Chemical and instrumental characterization of maturing kenaf core and bast

A limiting factor in the production of bast fiber from kenaf is retting, th e process by which the fiber is freed from the non-fibrous tissue. An objec tive of this study was to evaluate the variation in cell wall chemistry wit h maturity for different cultivars, particularly in relation to lignin and retting. Two cultivars of kenaf (Hibiscus cannabinus), Tainung-1 (T-l) and Everglades-41 (E41), were harvested at 96 and 151 days post planting DPP, a nd the top and bottom 15 cm of the stems were excised for analysis. The han d-separated core and bast portions were analyzed for guaiacyl and syringyl groups (indicative of lignin), and arabinose, xylose, mannose, galactose, g lucose, and uronic acids. Cell walls from bast were examined for aromatics by ultraviolet (UV) absorption micro spectrophotometry. Bottom core contain ed significantly higher amounts of aromatics. Results suggested that the to p bast tissue was not completely lignified by the early harvest. Cellulose deposition, as indicated by the glucose content at 96 DPP, also was not ful ly complete until the later harvest. UV absorption microspectrophotometry d emonstrated that, while the entire bast fiber cell walls were all lignified , the middle lamelIa had higher absorbance indicative of more aromatic comp ounds. The lambda(max) at 274-276 nm was consistent with a predominance of syringyl lignin. In a second study, three cultivars of kenaf, Tainung-2 (T- 2), E41, and SF459, were harvested at 30, 60, 90, 120, and 180 DPP. The cen ter 15 cm of the stems was excised for analysis. All three cultivars were s imilar to each other in components within a maturity period. East and core fractions were relatively high in lignin even at 30 DPP, and both secondary layers and middle lamellea contained lignin. The core had more lignin than fiber at maturity. Plants increased in lignin to ca 60 DPP and did not inc rease thereafter, while carbohydrates continued to vary with maturity. Gluc ose concentrations became stable at about 90 DPP and xylose concentrations remained constant at 60 DPP. East fibers had unusually high syringyl:guaiac yl ratios (6.3-9.4). Principal component analysis (PCA) of the mass spectra l data of the bast indicated that 30 DPP samples were distinctly different from those harvested at 60-120 DPP, with the 180 DPP sample different from all others. In enzymatic retting studies of Everglades-41 bast from younger plants, i.e. 30-60 DPP were more easily retted than other harvests and mec hanical disruption improved retting of more mature bast. Enzymatic retting resulted in separation of fiber bundles, rather than ultimate fibers. (C) 1 999 Elsevier Science B.V. All rights reserved.