Hydrolytic efficiency of two commercial enzymes and its impact on ethanol production from alkali-acid-pretreated rice straw

Recently, the discovery of oxidative enzymes role on lignocellulosic conversion has contributed to the development of new cellulase cocktails, such as Cellic CTec2, which also contains polysaccharide monooxygenases (LPMO’s). In the present work, this commercial enzyme was compared to mixture composed by Cellubrix (negligible LPMO’s) and Novozyme 188, regarding to cellulose digestibility and ethanol production by simultaneous saccharification and fermentation (SSF). Rice straw was sequentially pretreated with alkali and acid and the pretreated solids were submitted to hydrolysis (8% m/v solids, 12.5 FPU/g of each enzyme, pH 4.8, 100 rpm and 43 °C) aiming to evaluate the kinetic profile of both enzymes. In addition, under the same conditions, ethanol fermentation employing Kluyveromyces marxianus NRRL Y-6860 was performed by SSF. The results showed the same hydrolysis profiles, being attained digestibility of 90.3 and 83.6% at 48h from CTec2 and Cellubrix, respectively. Moreover, the maximum ethanol production by SSF was similar for both enzymes (23 g/L at 48h). Therefore, under the experimental conditions employed, the use of CTec2 did not show positive impact on hydrolytic efficiency as well as in the fermentation of pretreated rice straw. This behavior can be attributed to the pretreatment conditions that provided more accessible substrate, with lower content of lignin and hemicellulose, thus not requiring higher loadings of LPMO’s and accessory enzymes to enhance cellulose digestibility. Efforts will be directed to optimize enzyme loading at increasing solids content to attain higher ethanol titer in SSF.

Acknowledgments: FAPESP (N° 2013/13953-6), CAPES and CNPq