Optimization of sugar recovery efficiency using microwave assisted alkaline pretreatment of cassava stem using response surface methodology and its structural characterization

Abstract

Cassava stem is one of the prominent lignocellulosic wastes and has potential as a feedstock for fermentable sugar production. In this study, response surface methodology (RSM) with Box-Behnken design (BBD) was employed to investigate optimum conditions for microwave assisted alkaline pretreatment of cassava stem. Effect of four variables such as reaction time (60–120 s), NaOH concentration (2–4% w/v), solid to liquid ratio (1:25–1:75 g/ml), and microwave frequency (360–720 Hz) were evaluated to improve the sugar recovery. The quadratic model indicated that, reaction time of 116.4 s, NaOH concentration of 3.21% (w/v), substrate to liquid ratio of 1:62.07 g/ml and microwave frequency of 719.86 Hz was found to be optimum and obtained a maximum yield of 43.60 μg/ml of reducing sugar and 91.71 μg/ml of xylose. Under this condition, the cellulose content of cassava stem was increased from 33.27% to 52.34%, while the hemicellulose and lignin content was decreased from 32.30% to 27.15% and 27.15% 14.59%, respectively. Moreover, to evaluate the effectiveness of the pretreatment, Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD) and scanning electron microscope (SEM) analysis were employed on the untreated and pretreated cassava stem. These results suggest that the microwave assisted alkaline NaOH pretreatment (MAASHP) influences the fermentable sugar production significantly and further it can be utilized effectively for bioethanol production.