enzyme adsorption

BackgroundCellulase adsorbed on cellulose is productive and helpful to produce reducing sugars in enzymatic hydrolysis of lignocellulose; however, cellulase adsorbed on lignin is non-productive. Increasing productive adsorption of cellulase on cellulose would be beneficial in improving enzymatic hydrolysis. Adding lignin that was more hydrophilic in hydrolysis system could increase productive adsorption and promote hydrolysis. However, the effect mechanism is still worth exploring further. In this study, lignosulfonate (LS), a type of hydrophilic lignin, was used to study its effect on cellulosic hydrolysis.ResultsThe effect of LS on the enzymatic hydrolysis of pure cellulose (Avicel) and lignocellulose [dilute acid (DA) treated sugarcane bagasse (SCB)] was investigated by analyzing enzymatic hydrolysis efficiency, productive and non-productive cellulase adsorptions, zeta potential and particle size distribution of substrates. The result showed that after adding LS, the productive cellulase adsorption on Avicel reduced. Adding LS to Avicel suspension could form the Avicel–LS complexes. The particles were charged more negatively and the average particle size was smaller than Avicel before adding LS. In addition, adding LS to cellulase solution formed the LS–cellulase complexes. For DA-SCB, adding LS decreased the non-productive cellulase adsorption on DA-SCB from 3.92 to 2.99 mg/g lignin and increased the productive adsorption of cellulase on DA-SCB from 2.00 to 3.44 mg/g cellulose. Besides, the addition of LS promoted the formation of LS–lignin complexes and LS–cellulase complexes, and the complexes had more negative charges and smaller average sizes than DA-SCB lignin and cellulase particles before adding LS.ConclusionsIn this study, LS inhibited Avicel’s hydrolysis, but enhanced DA-SCB’s hydrolysis. This stemmed from the fact that LS could bind cellulase and Avicel, and occupied the binding sites of cellulase and Avicel. Thus, a decreased productive adsorption of cellulase on Avicel arose. Regarding DA-SCB, adding LS, which enhanced hydrolysis efficiency of DA-SCB, increased the electrostatic repulsion between DA-SCB lignin and cellulase, and therefore, decreased non-productive adsorption of cellulase on DA-SCB lignin and enhanced productive adsorption of cellulase on DA-SCB cellulose.Graphical abstract

Lignin-Only Polymeric Materials Based on Unmethylated Unfractionated Kraft and Ball-Milled Lignins Surpass Polyethylene and Polystyrene in Tensile Strength.

Functional polymeric materials composed solely of lignin preparations appeared only very recently. A gradual paradigm shift spanning 56 years has revealed how lignin–lignin blends can upgrade the performance of 100 wt% lignin-based plastics. The view, first espoused in 1960, that lignin macromolecules are crosslinked reduces the plausibility of creating functional polymeric materials that are composed only of lignin preparations. Lignin-based materials would be much weaker mechanically if interstices remain in significant numbers between adjoining macromolecular structures that consist of rigid crosslinked chains. In 1982, random-coil features in the hydrodynamic character of kraft lignin (KL) components were evident from ultracentrifugal sedimentation equilibrium studies of their SEC behavior. In 1997, it was recognized that the macromolecular species in plastics with 85 wt% levels of KL are associated complexes rather than individual components. Finally, in 2016, the first polymeric material composed entirely of ball-milled softwood lignin (BML) was found to support a tensile strength above polyethylene. Except in its molecular weight, the BML was similar in structure to the native biopolymer. It was composed of associated lignin complexes, each with aromatic rings arranged in two domains. The inner domain maintains structural integrity largely through noncovalent interactions between cofacially-offset aromatic rings; the peripheral domain contains a higher proportion of edge-on aromatic-ring arrangements. Interdigitation between peripheral domains in adjoining complexes creates material continuity during casting. By interacting at low concentrations with the peripheral domains, non-lignin blend components can improve the tensile strengths of BML-based plastics to values well beyond those seen in polystyrene. The KL-based plastics are weaker because the peripheral domains of adjoining complexes are less capable of interdigitation than those of BML. Blending with 5 wt% 1,8-dinitroanthraquinone results in a tensile strength above that of polyethylene. Analogous effects can be achieved with 10 wt% maple γ-valerolactone (GVL) lignin which, with a structure close to the native biopolymer, imparts some native character to the peripheral domains of the KL complexes. Comparable enhancements in the behavior of BML complexes upon blending with 10 wt% ball-milled corn-stover lignin (BMCSL) result in lignin-only polymeric materials with tensile strengths well beyond polystyrene.

