Integration of Subcritical Water and Enzymatic Hydrolysis to Obtain Fermentable Sugars and Second-Generation Ethanol from Sugarcane Straw

Abstract

Converting the chemical and biologically resistant structure of cellulosic biomass into useful products is a technological and economic challenge. The use of sustainable and efficient conversion technology sustainably has been gaining prominence. This work aimed to evaluate the integration of subcritical water hydrolysis (SWH) and enzymatic hydrolysis of sugarcane straw to obtain fermentable products and second-generation ethanol. SWH was conducted under semi-continuous conditions at 200 °C, 5 mL/min, and a 19-min retention time. Enzymatic hydrolysis using an enzyme blend produced simple sugars that were evaluated for fermentation using two strategies: (i) Saccharomyces cerevisiae SA-1; and (ii) co-culture of S. cerevisiae SA-1 and Scheffersomyces stipitis NRRLY-7124. Integrating subcritical water and enzymatic hydrolysis resulted in greater fermentable sugar yield (2.40 ± 0.01 g/L of xylose and 8.9 ± 0.6 g/L of glucose) than subcritical pre-treatment on its own. The co-culture fermentation strategy was more efficient than fermentation by S. cerevisiae on its own, resulting in 2.8 ± 0.3 g/L of ethanol. Subcritical water hydrolysis pre-treatment disrupted the lignocellulosic matrix, leading to increased enzymatic accessibility of cellulose, with minimal formation of degradation compounds, and with minimal waste formation. Future work can optimize the hydrolysis process to increase the yield of simple sugars, increase fermentation efficiency by controlling oxygen concentration and removing acetic acid, and consequently increase the yields of second-generation ethanol.