Cellulases and xylanase production by 'Aspergillus fumigatus' SK1 through solid state fermentation for ethanol fermentation

Abstract

Direct utilization of oil palm trunk (OPT) without chemical pretreatment for cellulases and xylanase production under solid state fermentation (SSF) was conducted in batch culture. A total of 12 fungal strains from Biorefinery Laboratory collections and 5 strains isolated from wooden board were able to secrete cellulases and xylanase based on the clear zones formed on selective agar plates. Aspergillus fumigatus SK1 showed significant enzymes productivities with the xylanase activity of 648.448 U g-1, CMCase of 48.006, FPase of 6.860, ß-glucosidase of 16.328 U g-1 and lignin peroxidase of 4.820 U g-1, respectively. Secretion of cellulases and xylanase by Aspergillus fumigatus SK1 was further confirmed by zymographic analysis. The crude cellulases-xylanase cocktail was highly stable at temperature lower than 40°C. The optimum temperature for FPase was 60°C and 70°C for CMCase, ß-glucosidase, and xylanase. Statistical optimization of cellulases and xylanase production was carried out involving General Factorial Design (GFD), 2- Level-Factorial Design (2LFD), and Central Composite Design (CCD). The GFD optimization demonstrated significant improvement of cellulases and xylanase production in medium supplemented with ammonium sulphate. The significant factors for xylanase production were incubation time and temperature, inoculum size, and ammonium sulphate concentration. These factors were optimized through CCD which produced approximately 4.28 fold higher xylanase activity (1792.43 U/g) compared to that before optimization. The enzymes cocktail produced from SSF was successfully applied in saccharification of chemical untreated OPT, producing a hydrolysate containing a maximum of 15.06 g/L reducing sugars after 24 hours incubation at 40ºC. Alcoholic fermentation of the hydrolysate by Candida tropicalis RETL-Crl and Saccharomyces cerevisiae were resulted in release of 3.067 g/L and 3.151 g/L of ethanol, respectively. The higher ethanol productivity (0.263 g/L/h), Yp/s (0.476 g/g) and specific ethanol productivity (0.0947 g/L/h/g of biomass) of Saccharomyces cerevisiae showed a great potential to be used in ethanol fermentation process.