Enzymic saccharification of sugarcane bagasse pretreated by autohydrolysis–steam explosion

Abstract

Pretreatment of bagasse by autohydrolysis at 200°C for 4 min and explosive defibration resulted in the solubilization of 90% of the hemicellulose (a heteroxylan) and in the production of a pulp that was highly susceptible to hydrolysis by cellulases from Trichoderma reesei C-30 and QM 9414, and by a comercial preparation, Meicelase. Saccharification yields of 50% resulted after 24 h at 50°C (pH 5.0) in enzymic digests containing 10% (w/v) bagasse pulps and 20 filter paper cellulase units (FPU). Saccharifications could be increased to more than 80% at 24 h by the addition of exogenous β-glucosidase from Aspergillus niger. The crystallinity of cellulose in bagasse remained unchanged following autohydrolysis-explosion and did not appear to hinder the rate or extent of hydrolysis of cellulose. Autohydrolysis-exploded pulps extracted with alkali or ethanol to remove lignin resulted in lowere conversions of cellulose (28–36% after 25 h) than unextracted pulps. Alkali extracted pulps arising from autohydrolysis times of more than 10 min at 200°C were less susceptible to enzymic hydrolysis than unextracted pulps and alkali-extracted pulps arising from short autohydrolysis times (e.g., 2 min at 200°C). Autohydrolysis-explosion was as effective a pretreatment method as 0.25M NaOH (70°C/2 h) both yielded pulps that resulted in high cellulose conversions with T. reesei cellulase preparations and Meicelase. Supplementation of T. reesei C-30 cellulose preparations with A. niger β-glucosidases was effective in promoting the conversion of cellulose into glucose. A ration of FPU to β-glucosidase of 1:1.25 was the minimum requirement to achieve more than 80% conversion of cellulose into glucose within 24 h. Other factors which influenced the extent of saccharification of autohydrolysis-exploded bagasse pulps were the enzyme-substrate ratio, the substrate concentration, and the saccharification mode.