An improved acetyl bromide procedure for determining lignin in woods and wood pulps

Summary The acetyl bromide procedure for spectrophotometrically determining lignin in wood and wood pulp samples has been modified by adding perchloric acid to the digestion medium. This enables faster dissolution of the materials and the use of coarser samples. Subsequent treatment of the digestion mixtures with larger amounts of sodium hydroxide than those used in the conventional procedure has obviated the need for treatment with hydroxylamine. The modified method has been successfully applied to woodmeals and pulps from Pinus radiata and Eucalyptus species.     

Distribution of lignin in normal and tension wood

Summary The distribution of lignin in normal and tension wood of four hardwood species has been studied by examination in the electron microscope of the lignin skeletons remaining after removal of the polysaccharides with hydrofluoric acid. In normal wood fibers, the S₁ had a higher lignin concentration than the S₂ layer, which was not as highly lignified as in conifer tracheids. Vessels had a high concentration of lignin in both normal and tension wood, while the extent of lignification of the parenchyma was variable.In tension wood fibers, the S₁ and S₂ layers were highly lignified. A thick, unlignified G-layer was often associated with an extremely thin S₂ layer with a high concentration of lignin. In both normal and tension wood, the lignin had the same orientation as the cellulose micro-fibrils in the different cell wall layers. The results confirm the earlier conclusion that, in the species investigated, the same amount of lignin is present in gelatinous as in normal fibers. Evidently, the lignification mechanism operates normally in the non-gelatinous layers of the fibers, as well as in the vessels and in the parenchyma of tension wood.

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Zusammenfassung Die Ligninverteilung im Normalholz und im Druckholz von vier Laubhölzern wurde untersucht. Die Ligningerüste, die nach der Entfernung der Polysaccharide durch Fluorwasser-stoffsäure übrigblieben, wurden im Elektronenmikroskop beobachtet. In den Normalholzfasern hatte die S₁-eine höhere Ligninkonzentration als die S₂-Schicht, die weniger lignifiziert war als in den Koniferentracheiden. Die Gefäße hatten eine hohe Ligninkonzentration in sowohl Normal-als in Zugholz, während der Lignifizierungsgrad der Parenchymzellen variierte.In den Zugholzfasern waren die S₁- und S₂-Schichten völlig lignifiziert. Eine dicke, unlignifizierte G-Schicht war oft mit einer außerordentlich dünnen S₂-Schicht, die eine hohe Ligninkonzentration zeigte, verbunden. Sowohl im Normal- wie auch im Zugholz besaß das Lignin dieselbe Orientierung wie die Cellulosemikrofibrillen in den verschiedenen Zellwandschichten. Die Ergebnisse bestätigen den früheren Schluß, daß in den hier untersuchten Laubhölzern in den gelatinösen und in den normalen Fasern dieselbe Ligninmenge vorliegt. Offenbar läuft der Mechanismus der Lignifizierung in den S₁- und S₂-Schichten der gelatinösen Fasern des Zugholzes normal ab.

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This investigation was supported by the United States Department of Agriculture, Forest Service, through Forest Service Research Grant No. 1, which is hereby gratefully acknowledged.     

Condensation of lignin during heating of wood

Summary The structural change of lignin during heating of wood was investigated quantitatively by a method combining nucleus exchange and nitrobenzene oxidation. Lignin modification during heating was mainly a diphenylmethane type condensation. About 40 and 75% of noncondensed units in protolignins were converted to diphenylmethane type units by heating of dry and wet wood meals up to 220 °C, respectively. On the other hand, during heating of modified lignin (dioxane lignin) various types of modifications in addition to diphenylmethane type condensation occurred. Lignin modification via the diphenylmethane type condensation was proposed as a new route for its utilization.     

Studies on milled wood lignin from spruce Part II. Electron microscopic observations on the milled wood

Summary Wood samples that had been ball-milled and extracted with dioxane under various conditions were observed in the scanning electron microscope. Ball milling causes not only a reduction in size of the cells and cell walls but also an internal disruption of the particles. The intensity of milling has a larger influence on the particle size than a prolongation of the milling time. A direct relation was established between particle size and the previously determined yield of purified milled wood lignin.Submitted by Erwin-Riesch-Stiftung zur Förderung der wissenschaftlichen Forschung.     

