Optimization of dilute NaOH pretreatment at mild temperatures for monomeric sugar release from sorghum pith using response surface methodology

Abstract

Response surface methodology (RSM) was used to optimize the alkali pretreatment conditions for maximum fermentable sugar yield from sorghum pith with respect to NaOH concentration (0.5% to 2%), reaction temperature (20 °C to 40 °C), and pretreatment time (2 h to 20 h). The pretreatment caused a slight loss of glucan, but a significant removal of lignin and xylan from the pith, particularly lignin. The optimized results showed that the pretreatment conditions for the maximum predicted enzymatic glucose yield (90.5%) were 19.5 h, 2% NaOH, and 40 °C, while that for the maximum predicted enzymatic xylose yield (57.7%) were 9.9 h, 1.4% NaOH, and 37.5 °C. The optimized pretreatment conditions for the maximum total sugars yield were 16.2 h, 2% NaOH, and 37 °C, under which 72.4% of the glucan and xylan present in the raw material were experimentally hydrolyzed to release monomeric sugars. Additionally, with the optimized combination of 15 FPU cellulase and 7.5 CBU β-glucosidase per gram of pretreated material, the saccharification efficiency approached 90% glucan and 74% xylan present in the pretreated material (obtained at 37 °C for 16 h with 2% NaOH). This study may provide useful information for the development of novel alternative feedstocks for cellulosic ethanol production.