Blend configuration in functional polymeric materials with a high lignin content.

Lignins upgrade the lignocellulosic cell-wall domains in all vascular plants; they embody 20-30% of terrestrial organic carbon. For 50 years, mistaken assumptions about the configuration of lignin have hindered the development of useful polymeric materials with a lignin content above 40 wt%. Now, polymeric materials composed only of methylated softwood lignin derivatives can exhibit better tensile behavior than polystyrene. Marked improvements may be achieved with small quantities (5-10 wt%) of miscible blend components as simple as poly(ethylene glycol). These observations contradict commonly held views about crosslinking or hyper-branching in lignin chains. The hydrodynamic compactness of the macromolecular lignin species arises from powerful noncovalent interactions between the lignin substructures. Individual lignin components undergo association to form macromolecular complexes that are preserved in plastics with a very high lignin content. Material continuity results from interpenetration between the peripheral components in adjoining lignin complexes. Through interactions with the peripheral domains, miscible blend components modulate the strength and ductility of these utterly original lignin-based plastics.

Investigation of three lignin complexes with antioxidant and immunological capacities from Inonotus obliquus

Mushroom Inonotus obliquus (I. obliquus), a folk medicine, has been widely used to treat several human malicious tumors since 16th century. In this study, three homogenous biomolecules (designated IOA1, IOA2 and IOA3) were prepared from the alkali extract of I. obliquus. Their molecular weights were measured to be 6.1×104, 2.9×104 and 3.5×104g/mol respectively and all of them were characterized as lignin–carbohydrate complexes mainly comprised lignin as well as −25% carbohydrates. Antioxidant assays indicated that all of them exhibited pronounced reductive power and strong scavenging activities on DPPH and hydroxyl radicals in vitro. Immunological tests showed that they could also significantly stimulate nitric oxide production and phagocytic activity in RAW 264.7 macrophages. These results implied that the lignin–carbohydrate complexes extracted from I. obliquus might be used as novel natural antioxidants or immunostimulants in functional foods or pharmaceutical candidates.

On the formation of lignin polysaccharide networks in Norway spruce

In this study we were mirroring suggested in vivo phenomena of lignin–hemicellulose complex formation in vitro, by cross-linking Norway spruce (Picea abies) galactoglucomannans, xylans and lignin moieties to high molecular weight complexes by laccase treatment. We were able to observe the oxidation and cross-linking of non-condensed guaiacyl-type phenolic moieties attached to both of the hemicelluloses by 31P NMR and size-exclusion chromatography. We suggest that hemicelluloses–lignin complexes form covalently linked structural units during the early stages of lignification via radical enzymatic cross-linking catalyzed by laccase. This work shows that the hemicellulose molecules in wood are covalently linked to two or more lignin units thereby making them suited for forming network structures.

Analysis of lignin–carbohydrate and lignin–lignin linkages after hydrolase treatment of xylan–lignin, glucomannan–lignin and glucan–lignin complexes from spruce wood