Analysis of birch wood lignin by oxidative degradation

Summary The lignin in birch wood has been subjected to analysis by oxidative degradation after milling of wood samples in a Wiley mill and in a rotatory ball mill. For comparison milled wood lignin from birch has also been analysed. It was found that the type of milling affects both the total yield of degradation acids and their relative frequency of occurrence. These findings are interpreted as being due to a cleavage of aryl ether linkages during the milling procedure. The oxidative degradation analysis gave rise to a total of 30 different acids which were tentatively identified on the basis of their mass spectral fragmentation patterns. Among the acids some were found having either a phenoxyacetic acid or a phenoxypropionic acid structure. The possible origin of these is discussed. A grant to one of us (R.A.N.) from Jacob Wallenbergs Forskningsstiftelse is gratefully acknowledged. Thanks are due to Kristina Gustafsson for skilful experimental assistance     

Residual extractives in aspen kraft pulps and their impact on kappa number and Klason lignin determination

We investigated the impact of residual extractives on lignin determination by lignin content difference between unextracted and extracted pulps, residual extractives analysis, and lignin content contribution from model extractive compounds. There were two different kinds of extractives in aspen kraft pulp. The extractives impacting on kappa number determination were well removed in oxygen delignification; these were mainly unsaturated fatty acids. However, the extractives impacting on Klason lignin determination were largely resistant to oxygen delignification; these were mainly saturated fatty acids, sterols, and hydrocarbons. Oxidation of unsaturated fatty acids was the main reaction in oxygen delignification. These trends were confirmed by simulation of lignin content determination with three model extractive compounds (β-sitosterol, linoleic acid, and palmitic acid). The publication of this article was made possible by an Emachu Research Fund. The authors are grateful for the fund.     

Anatomy and lignin distribution of reaction wood in two Magnolia species

Summary Anatomical features of reaction wood formed in two Magnolia species, M. obovata Thunb. and M. kobus DC. which are considered to be among the primitive angiosperms, were observed. In addition, the distribution of guaiacyl and syringyl units of lignins in the cell walls of normal and reaction wood was examined using ultraviolet (UV)- and visible light (VL)- microspectrophotometry coupled with the Wiesner and M?ule reactions. The two Magnolia species formed a tension-like reaction wood without possessing the typical gelatinous layer (G-layer) on the upper side of the inclined stem or branch, in which a radial growth promotion occurred. Compared with the normal wood, the reaction wood had the following anatomical features: (1) the secondary walls of fiber tracheids lacked the S₃ layer, (2) the innermost layer of fiber-tracheid walls showed a small microfibril angle, a fact being similar to the orientation of the microfibril angle of the G-layer in tension wood, and (3) the amounts of lignin decreased in the cell walls of fiber tracheids, especially with great decrease in proportion of guaiacyl units in lignins. In addition, VL-microspectrophotometry coupled with the Wiesner and M?ule reactions adopted in the present study showed potential to estimate the lignin contents in the cell walls and the proportion of guaiacyl and syringyl units in lignins. Received: 15 July 1998     

Chemical structural elucidation of total lignins in woods I: fractionation of the lignin in residual wood meal after extraction of milled wood lignin

Residual wood meal after extraction of milled wood lignin (WMEM) ofEucalyptus globulus was extracted with alkali and LiCl/N,N-Dimethylacetamide (DMAc). These agents dissolve mainly hemicellulose and cellulose, respectively. The extractability of WMEM in alkali solutions was influenced by the degree of swelling of the cellulose. Under good swelling conditions, considerable amounts of cellulose and lignin were extracted with the hemicellulose. Maximum extractability was about 60% of the WMEM under optimum conditions (3 M or 5 M LiOH or 3M NaOH solution). Some portion of cellulose was extracted with LiCl/DMAc at room temperature. Thus, lignin inE. globulus WMEM was divided into three fractions: hemicellulose-lignin fraction, cellulose-lignin fraction, and insoluble-lignin fraction.Part of this work was presented at the 49th annual meeting of the Japan Wood Research Society, Tokyo, April 1999; and at the 50th annual meeting of the Japan Wood Research Society, Kyoto, April 2000     

Notes on the spectrophotometric determination of lignin in wood samples

A method developed by Johnson et al. whereby lignin is determined spectrophotometrically was found to yield lignin values which showed variations of up to 10%. The presence of water in, and the freshness of the digestive mixture together with the grade of acetic acid used and the time between dilution and measurement of absorbance were found to affect the reproducibility of the method.     

