Optimization of Steam Explosion to Enhance Hemicellulose Recovery and Enzymatic Hydrolysis of Cellulose in Softwoods

Abstract

A combination of Douglas fir heartwood and sapwood chips were steam pretreated under three conditions as measured by the Severity Factor (log R_°), which incorporated the time, temperature/pressure of pretreatment. By adjusting the steam pretreatment conditions, it was hoped to recover the majority of the hemicellulose component as monomers in the water-soluble stream, while providing a cellulosic-rich, water-insoluble fraction that could be readily hydrolyzed by cellulases. These three conditions were chosen to represent either high hemicellulose sugar recovery (low severity [L], log R_° = 3.08), high-enzyme hydrolyzability of the cellulosic component (high severity [H], log R_° = 4.21), and a compromise between the two conditions (medium severity [M], log R_° = 3.45). The medium-severity pretreatment conditions (195°C, 4.5 min, 4.5% SO₂. logR_° = 3.45) gave the best compromise in terms of relatively high hemicellulose recovery after stream pretreatment and the subsequent efficiency of enzymatic hydrolysis of the water-insoluble cellulosic fraction. The percent recovery of the original hemicellulose in the water-soluble fraction dropped significantly when the severity was 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 pretreatment conditions provided optimum hydrolysis of the cellulosic component, less severe conditions resulted in better recovery of the combined hemicellulose and cellulosic components.