Xylan-lignin (XL), glucomannan-lignin (GML) and glucan-lignin (GL) complexes were isolated from spruce wood, hydrolyzed with xylanase or endoglucanase/β-glucosidase, and analyzed by analytical pyrolysis and 2D-NMR. The enzymatic hydrolysis removed most of the polysaccharide moieties in the complexes, and the lignin content and relative abundance of lignin-carbohydrate linkages increased. Analytical pyrolysis confirmed the action of the enzymatic hydrolysis, with strong decreases of levoglucosane and other carbohydrate-derived products. Unexpectedly it also revealed that the hydrolase treatment alters the pattern of lignin breakdown products, resulting in higher amounts of coniferyl alcohol. From the anomeric carbohydrate signals in the 2D-NMR spectra, phenyl glycoside linkages (undetectable in the original complexes) could be identified in the hydrolyzed GML complex. Lower amounts of glucuronosyl and benzyl ether linkages were also observed after the hydrolysis. From the 2D-NMR spectra of the hydrolyzed complexes, it was concluded that the lignin in GML is less condensed than in XL due to its higher content in β-O-4' ether substructures (62 % of side chains in GML vs 53 % in XL) accompanied by more coniferyl alcohol end units (16 vs 13 %). In contrast, the XL lignin has more pinoresinols (11 vs 6 %) and dibenzodioxocins (9 vs 2 %) than the GML (and both have ~13 % phenylcoumarans and 1 % spirodienones). Direct 2D-NMR analysis of the hydrolyzed GL complex was not possible due to its low solubility. However, after sample acetylation, an even less condensed lignin than in the GML complex was found (with up to 72 % β-O-4' substructures and only 1 % pinoresinols). The study provides evidence for the existence of structurally different lignins associated to hemicelluloses (xylan and glucomannan) and cellulose in spruce wood and, at the same time, offers information on some of the chemical linkages between the above polymers.

Investigation of trace metal binding properties of lignin by diffusive gradients in thin films

The binding behavior of lignin for Pb, Cu, Co, Mn, Cd and Ni was studied using the diffusive gradients in thin films technique (DGT). Samplers with different structures of diffusive gel were used in the well-stirred systems containing known concentrations of metals along with (a) 10, 20 and 40μM lignin and; (b) 0.64 and 6.47μM Suwannee river fulvic acid+40μM lignin at an ionic strength of 0.01M (NaNO3) and pH=7. Diffusion coefficients of lignin complexes in acrylamide gels were estimated and found to be less than 5% of the equivalent coefficients for the uncomplexed metal ions. These values were used to calculate concentrations of labile metals from DGT measurements in solutions, where lignin could discriminate metals in the order of Pb+2>Cu+2>Cd+2>Ni+2>Co+2>Mn+2. Stability constants (LogK) were calculated using Visual MINTEQ II and WHAM V software. The K values were compared with the stability constants from titration of Pb and Cd with 10μM lignin aqueous samples and with those of humic substances in natural waters. The constants obtained from measurement of complexing capacities might bias the real corresponding values unless two line regression analyses on titration data are considered. The DGT study of fractionation of metal species at varying ratios indicated that the proportion of organic complexes decreased with increasing ratios and gradually more metals were exchanged with inorganic phases. Speciation of Pb and Cd is affected by the concentrations of FA, Cd is dominantly bound with FA while Pb is evenly partitioned between the ligands. The comprehensive knowledge of metal–lignin complexes sheds some light on in situ operational speciation information that can be achieved by DGT.

Characterization of the effects of lignin and lignin complex particles as filler on a polystyrene film

The work in this research outlines the use of lignin precipitated from lignocellulosic substrate as fillers after modified with transition metal cations, Fe(III), Ni(II) and Co(II), in the production of a polystyrene based composite for polymer packaging applications. Virgin polystyrene was compared with lignin and lignin complex filled composites with loading of 5% by weight prepared using twin screw extrusion. The lignin complexes were first characterized by the UV spectra to identify the new absorption bands occurred due to the complex formation. Moreover, lignin model, namely vanillin, was used to notify the geometric structure of the resulting complexes applying the GC mass spectra. Scanning electron microscopy was used to indicate the change in the morphological structure of the filler particles. On the other hand, the mechanical and thermal analysis for the resulting polymer composites was studied and it was noticed that the type of lignin or lignin complex plays a roll in the results. The inclusion of the Co(II)–lignin complex was observed to increase the tensile strength of the resulting polymer composite and a decrease of the glass transition temperature. Furthermore, light wave lengths and UV fluorescent microscope were used to identify the change of color for the resulting polymer film.

