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In this paper we present an overview of the main strategies for physicochemical fractionation, sulphur- and chlorine-free delignification, and intelligent characterisation of various types of plant fibres, which have been experimentally investigated, as well as further developed and assessed by this group over the last ten years. To cite this article: E. Avgerinos et al., C. R. Biologies 327 (2004).

In this paper, multivariate chemometric analysis of fluorescence spectra of juvenile and mature eucalyptus wood samples is used in order to investigate the existence of any correlation between fluorescence data and, the physical/chemical properties, as well as the pulping data of eucalyptus wood. The results obtained demonstrated that the wood of juvenile and adult trees could be discriminated by fluorescence spectroscopy. Data analysis also has shown the existence of good statistical correlation between fluorescence data on the one hand, and ash content, sodium hydroxide solubility, and lignin monomeric composition of the wood samples on the other hand. Moreover, fluorescence spectral data were found to be well correlated with the pulping yield and the Kappa number of the kraft pulps issued from the adult and juvenile eucalyptus wood.

Fluorescence spectroscopy is a rapid, cheap, sensitive, specific and non-destructive method suitable for the characterization of autofluorescence-exhibiting lignins. Fluorescence spectral information, treated according to Principal Components Analysis (PCA), was used to study paper pulps from different biological sources. Wheat straw, fiber sorghum and sweet sorghum stalks were pulped with an aqueous ethanol solution under acid or alkali conditions. The results indicated that the fluorescence emission spectra of solid paper pulps through PCA could give information about the origin of the pulp sample and the type of pulping method used. Additional findings included the existence of a good correlation between the fluorescence data and the kappa number, as well as the lignin contents in the pulps, from the various raw materials.

Wheat straw was refined into fibres, hemicellulosic sugars and lignin oligomers by means of a multi-stage fractionation treatment: mechanical separation of the botanical components followed by one or two stage chemical fractionation process. Yellow winter wheat straw was initially separated into two fractions by milling in a Disc Mill: (a) chips, containing mostly internodes and (b) meal, consisting mainly of ground leaves and nodes. The internode fraction (63% of the whole straw) contained 8% more cellulose, 9% more lignin and 10% less ash compared with the unfractionated material. Disc mill fractionation was particularly effective with respect to the separation of the non-lignocellulosic components, i.e. protein, extractives, etc. Wheat straw internodes as well as whole straw were treated with acidic or alkaline aqueous ethanol in one or two stages; two-stage treatment involved the introduction of a pretreatment step with dilute acid improving, both, the production of hemicellulose derived sugars, and the aqueous-ethanol delignification to follow. The fibre yield in the case of internode chemical fractionation was generally higher in comparison with the one of the whole straw. The fibrous fractionation residues from internodes were in all cases enriched in cellulose compared with whole straw fibres. In spite of the higher initial lignin content of the internodes, delignification was more extensive than in the case of whole straw. Moreover, variations in the residual lignin composition were found, reflecting the intensity of the chemical fractionation process, as well as the origin of the raw material.

The monomeric composition of in situ lignins in sorghum, maize and wheat straw stems is investigated by two chemical degradation methods: thioacidolysis and alkaline nitrobenzene oxidation. It is shown that the structural information obtained in the case of lignin characterisation by alkaline nitrobenzene oxidation is strongly dependent on the reaction conditions, namely the temperature and the reaction time, as indicated by the significant variations in the yields of aromatic aldehydes that correspond to the three main constitutive lignin units, p -hydroxyphenyl (H), guaiacyl (G) and syringyl (S). Moreover, the complementary character of the two methods that stems from their different specificities is discussed in view of the molecular heterogeneity of lignins. Finally, the results obtained indicate a significant structural variability of the lignins of the three crops examined.

