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 f rom timothy grass, were degraded using a commercial cellulase enzyme prepar ation. Timothy and lucerne were extensively degraded (60-70% loss of dry ma tter) 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 an d surface area by nitrogen adsorption. The pore regime of all of the sample s fell within the range 0.5-5 nm radius with pores of 1-2nm radius predomin ating. Degradation had little impact on pore regimes or mean pore radius wh ich were essentially the same as the starting material. The exception was t imothy where the mean pore radius fell from 2.80 to 2.36 nm (P < 0.001) acc ompanied by a fall in available surface area (2.49 to 1.39m(2)g(-1) P < 0.0 01). Wheat showed a similar fall in available surface area after 72h degrad ation (3.29 to 2.49m(2)g(-1) P < 0.05) but no significant differences were seen in the remaining samples. In a separate experiment, effect of lignific ation on pore structure was examined using sequential sections taken from a newly extended maize internode. The phenolic content differed two-fold bet ween the youngest and oldest sections and there was an accompanying steady decrease in electronegativity which weakly correlated with the phenolic con tent (r(2) = 0.504). Na significant differences were seen in pore volume, m ean pore radius or pore regimes along the internode, but the available surf ace area increased significantly with maturity (P < 0.01) and was strongly correlated with the phenolic content (r(2) = 0.896). Since porosity is defi ned 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 degradat ion. 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. (C) 1999 Society of Chemical Industry.