Effect of degradation on the porosity and surface area of forage cell walls of differing lignin content

Cell walls, prepared from the stems of wheat, maize, lucerne and rape and from timothy grass, were degraded using a commercial cellulase enzyme preparation. Timothy and lucerne were extensively degraded (60-70% loss of dry matter) while dry matter losses from the more lignified maize, rape and wheat samples were substantially less (30-40%). Residues obtained after 6 and 72 h degradation and original preparations were examined for pore structure and surface area by nitrogen adsorption. The pore regime of all of the samples fell within the range 0.5-5 nm radius with pores of 1-2 nm radius predominating. Degradation had little impact on pore regimes or mean pore radius which were essentially the same as the starting material. The exception was timothy where the mean pore radius fell from 2.80 to 2.36 nm (P < 0.001) accompanied by a fall in available surface area (2.49 to 1.39 m2 g-1 P < 0.001). Wheat showed a similar fall in available surface area after 72 h degradation (3.29 to 2.49 m2 g-1 P < 0.05) but no significant differences were seen in the remaining samples. In a separate experiment, effect of lignification on pore structure was examined using sequential sections taken from a newly extended maize internode. The phenolic content differed two-fold between the youngest and oldest sections and there was an accompanying steady decrease in electronegativity which weakly correlated with the phenolic content (r2 = 0.504). No significant differences were seen in pore volume, mean pore radius or pore regimes along the internode, but the available surface area increased significantly with maturity (P < 0.01) and was strongly correlated with the phenolic content (r2 = 0.896). Since porosity is defined by the spacing between polymers of the wall, a considerable commonality of three-dimensional structure evidently existed amongst the cell walls of the plants examined which was little affected by lignification or degradation. This is consistent with exclusion of enzymes by the wall which ensures that bacterial attack in the rumen and elsewhere proceeds by a process of surface erosion.