In lignified secondary walls of grasses and in hardwoods and softwoods, hydrophobic lignins overlie and encrust the cellulose microfibrils and matrix polysaccharides and proteins, and are variously covalently complexed, to a greater or lesser extent, with these wall polymers. Lignins can be regarded as hydrophobic fillers that replace the water in the developing wall (133). Because water is displaced during lignification, increased hydrogen bonding is favored both between non-cellulosic polysaccharides and between these and cellulose microfibrils. Moreover, the chemical bonds between lignin and the non-cellulosic matrix components cross-link the matrix phase and the cellulose microfibrils ensuring coherence (134). North — cote (133) likened lignified walls to a synthetic glass-fiber composite, which is rigid because of the lignified matrix and has great strength because of the cellulose microfibrils. This composite has a porosity [20-50 nm in wheat straw and pericarp (135,136)], that limits the approach of polysaccharide hydrolases to their substrates contributing to the recalcitrance of these wall types to enzymatic digestion.

Covalent cross-linking of wall polymers, in particular the lignin-polysaccharide associ­ations, prevents extraction of matrix polysaccharides from cell walls by neutral aqueous solvents and hydrogen bond breaking reagents. Alkaline reagents, which cleave most of the lignin-polysaccharide linkages and also solubilize some lignins (93,137), are required. Thus, extraction of heteroxylans from lignified secondary walls of grasses can be achieved by alkali at room temperature (83), alkaline H2O2 (138) or oxidative degradation with chlorite(139), which however leads to polysaccharide breakdown (140). Ammonia fiber/freeze explosion effective in pretreatment of e. g., maize stover, prior to enzymatic digestion, cleaves alkali la­bile LCC complexes (99, 141). (See also Chapter 14, Pretreatments for Enhanced Digestibility of Feedstocks)