Chemical composition and structural features of the macromolecular components of Hibiscus cannabinus grown in Portugal

Different morphological regions of Hibiscus cannabinus plants grown in Portugal were submitted to chemical composition studies. General chemical composition was determined by established methods. The polysaccharides were fractionated by successive extractions of holocellulose with aqueous KOH solutions. The sugar composition was determined by hydrolysis of polysaccharides followed by gas chromatography (GC) analysis of neutral sugars and spectrophotometric determination of uronic acids. In situ lignins and milled wood lignins (MWL) were characterised by permanganate oxidation followed by GC and GC-MS (mass spectrometry) analysis of the methylated oxidation products. The results of general chemical analysis have evidenced the different relative abundance of holocellulose, lignin. proteins, extractives and ashes in bark, core and foliage, at different stages of maturity. About 70-80% of the core hemicelluloses (about 20% o.d. material) was easily extracted with 5% KOH aqueous solutions against 60-70% (about 15% o.d. material) for the hemicelluloses of the bark. This hemicellulose fraction composed mainly by glucuronoxylans with high content of uronic acids (xylose:uronic acid: 3-5:1 for bark and 5-10:1 for core). The hemicellulose fraction extracted with 24% KOH aqueous solutions was composed mainly by glucuronoxylans (80-90%) and glucomannans (10-15%). The results obtained by the permanganate oxidation method indicated that kenaf lignins are H-G-S type with approximate H:G:S molar proportions of (9-13):(55-60):(27-34) in bark and (14-20):(57)-(74):(12-23) in core, which evidences a high content of H and G units and a relatively low content of S units when compared with traditional dicotyledons. The relative proportion of H, G and S units as well as the structural features of lignins depends on the stage of maturity and on the morphological region of the plant. Kenaf lignins present a lower degree of condensation when compared with traditional wood lignins.