First isolation of pine wood nematode from Pinus tabuliformis forests in China

Previous risk prediction studies have mostly shown that suitable areas for pine wood nematode, Bursaphelenchus xylophilus (PWN), in China are mainly concentrated in eastern and southern China, with pest reaching no further northern than the Yangtze River basin. The Chinese pine (Pinus tabuliformis, Chinese pine or Chinese red pine) is the main, native, afforestation species in areas to the north of the Yangtze River basin. To our knowledge, there is no report on PWN infecting Chinese pines in the north of the Yangtze. In this study, we used Baermann funnel method and several PCR primers to detect the presence of PWN in Chinese pine forest fields in 2010 in Shaanxi, China, which is far to the north of the Yangtze River and a cause for concern. Pinus koraiensis (Korean pine) may also be at risk and should also be monitored to prevent invasion by PWN.     

New approaches to wood bonding A base-activated lignin adhesive system

Current knowledge of wood surface characteristics and surface modification are briefly reviewed and the postulated effects of chemical activation are summarized. It was found that aqueous sodium hydroxide can effectively activate wood surfaces to give strong dry autohesive bonds, but only low wet strength was obtained. However, excellent dry and wet wood bond strengths, equivalent to phenol-formaldehyde bonded samples, were obtained when methylolated lignin was used in combination with 3N sodium hydroxide activation. Several mechanisms of base activation are suggested, including enhanced wood surface contact and reactivity.     

Structure of small lignin fragments retained in water-soluble polysaccharides extracted from birch MWL isolation residue

To analyze the structural features of lignin in the vicinity of lignin–carbohydrate linkages, water-soluble lignin–carbohydrate complex (LCC) with low lignin content was prepared from residual birch wood meal after the extraction of milled wood lignin (MWL). The molecular weight distribution of lignin in this LCC appeared together with carbohydrate in the relatively high molecular weight region of the gel permeation chromatogram. This result was consistent with our previous results obtained for the same fraction of Japanese cedar (sugi); however, after treatment with polysaccharide-degrading enzyme, the molecular weight distribution of carbohydrate and that of lignin shifted significantly to the lower region. These results demonstrated that molecular size of this LCC is determined by carbohydrates while lignin is present as a minor fragment in this fraction. The syringyl/guaiacyl (S/V) ratio of this LCC was higher than other lignin fractions. Ozonation analysis implied that this LCC has a relatively high number of β-1 structures. It is likely that lignin that exists near lignin–carbohydrate linkages has more endwise-type features than other lignin fractions.This paper was presented in part at the 48th Lignin Symposium, Fukui, Japan, October 2003 and at the 12th International Symposium on Wood and Pulping Chemistry, Madison, USA, June 2003     

Ester linkages between lignin and glucuronoxylan in a lignin-carbohydrate complex from beech (Fagus crenata) wood

Summary A water-soluble lignin-carbohydrate complex (LCC) isolated from beech (Fagus crenata) MWL was investigated. Results from gelfiltration chromatography and the infrared spectrum of the LCC treated with alkali under mild conditions indicated that the LCC contained alkali-labile bonds. Decrease of uronic acid content and the detection of 4-O-methylglucose in the sodium borohydride-reduced LCC suggested the presence of an ester linkage between lignin and glucuronic acid in the glucuronoxylan. Conductometric titration also indicated the existence of glucuronic acid ester linked to lignin. From these results, it is concluded that the LCC contained an ester linkage between lignin and glucuronoxylan and that about one-third of the glucuronic acid present in the LCC was involved in this ester linkage.     

Structure of small lignin fragment retained in water-soluble polysaccharide extracted from sugi MWL isolation residue

In order to analyze the structural features of small lignin fragments that are closely associated to polysaccharides, lignin carbohydrate complex (LCC) with low lignin content was extracted with water from the residual wood meal (sugi, Cryptomeria japonica) of milled wood lignin (MWL) isolation. This LCC exhibited almost the same neutral sugar composition as those extracted by other LCC solvents (dimethylformamide, dimethylsulfoxide, and others) but the lignin content was only 5.3%, which was much lower than others. Gel filtration chromatography demonstrated that lignin in this LCC was found together with carbohydrates at the higher molecular weight region, but after the treatment with carbohydrate-degrading enzymes the apparent molecular weight of both lignin and carbo-hydrates decreased significantly. Using a mild alkaline treatment, the apparent molecular weight of lignin also decreased while that of polysaccharide was not affected. These data indicated that lignin in this LCC is present as small fragments attached to high molecular weight polysaccharide at least partly by alkali-unstable linkages. Structural analysis by ozonation method revealed that the lignin in this LCC was lower in erythro/threo ratio of -O-4 structure and relatively richer in the threo type structure carrying C-aryl linkages at -position (-5 and/or -1) than other lignin fractions present in MWL, LCCs extracted by other solvents, and their extraction residues. It was suggested that the chemical structure of lignin closely associated to carbohydrates was different from that of the main part of lignin.This paper was presented in part at the 52nd Annual Meeting of the Japan Wood Research Society, Gifu, Japan, April 2002, at the 47th Lignin Symposium, Gifu, Japan, October 2002, and at the 12th International Symposium on Wood and Pulping Chemistry, Madison, USA, June 2003     

