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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.

Publisher Summary A convenient method for the preparation of lignin fractions from biomass is provided by the combination of mild solvolytic conditions with a good lignin solvent. Dioxane is a particularly interesting isolating agent of lignin fractions because of its solubility parameter, which is very close to the optimum value for isolated lignin preparations. Preparation of dioxane acidolysis lignins has the main advantage of providing, in the shortest time, lignin fractions in high yields and in a form suitable for further chemical and biotechnological investigations. This chapter discusses two procedures for preparation of dioxane lignins with their quantitative and qualitative comparisons. Two procedures for isolation have been adapted: the first one is carded out at ambient temperature according to Freudenberg, and the second is performed at reflux temperature, approximating the procedure of Pepper et al. Acidolysis conditions are, however, similar to those chosen by Lapierre et al. for characterization of lignins by analysis of acidolysis products.