Optimization of simultaneous saccharification and fermentation parameters for sustainable production of ethanol from wheat straw by Pichia stipitis NCIM 3498

ABSTRACT

Bioethanol is a potentially safe and renewable alternate source of energy. However, ethanol production from value added food and feedstock has not shown growth as estimated. Of late, the second generation processes of production of ethanol, such as from lignocellulosic biomass out of agricultural/domestic waste, has been gaining considerable momentum. Here, we attempted optimizing the conditions of physiochemical pretreatment as well as fermentation process using wheat straw by Pichia stipitis NCIM 3498 (now known as Schefferomyces stipitis). We have also studied the influence of process variables, such as incubation temperature, inoculum concentration and different nutrients, on ethanol production. Pulverized wheat straw consists of 32±0.31% cellulose, 48±0.37% hemicellulose and 17±0.15% lignin on dry solid (DS) basis. Wheat straw delignified with 1% HNO3, yielded 11.54% xylose and 1.54% glucose under steam explosion [15 psi (121°C) for 60 min], with a hydrolytic efficiency of 59.56%. Simultaneous Saccharification and Fermentation (SSF) of pretreated wheat straw by crude cellulose (produced by Trichoderma reesei NCIM 1052) and S. stipitis were investigated in the present study. Important process variables for ethanol production from pretreated wheat straw were optimized using response surface methodology (RSM) based on central composite design (CCD) experiments. A three level CCD experiments with central and axial points was used to develop a statistical model for optimization of process variables viz. incubation temperature (30, 32 and 34°C) X1, inoculum level (2, 4 and 6%) X2 and nutrients (1/2/3) X3. Data obtained from RSM on ethanol production were subjected to the analysis of variance (ANOVA), analyzed using a second order polynomial equation, and contour plots were used to study the interactions among three relevant variables of the fermentation process. The fermentation experiments were carried out at flask level. The processing parameters setup for reaching a maximum response for ethanol production was obtained when applying the optimum values for temperature (34°C), inoculum level (6%) and fermentation medium (ammonium sulphate, KH2PO4, peptone and yeast extract) for S. stipitis. Maximum ethanol concentration 7.15 g/L was obtained after 72 h from S. stipitis at the optimized process conditions in anaerobic batch fermentation.