Ultrastructure of steam-exploded wood

Summary The ultrastructure of steam-exploded wood from the softwood Pinus radiata D. Don was examined by electron microscopy in order to determine the reasons for increased porosity and enhanced susceptibility to enzymatic hydrolysis. Ultrastructural changes were observed in the form of lignin redistribution and swelling of the cellulose framework. Lignin showed evidence of melting, having contracted into well defined agglomerates suspended in a web of cellulose. Using lanthanum and gold tracers of known particle size the pores in the microfibrillar cell wall have been examined. Cellulose regions were shown to contain numerous pores greater than 2 nm, while lignin agglomerates did not contain such pores. Treatment with NaOH resulted in lignin being smeared over the porous cell wall material — hence blocking pores and reducing digestibility.The authors gratefully acknowledge the assistance of T. A. Clark and K. F. Deverell during the course of this work