Physical properties affecting the bioconversion of maize stems and other fibre-rich crop residues
The aim of this work was to develop methods of measuring pore size and the related surface area and pore volume, and then to apply these methods in studying the effects of degradation and lignification on these properties. Colloidal gold particles of prescribed sizes (2-10 nm in diameter) were used as molecular probes to determine pore size in maize and rape walls of different cell types. The particles were visualised by electron microscopy, and this indicated that the pore size in maize parenchyma walls was somewhere between 3 and 5 nm. Gas adsorption analysis provides information on pore size, surface area and pore volume, and was used to study cell walls of maize, alfalfa, wheat, timothy, and rape. This showed that the overall porous structure of the plants selected was very similar. The pore size distributions showed that the vast majority of pores had radii less than 3 nm, although wheat and timothy appeared to have a greater number of larger pores. The physical properties of plants containing type 1 walls were also investigated by this method. This illustrated how gas adsorption could be used to detect the changes in porosity and surface area which arise from the extraction of pectic polysaccharides. The effect of degradation on porosity was investigated using the same five plant types as had been studied previously. It was found that there was no great change in the porous structure as the walls were digested, except for wheat and timothy which both lost the larger pores they possessed before degradation. This resulted in pore size distributions more typical of the other three plant types. The data indicated that degradation occurred by a surface erosion process. Porosity and surface area did not alter greatly between cell walls of varying levels of lignification.