Pulpability and Bleachability of Xylan-Depleted Eucalyptus Wood Chips

Wood autohydrolysis for xylan removal prior to kraft pulping and the consequences of this practice downstream in a kraft pulp mill has been investigated. Chip autohydrolysis for 60 min at 165 °C removed 61% of the xylans, but subsequent kraft cooking produced a 9.4% lower yield than reference. Pulp from 60-min autohydrolyzed chips showed 75% efficiency in oxygen delignification compared to 43.6% from the reference, resulting in a subsequent ECF bleaching cost reduction of 36% and significant effluent improvement. The chip autohydrolysis process resulted in a pulp of higher bulk, lower beatability and strength in relation to the reference, and an acid hydrolysate of low xylan concentration. Pulp cellulose and xylan derived from autohydrolyzed chips are less stable during kraft pulping because of their low molecular weight and high polydispersity. Xylan−lignin and cellulose−lignin complexes from kraft pulps derived from autohydrolyzed chips contain lignin of higher molecular weight than that of regular kraft pulp.

Spatial Correlation of Confocal Raman Scattering and Secondary Ion Mass Spectrometric Molecular Images of Lignocellulosic Materials

A detailed chemical and structural understanding of pre-enzymatic processing of lignocellulosic materials (LCMs) is a key objective in the development of renewable energy. Efficient rendering of biomass components into fermentable substrates for conversion into biofuel feedstocks would benefit greatly from the development of new technologies to provide high-quality, spatially resolved chemical information about LCMs during the various processing states. In an effort to realize this important goal, spatially correlated confocal Raman and mass spectrometric images allow the extraction of three-dimensional information from the perennial grass, Miscanthus x giganteus. An optical microscopy-based landmark registry scheme was developed that allows samples to be transferred between laboratories at different institutions, while retaining the capability to access the same physical regions of the samples. Subsequent to higher resolution imaging via confocal Raman microscopy and secondary ion mass spectrometry (SIMS), laser desorption-ionization mass spectrometry was used to place these regions within the overall sample architecture. Excellent sample registry was evident in the highly correlated Raman and SIMS images. In addition, the correlation of vibrational Raman scattering with mass spectra from specific spatial locations allowed confirmation of the assignment of intracellular globular structures to hemicellulose-rich lignin complexes, an assignment which could only be made tentatively from either image alone.

Intake, digestibility and nitrogen utilization of Robinia pseudoacacia foliage fed to growing goat wethers

Black locust (BL; Robinia pseudoacacia L.), a native tree of southeastern USA known to contain substantial levels of condensed tannins (CT), was fed to 32, 4 month old (20.4 kg BW) Boer cross wether goats in two randomized complete block design trials. The objectives were to examine the effects of feeding hay diets containing several levels of fresh BL foliage on intake, digestibility, and N metabolism. First year (1999) diets were H E (100% Eastern gamagrass [EGH; Tripsacum dactyloides L.] hay), H EG (70% EGH and a 30% mixture of 59% ground corn [GC; Zea mays L.], 36% soybean meal [SBM; Glycine max L.], and 5% minerals), 25BL99 (75% EGH and 25% BL leaves), and 50BL99 (50% EGH and 50% BL leaves). Second year (2000) diets were H O (100% orchardgrass [OGH; Dactylis glomerata L.] hay), H OG (70% OGH and a 30% mixture of 63% GC, 37% SBM, and 5% minerals), 50BL00 (50% OGH and 50% BL leaves), and 75BL00 (25% OGH and 75% BL leaves). In 1999, apparent digestibilities of the diets in the order listed above were 62.4, 68.2, 58.0, and 60.6% ( P = 0.001) for DM and 62.8, 72.5, 56.0, and 59.1% ( P = 0.001) for crude protein (CP). Acid detergent lignin digestibilities were negative for diets 25BL99 (−56.7%) and 50BL99 (−49.3%), apparently due to the formation of insoluble tannin and lignin complexes during passage through the digestive tract. Intakes of DM were similar across diets. In 2000, apparent digestibilities of diet DM (64.4, 71.7, 64.8 and 65.4%) and CP (70.0, 76.0, 66.6, and 66.5%) did not differ. Lignin digestibilities were positive for diets 50BL00 (9.4%) and 75BL00 (29.6%) unlike those for year 1999. Overall, BL contained 10% CT and 18–34% hydrolyzable tannins. In 1999, N intake, urinary N (UN) excretion and N retained were higher for diet H EG ( P = 0.01) than diet H E whereas fecal N excretion (FN) was similar for diets H EG, 25BL99 and 50BL99. In 1999, FN excretion as a percentage of N intake was higher ( P < 0.02) in the BL diets, although UN as a percentage of N intake did not differ among diets. In 2000, N intake and FN output were higher for BL diets compared to diets H O ( P = 0.01) and H OG ( P = 0.02). Fecal N as a percentage of N intake was lower ( P = 0.01) for diet H OG (24.0%) than for diets 50BL00 (33.4%) and 75BL00 (33.5%). Conversely, urinary N as a percentage of N intake was higher for diets H O and H OG compared to the BL diets ( P = 0.02). Increased levels of dietary BL increased FN, suggesting that tannins formed insoluble protein complexes thus hindering digestibility.

