Optimization of alkali pre-treatment of sweet sorghum [Sorghum bicolor (L.) Moench] residue to improve enzymatic hydrolysis for fermentable sugars

Abstract

Sweet sorghum was used as a bioenergy crop to improve the enzymatic hydrolysis for releasing fermentable sugars as compared to other crops. Effective pre-treatment with NaOH at 121 °C for up to 50 min was reported in this study before enzymatic hydrolysis. The response surface methodology approach was used to optimize the parameters for alkali pre-treatment. The optimized parameters of pre-treated sweet sorghum residue were 1–3 % NaOH concentration, 0.5–1.5 mm particle size, 10–50 min time studied using box-behnken design. The findings suggest that 2 % NaOH with a 1 mm reduction in particle size in 50 min leads to enzymatic saccharification for fermentable sugars with a maximum loading of 62.7 % of cellulose. Enzymatic hydrolysis was performed with the aid of enzymes cellulase and xylanase. The optimum conditions obtained for fermentable sugars were 50 °C, 72 h of hydrolysis duration with 20 FPU/g substrate of enzyme loading for 20 g/L sorghum residue from one factor at a time approach. Maximum concentration of 78.3 ± 0.23 g/L of glucose, 5.4 ± 0.47 g/L of xylose with 1.53 ± 0.11 g/L of cellobiose was obtained from hydrolysate after 72 h enzymatic treatment. The characterization, before and after pre-treatment were done by FTIR and FESEM shows effective pre-treatment is important for delignification and increase in cellulose digestibility for improving the enzymatic hydrolysis.