Enzymatic hydrolysis of pretreated rice straw

Abstract

California rice straw is being evaluated as a feedstock for production of power and fuel. This paper examines the initial steps in the process: pretreatment of rice straw and enzymatic hydrolysis of the polysaccharides in the pretreated material to soluble sugars. Rice straw was subjected to three distinct pretreatment procedures: acid-catalyzed steam explosion (Swan Biomass Company), acid hydrolysis (U.S. DOE National Renewable Energy Laboratory), and ammonia fiber explosion or AFEX (Texas A & M University). Standard conditions for each pretreatment were used, but none was optimized for rice straw specifically. Six commercial cellulases, products of Genencor International (USA), Novo (Denmark), Iogen (Canada) and Fermtech (Russia) were used for hydrolysis. The Swan- and the acid-pretreatments effectively removed hemicellulose from rice straw, providing high yields of fermentable sugars. The AFEX-pretreatment was distinctly different from other pretreatments in that it did not significantly solubilize hemicellulose. All three pretreatment procedures substantially increased enzymatic digestibility of rice straw. Three commercial Trichoderma-reesei-derived enzyme preparations: Cellulase 100L (Iogen), Spezyme CP (Genencor), and A1 (Fermtech), were more active on pretreated rice straw compared than others tested. Conditions for hydrolysis of rice straw using Cellulase 100L were evaluated. The supplementation of this enzyme preparation with cellobiase (Novozyme 188) significantly improved the parameters of hydrolysis for the Swan- and the acid-pretreated materials, but did not affect the hydrolysis of the AFEX-pretreated rice straw. The three pretreatment techniques were compared on a basis of a total yield and distribution of fermentable carbohydrates released by enzymatic hydrolysis (the highest possible substrate concentrations were used, 150 g/l for the Swan- and the acid- and 100 g/l for the AFEX-pretreated straw; enzyme loading of 6.7 filter paper units (FPU) and 6.7 cellobiase units (CBU) per gram of dry straw was the same for all pretreated materials). A combined yield of monosaccharides produced by a pretreatment step and by enzymatic hydrolysis was found to be 46, 42 and 37 g/l for the Swan-, the acid- and the AFEX-pretreated rice straw, respectively.