Attempts to understand the nature of phenolic and etherified components of wood lignin

Summary Periodate oxidation, because of its high selectivity in degrading phenolic nuclei, has been combined with nitrobenzene oxidation and phenyl nucleus exchange techniques to investigate the nature of wood lignin in situ. For both softwood and hardwood, the phenolic and etherified components of wood lignin have been shown to differ significantly in chemical composition, and the etherified lignin structure appears to be substantially more condensed.Financial support from the Empire State Paper Research Associate (ESPRA) and by the NRICGP of USDA (No. 93-37103-9318) is greatly appreciated     

Distribution of lignin in normal and compression wood of Pinus taeda L.

Summary The distribution of lignin in normal and compression wood of loblolly pine (Pinus taeda L.) has been studied by the technique of lignin skeletonizing. Hydrolysis of the wood carbohydrates with hydrofluoric acid left normal wood tracheids with a uniform distribution of lignin in the S1 and S2 cell wall layers. However, the S3 region of both earlywood and latewood tracheids consistently retained a dense network of unhydrolyzable material throughout, perhaps lignin.Lignin content in compression wood averaged about 7% more than in normal wood and appears to be concentrated in the outer zone of the S2 layer. The inner S2 region, despite helical checking, is also heavily lignified. The S1 layer, although thicker than normal in compression wood tracheids, contains relatively little lignin.Ray cells, at least in normal wood, appear to be lignified to the same extent, if not more so in certain cases, than the longitudinal tracheids. Other locations where lignin may be concentrated include initial pit border regions and the membranes of bordered pits.This report is a detailed excerpt from the Ph. D. dissertation of R. A. P. Financial support provided by the College of Forestry at Syracuse University and the National Defense Education Act is hereby gratefully acknowledged.     

The effect of lignin on the bending properties and fixation by cooling of wood

To clarify the effects of lignin on the fixation of bending deformation by cooling, cooling set for delignified woods with various lignin residues were investigated to compare with mechanical and dynamic viscoelastic properties. Bending tests showed that steep reductions occurred in the modulus of elasticity and modulus of rupture with delignification during the initial stage of delignification. The dynamic viscoelastic measurements revealed that the peak temperature of tan δ due to micro-Brownian motion of lignin was reduced with delignification, and the peak disappeared in the temperature range of 5°–100°C for the specimens that had lost more than 21% of their weight. On the other hand, no clear change in residual set was found in the range of 0%–15% of weight loss in spite of a marked reduction in lignin content. Subsequently, set decreased steeply for the specimens delignified beyond 15% of weight loss. It was suggested that cooling set is not determined solely by lignin content but is influenced by changes in the quality of lignin due to delignification. Lignin quality affects the balance of the elastic potential to recover from deformation and its viscosity, which is an indication of resistance against flow. Part of this report was presented at the 57th Annual Meeting of the Japan Wood Research Society, Hiroshima, August 2007     

Studies on opposite wood in conifers part III: Distribution of lignin

Summary The distribution of lignin in opposite wood has been studied by removing the polysaccharides with hydrofluoric acid and examining the resulting lignin skeletons in the electron microscope. The thick S₃ layer was more highly lignified than the S₁ and S₂ layers in Abies balsamea, Picea rubens, Pinus resinosa, and Tsuga canadensis. In Picea rubens, but not in the other species, there was, adjacent to the S₃ layer, a transition zone in S₂ with a high concentration of lignin. The S₃ layer varied considerably in thickness and was often buckled, especially in the latewood. The structure of the bordered pits was that observed in the original wood. The margo, the torus, and the initial pit border were all highly lignified.This paper is dedicated to Dean Edwin C. Jahn in honor of his 70th birthday.