Characterization of microwave-alkali-acid pre-treated rice straw for optimization of ethanol production via simultaneous saccharification and fermentation (SSF)

Abstract

Microwave-alkali-acid pre-treated rice straw was found most suitable feedstock for ethanol production via simultaneous saccharification and fermentation (SSF). Native rice straw had 42.54% cellulose, 24.51% hemicelluloses, and 9.16% lignin. Upon microwave-alkali-acid pre-treatment, cellulose content was increased to 60.07%, whereas hemicelluloses and lignin was decreased to 14.90% and 4.52%, respectively in solid fraction of the biomass. Physico-chemical characteristics of native and pre-treated rice straw were examined using SEM, XRD, TGA, FT-IR and solid state 13C CP/MAS NMR spectroscopy. SEM of pre-treated rice straw revealed distorted surface morphology and XRD analysis showed an increase (8.97%) in the crystallinity index (CrI) as compared to native biomass. Ethanol production was optimized for microwave-alkali-acid pre-treated rice straw using Taguchi orthogonal array design. An excellent correlation between predicted (18.74g/L) and experimental values (18.9g/L) for ethanol production was observed under optimized SSF conditions: substrate loading, 11% (w/v); enzyme concentration, 0.5% (v/v); inoculum size, 8% (v/v); pH, 4.5 at 30°C after 24h. A ∼1.3-fold gain in ethanol concentration (25.2g/L) was obtained which corresponded to a yield of 0.38g/g using a lab-scale bioreactor on scaling up the shake-flask SSF under optimal conditions.