Simultaneous saccharification and bioethanol production from underutilized biomass, cowpea haulm using co-cultures of Saccharomyces cerevisiae (BY4743) and Scheffersomyces stipitis (PsY633)

Abstract

Cowpea cultivation generates large quantities of biomass after pod harvest which are underutilized and could be exploited for energy generation. In this study, evaluation of sugar production by dilute sulfuric acid pretreatment of cowpea haulm for potential ethanolic fermentation was carried out. A central composite design (CCD) was used to investigate the effects of temperature (100–120 °C), time (30–90 min), and acid concentration (1.0–4.0%) on sugar yield and inhibitor formation. The model F-values of 25.42, 78.81, and 6.96 for xylose yield, glucose yield, and total inhibitor concentration, respectively, with low probability value (p < 0.05) suggest a high significance of the models. While the R2-values of 0.9581, 0.9861, and 0.8424 indicated a satisfactory agreement of the quadratic models in predicting the responses. The quadratic model predicted optimum conditions for maximum xylose (76.5%) and glucose yield (28.6%), at minimum inhibitor concentration (2.34 g/L) and temperature (110 °C), with an acid concentration of 3.1% and reaction time of 55 min. These conditions were validated experimentally suggesting the effectiveness of experimental design towards process optimization. The resulting sugar-rich prehydrolysate was detoxified and fermented to ethanol using co-cultures of Saccharomyces cerevisiae BY4743 and Scheffersomyces stipitis (PsY633), while the recovered pretreated solid was subjected to simultaneous saccharification and fermentation with prehydrolysis (PSSF). A total ethanol titer of 15.67 g/L was obtained which corresponds to an overall conversion efficiency of 75%. This suggests that cowpea haulm could also be potentially exploited for bioethanol production either singly or in combination with other lignocellulosic biomass.