Effect of hydrolytic lignin on formation of protein–lignin complexes and protein degradation by rumen microbes

Several methods have been developed to protect feed protein from rumen microbial degradation. The current study aimed to evaluate the potential use of an industrial lignin, namely hydrolytic lignin, to protect protein from rumen microbial degradation. The hydrolytic lignins assessed in this study were extracted from wheat straw previously subjected to various steam treatment conditions (pressure: 15, 17 and 19 bar; reaction time: 0, 5 and 10 min; use of acidic catalyst: without and with 2% H 2SO 4 on DM basis). Results indicated that hydrolytic lignin can precipitate protein when measured by a standard bovine serum albumin assay. It was also observed that protein-precipitating capacity of lignin increased with increasing harshness of steam treatment until a point from which no further effect was observed. The effect of lignin upon protein degradation in vitro was clearly detected. Both ammonia nitrogen and iso-acid concentration in vitro were significantly decreased ( P<0.01) when lignin was added to fermentation flask containing casein. Unlike tannins, hydrolytic lignins do not inhibit rumen microbial activity. Additionally, it was observed that lignin’s ability to bind and protect protein is a pH-dependent reaction. Protein binding to lignin is markedly reduced at pH<3.0.

Fractional Separation and Physicochemical Characterization of Polysaccharides from Poplar Chips

The polysaccharides in the cell walls of dewaxed poplar chips were separated into hemicellulose–lignin complexes (HLC), residual hemicelluloses, and cellulose by extraction with sodium hydroxide at various concentrations. Celluloses (43.9–44.5%), HLC (17.4–21.9%), and residual hemicelluloses (4.3–9.0%) were the major polysaccharide components. Initial extraction of the dewaxed poplar chips with 5% NaOH at 50°C for 4, 6, 8, and 12 hr, and 7.5 and 10% NaOH at 50°C for 6 hr yielded 17.4, 18.2, 18.4, 18.6, 20.3, and 21.9% HLC, respectively. Further extraction of the corresponding alkali-treated and delignified poplar chips with 17.5% NaOH–1% Na2B4O7·10H2O at 20°C for 2 hr released 9.0, 8.2, 7.9, 7.2, 5.5, and 4.3% residual hemicelluloses, respectively. Xylose was found to be a predominant sugar component (84.3–90.5%) in all of the more linear, acidic, and larger molecular size HLC preparations ([Mbar]w , 26,090–40,500 g·mol−1), in addition to 4.0–4.8% bound lignin. These lignin molecules were found to be chemically linked to hemicelluloses mainly via syringyl units. On the other hand, all of the residual hemicellulosic fractions contained less xylose (57.3–59.1%) and less uronic acids (4.5–8.9%), but had a higher content of mannose (24.9–26.5%) and glucose (13.3–16.0%). In comparison with the HLC, the residual hemicellulosic fractions had a small molecular size ([Mbar]w , 18,300–29,730 g·mol−1), and had less than half the amount of bound lignin (1.5–1.7%). These lignin molecules were associated with residual hemicelluloses mainly via guaiacyl units. All of the cellulose fractions were relatively free of noncellulosic sugars (contained more than 99.6% glucose), and no considerable degradation of the cellulose was observed under the various separation conditions given.

