Control of pH by CO₂ Pressurization for Enzymatic Saccharification of Ca(OH)₂-Pretreated Rice Straw in the Presence of CaCO₃

Abstract

The aim of this study was to investigate the effect of pH control by CO₂ pressurization on the enzymatic hydrolysis of herbaceous feedstock in the calcium capturing by carbonation (CaCCO) process for fermentable sugar production. The pH of the slurry of 5% (w/w) Ca(OH)₂-pretreated/CO₂-neutralized rice straw could be controlled between 5.70 and 6.38 at 50 °C by changing the CO₂ partial pressure (pCO₂) from 0.1 to 1.0 MPa. A mixture of fungal enzyme preparations, namely, Trichoderma reesei cellulases/hemicellulases and Aspergillus niger β-glucosidase, indicated that pH 5.5–6.0 is optimal for solubilizing sugars from Ca(OH)₂-pretreated rice straw. Enzymatic saccharification of pretreated rice straw under various pCO₂ conditions revealed that the highest soluble sugar yields were obtained at pCO₂ 0.4 MPa and over, which is consistent with the expected pH at the pCO₂ without enzymes and demonstrates the effectiveness of pH control by CO₂ pressurization.