Stem bark and pith of sweet sorghum were analyzed with reference to their sucrose, simple reducing sugars, cellulose, hemicelluloses, lignin and associated phenolic acids contents. Moreover, lignin monomeric units (guaiacyl and syringyl) engaged in non-condensed structures were characterized by thioacidolysis, whereas cell wall associated phenolic acids ( p- coumaric and ferulic acids) were estimated by alkaline hydrolysis at 170°C. The results obtained showed that bark and pith are heterogeneous as far as their chemical composition and the structure of their chemical components are concerned. In particular, the pith content in water soluble sugars is twice as high compared with the one in the bark, whereas bark is enriched in lignocellulosic fibres. Bark lignin is twice as important in content and less condensed in structure compared to pith lignin. p- Coumaric acid is the predominant p- hydroxycinnamic acid associated to the cell walls, whereas ferulic acid is present in significant quantities.

The effect of temperature and time on the recovery yield of the products of alkaline nitrobenzene oxidation of in situ lignins in wheat straw inter-nodes and corresponding leaves were studied. The results showed that significant variations in the yields of aromatic aldehydes (p-hydroxybenzaldehyde, vanillin and syringaldehyde) as well as hydroxycinnamic acids (p-coumaric and ferulic acids) arise from changes in the temperature and the reaction time. The conditions for maximum yield depend on the aldehyde concerned. For example, the yield of p-hydroxybenzaldehyde increases with increasing temperature and reaction time. This phenomenon could be attributed to the low relative reactivity related to the specific electronic structure of the non-methoxylated units and/or to the existence of specific condensation reactions involving the p-hydroxyphenyl aromatic ring. Finally, the results indicate a much higher complexity of Gramineae lignins compared to those derived from hardwoods and softwoods.

Wheat straw was fractionated into cellulosic fibres, hemicellulose sugars and solid lignin oligomers through a two-stage, acid-catalyzed process. During the first stage, raw material was treated with dilute H2SO4 (0·5–2·5 n ) at reflux temperature (99·5 ± 1·0°C) for 10–60 min. Subsequently, delignification was conducted with H2SO4 (2 n ) in aqueous ethanol (62·5–87·5% EtOH v/v) at reflux temperature (81·0 ± 2·0°C) for 90 min. Selective hydrolysis of about 50% of the straw hemicelluloses (w/w on original straw hemicelluloses), converted to water-soluble oligo- and monosaccharides (first stage), followed by delignification in 87·5% v/v EtOH (second stage), led to optimal component fractionation efficiency with negligible cellulose loss (less than 2% w/w on original straw cellulose) and high lignin removal (more than 70% w/w on original straw lignin). By this two-stage process, high overall fibre yields (more than 60% on original raw material) and good pulp mechanical properties were achieved.

Stem bark and pith of sweet sorghum were analysed with reference to their sucrose, simple reducing sugars, cellulose, hemicelluloses, lignin and associated phenolic acids contents. Moreover, lignin monomeric units (guaiacyl and syringyl) engaged in non-condensed structures were characterized by thioacidolysis, whereas cell wall associated phenolic acids (p-coumaric and ferulic acids) were estimated by alkaline hydrolysis at 170°C. The results obtained showed that bark and pith are heterogeneous as far as their chemical composition and the structure of their chemical components are concerned. In particular, the pith content in water soluble sugars is twice as high compared to the one in the bark, whereas bark is enriched in lignocellulosic fibres. Bark lignin is twice more important in content and less condensed in structure compared to pith lignin. P-coumaric acid is the predominant p-hydroxycinnamic acid associated to the cell walls, whereas ferulic acid is present in significant quantities.

Occurrence and accumulation of mineral silicon has been frequently reported as a characteristic feature of gramineae. Trying to find evidence on the occurrence of chemical associations between silicon and lignin and/or associated polysaccharide in lignin-carbohydrate complexes (LCC), silicon content has been determined by a colorimetric procedure with reference to atomic absorption and X-ray fluorescence analysis. The distribution of silicon has been studied after fractionation of the LCC by alkaline hydrolyses. Content in neutral component sugars and in p -coumaric and ferulic acids associated as ester and ether to these LCC are also reported.