Fractional isolation and physico-chemical characterization of alkali-soluble polysaccharides from sugar beet pulp

Pectin rich polysaccharide fractions were extracted with 2% sodium hydroxide at 45°C for 0.5, 1, 2, 3, 4 and 5 h, respectively, from sugar beet pulp. The neutral sugar rich polysaccharide preparations were obtained by treatment of the alkali extracts with pectinase. The pectin rich alkali-soluble polysaccharide–lignin complexes contained 36.7%–46.7% galacturonic acid, 13.7%–16.0% neutral sugars, 9.2%–14.8% protein, 3.6%–9.1% lignin and 18.9%–22.9% ash. The hemicellulose–lignin complex preparations were composed mainly of neutral sugars, in which galactose, rhamnose and arabinose were detected as the major sugar constituents. The weight-average molecular weights of the alkali-soluble polysaccharide–lignin complex fractions (38400–184900 Da) were much higher than those of the hemicellulose–lignin complex preparations (21100–26400 Da). The acetyl groups and methyl esters were found to be fully saponified during the alkali treatments, while feruloyl and p-coumaroyl esters appeared to be more resistant to the alkali conditions. Besides ferulic and p-coumaric acids, six other phenols were also firstly identified in the nitrobenzene oxidation products from all the isolated 12 fractions. These findings indicated that coumaryl, coniferyl and sinapyl alcohols are tightly associated with hemicelluloses and pectin in sugar beet pulp cell walls.

Studies of the degradation and protection of wood surfaces

Large weight losses occurred in thin veneers of radiata pine (P. radiata) during natural weathering due mainly to loss of lignin and hemicelluloses. This was demonstrated by direct chemical analysis and by spectrocopic techniques. Treatment of veneers with dilute aqueous solutions of the recognized surface stabilizing compound chromium trioxide was found to dramatically restrict weight loss during weathering, but similar applications of ferric compound were less effective (Evans, Schmalzl 1989). In order to rationalize these protective effects FTIR internal reflectance spectroscopy was used to study chemical changes taking place at the wood surface upon treatment and during weathering. The spectra obtained provide direct evidence for the modification and stabilization of the lignin aromatic system with aqueous chromium trioxide and to a lesser extent with ferric salts. It is postulated that photostable lignin complexes are formed. The implications of these findings for the development of improved surface stabilizing compounds for wood are discussed briefly.

Soluble lignin complexes isolated from wheat straw (Triticum arvense) and red clover (Trifolium pratense) stems by an in‐vitro method

AbstractSoluble degraded lignin complexes (LCs) were prepared using a two‐step incubation with buffered rumen fluid, and isolation of the LC from the latter liquid phase by filtration, dialysis and freeze drying. The yields of crude LC were 19 and 10 g kg−1 of initial dry matter for wheat straw (Triticum arvense L cv Holme) and red clover (Trifolium pratense L cv Hermes II) stems, respectively. Gel permeation chromatography of both LCs showed similar patterns with about 95% of the material having a molecular weight of 12 000 to 16 000 when compared with dextran standards. The total content of carbohydrates in the complexes was 82 g kg−1 DM in wheat straw LC and 50 g kg−1 DM in red clover stem LC. Uronic acid and arabinose residues dominated both species with xylose, galactose and glucose as important residues in wheat straw LC and galactose and glucose residues in red clover stem LC. The lignin content estimated by the acetyl bromide method was 328 g kg−1 DM in wheat straw LC and 139 g kg−1 DM in red clover stem LC, and nitrobenzene oxidation of both LCs yielded vanillin and syringic aldehyde in approximately equal amounts. NMR spectroscopy gave glycosidic, but not aromatic, signals, and ESR spectroscopy indicated the presence of stable radicals.