Enzyme adsorption on SO 2 catalyzed steam-pretreated wheat and spruce material

Abstract

Lignocellulose is widely recognized as a sustainable substrate for biofuels production, and the enzymatic hydrolysis is regarded as a critical step for the development of an effective process for the conversion of cellulose into ethanol. One key factor affecting the overall conversion rate is the adsorption capacity of the cellulase enzymes to the surface of the insoluble substrate. Pretreatment has a strong impact on hydrolysis, which could be related to both chemical changes and morphological changes of the material. In the current work, the accessibility of four differently pretreated wheat straw substrates, two differently pretreated spruce materials, and Avicel cellulose was investigated. Adsorption isotherms (at 4 °C and 30 °C) for a cellulase preparation were obtained, and the rates of hydrolysis were determined for the different materials. Furthermore, the surface area and pore size distribution of the various materials were measured and compared to adsorption and hydrolysis properties, and the structures of the pretreated materials were examined using scanning electron microscopy (SEM). The results demonstrated a positive correlation between enzyme adsorption and the substrate specific surface area within each feedstock. Overall, the amount of enzyme adsorbed was higher for pretreated spruce than for the pretreated wheat straw, but this was not accompanied by a higher initial rate of hydrolysis for spruce. Also, the difference in the measured endoglucanase adsorption and overall FPU adsorption suggests that a larger fraction of the enzyme adsorbed on spruce was unproductive binding. The SEM analysis of the material illustrated the structural effects of pretreatment harshness on the materials, and suggested that increased porosity explains the higher rate of hydrolysis of more severely pretreated biomass.