Ammonium Hydroxide Pretreatment of Sugarcane Bagasse and Corn Cob: Saccharification Enhancement and Ethanol Production by C6/C5 Cofermentations

The pretreatment of lignocellulosic biomass is a key step in second-generation (2G) ethanol production. In this study, sugarcane bagasse (SCB) and corn cob (CC) were pretreated with 28–30% ammonium hydroxide (1:10 w/v ratio, 30 min, 180 °C) to promote lignin removal and enhance enzymatic hydrolysis. Delignification levels reached 65.5 and 73.5% in pretreated (PT) SCB and CC, (PTSCB and PTCC, respectively), with preservation of the polysaccharide fraction. The biomasses were submitted to enzymatic saccharification using Cellic CTec3 (12.5 FPU/g cellulose) at solid concentrations of 10–30%. Fed-batch hydrolysis with 30% solids released the highest total reducing sugars (TRS), achieving 65.15 g/L in PTSCB and 70.18 g/L in PTCC, with increases of 238 and 137% in comparison to the batch at 10% of substrate. The hydrolysates presented low levels of fermentative inhibitors and high concentrations of glucose (43.75 and 41.63 g/L) and xylose (18.12 and 18.75 g/L) in PTSCB and PTCC, respectively. The fermentation of PTCC hydrolysate with the nonconventional Spathaspora passalidarum Y7204 monoculture reached the ethanol production (26.34 g/L, Yp/s = 0.44, and Qp = 0.73 g/L·h), while the commercial Saccharomyces cerevisiae Y904 yielded 19.18 g/L (Yp/s = 0.47; Qp = 0.80 g/L·h). The cocultures of both yeasts generated around 20 g/L of ethanol; however, metabolic interactions and catabolic repression may have occurred. These findings evidenced the NH4OH pretreatment as a promising strategy for enhancing saccharification. In addition, the study highlighted the positive role of S. passalidarum in the hexose (C6) and pentose (C5) cofermentation of PTCC hydrolysate.