Weighing the factors behind enzymatic hydrolyzability of pretreated lignocellulose

The major factors determining enzymatic hydrolyzability of pretreated wheat straw were analyzed and their relative importance quantified. The effects of NaOH-delignification, autohydrolysis and their combination at different severities were analyzed by determining the pore size distribution (DSC-thermoporometry), the cellulose surface area and the accessible phenolic hydroxyls on the lignin surface (adsorption of Congo Red and Azure B; ATR-FTIR) and crystallinity (WAXD). The correlation of these factors with initial and overall enzymatic hydrolyzability was studied and further arranged in order through principal component analysis. The major positive factors affecting hydrolyzability were the cellulose surface area and the accessibility of the pore system, while the lignin content was the major negative factor accompanied by cellulose crystallinity. Autohydrolysis effectively increased the cellulose surface area by hemicellulose dissolution, but the high lignin content associated with small pores led to a lower hydrolyzability compared to delignified straw. Besides the removal of lignin, delignification led to a more accessible pore structure, which was supported by the remaining hemicellulose. Additionally, delignification increased the hydrophilicity of the remaining lignin, which also increased hydrolyzability. All pretreatments decreased cellulose crystallinity, which particularly increased the initial hydrolysis, and also improved the final carbohydrate conversion. The established weighed order of the factors behind enzymatic carbohydrate conversion is an important milestone in the path towards more efficient lignocellulosic sugar utilization in biorefineries.