Cp. Neto et al., CHEMICAL-COMPOSITION AND STRUCTURAL FEATURES OF THE MACROMOLECULAR COMPONENTS OF HIBISCUS-CANNABINUS GROWN IN PORTUGAL, Industrial crops and products, 5(3), 1996, pp. 189-196
Different morphological regions of Hibiscus cannabinus plants grown in
Portugal were submitted to chemical composition studies. General chem
ical composition was determined by established methods. The polysaccha
rides were fractionated by successive extractions of holocellulose wit
h aqueous KOH solutions. The sugar composition was determined by hydro
lysis 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) an
alysis of the methylated oxidation products. The results of general ch
emical analysis have evidenced the different relative abundance of hol
ocellulose, lignin. proteins, extractives and ashes in bark, core and
foliage, at different stages of maturity. About 70-80% of the core hem
icelluloses (about 20% o.d. material) was easily extracted with 5% KOH
aqueous solutions against 60-70% (about 15% o.d. material) for the he
micelluloses of the bark. This hemicellulose fraction composed mainly
by glucuronoxylans with high content of uronic acids (xylose:uronic ac
id: 3-5:1 for bark and 5-10:1 for core). The hemicellulose fraction ex
tracted with 24% KOH aqueous solutions was composed mainly by glucuron
oxylans (80-90%) and glucomannans (10-15%). The results obtained by th
e permanganate oxidation method indicated that kenaf lignins are H-G-S
type with approximate H:G:S molar proportions of (9-13):(55-60):(27-3
4) in bark and (14-20):(57)-(74):(12-23) in core, which evidences a hi
gh content of H and G units and a relatively low content of S units wh
en 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 w
ith traditional wood lignins.