Analyses of carbohydrates and lignin in the surface and inner layers of softwood pulp fibers obtained employing various alkaline cooking processes

The chemical compositions of the surface and inner layers of softwood fibers, obtained employing various alkaline cooking processes, have been characterized in the present study. The surface and inner layers of fibers in soda, soda-anthra-quitione and soda-polysulfide pulps, as well as three different kraft pulps were collected by mechanical peeling. Subsequently, the carbohydrate and lignin compositions of these materials were determined utilizing enzymatic hydrolysis followed by capillary electrophoretic analysis (referred to herein as enzyme/CE), as well as by analytical pyrolysis in combination with gas chromatography and mass spectrometry (pyrolysis-GC/MS). In the case of all of the pulps investigated, the fiber surface layer was found to contain significantly higher contents of arabinohexenuronoxylan and lignin than were present in the inner layers. Depending on the cooking process employed, the xylan and lignin contents of the surface layer were 15-35% and 120-180%, respectively, greater than the corresponding values for the inner layer. The pulps obtained using kraft cooking processes demonstrated the highest surface contents of arabinohexenuronoxylan. The surface material isolated from the soda pulp exhibited the lowest xylan content. Furthermore, these same soda pulp fibers demonstrated the most pronounced difference between the lignin contents of the surface and inner layers. In all cases, the hemicelluloses and lignin in the surface layer were shown to have chemical compositions that differed from those of the corresponding polymers in the inner layers of the fibers.