Optimization of steam explosion to enhance hemicellulose recovery and enzymatic hydrolysis of cellulose in softwoods

A combination of Douglas fir heartwood and sapwood chips were steam pretrea ted under three conditions as measured by the Severity Factor (log R-o), wh ich incorporated the time, temperature/pressure of pretreatment. By adjusti ng the steam pretreatment conditions, it was hoped to recover the majority of the hemicellulose component as monomers in the water-soluble stream, whi le providing a cellulosic-rich, water-insoluble fraction that could be read ily hydrolyzed by cellulases. These three conditions were chosen to represe nt either high hemicellulose sugar recovery (low severity [L], log R-o = 3. 08), high-enzyme hydrolyzability of the cellulosic component (high severity [H], log R-o = 4.21), and a compromise between the two conditions (medium severity [M], log R-o = 3.45). The medium-severity pretreatment conditions (195 degrees C, 4.5 min, 4.5 % SO2; log R-o = 3.45) gave the best compromis e in terms of relatively high hemicellulose recovery after stream pretreatm ent and the subsequent efficiency of enzymatic hydrolysis of the water-inso luble cellulosic fraction. The percent recovery of the original hemicellulo se in the water-soluble fraction dropped significantly when the severity wa s increased (L-76.8%, M-64.7%, and H-37.5%). However, the ease of enzymatic hydrolysis of the cellulose-rich, water-insoluble fraction increased with increasing severity (L-24%, M-86.6%, and H-97.9%). Although more severe pre treatment conditions provided optimum hydrolysis of the cellulosic componen t, less severe conditions resulted in better recovery of the combined hemic ellulose and cellulosic components.