Response Surface Optimization of Reducing-Sugar Release and Bioethanol Production from Cassava Peels Using Oven-Assisted Acetic–Citric Acid Hydrolysis

Abstract

Abstract
Cassava processing generates large quantities of peel waste that is commonly discarded or burned, yet its high carbohydrate content makes it a candidate feedstock for fermentable-sugar and bioethanol production. This study examined the oven-assisted hydrolysis of cassava peels using a combined acetic–citric acid system, followed by fermentation of the hydrolysate with Saccharomyces cerevisiae. A three-factor, modified central composite design with 20 runs evaluated the effects of acetic–citric acid concentration (A), oven temperature (B), and hydrolysis time (C) on reducing-sugar concentration. Reducing-sugar concentrations ranged from 9.38 to 24.76 g/L. The fitted quadratic model was highly significant (F = 47.50, p < 0.0001) and explained 97.71% of the variation (R² = 0.9771; adjusted R² = 0.9566; predicted R² = 0.8353). Hydrolysis time exerted the strongest effect, followed by oven temperature and then acid concentration; the acid and temperature quadratic terms were significant, whereas none of the two-factor interactions was significant. The lack-of-fit was statistically significant (p = 0.0001), a consequence of the very small pure error of the highly repeatable center-point runs rather than gross model inadequacy, and the model predicted an independent validation condition to within 6.50%. Numerical optimization identified a high-yield validation condition of 2.72% acetic–citric acid, 151 °C and 78 min, with a model-predicted reducing-sugar concentration of 26.34 g/L; triplicate validation gave 24.63 ± 0.41 g/L. Fermentation of the optimized hydrolysate yielded a maximum ethanol concentration of 8.92 g/L at 96 h, with a slight decline thereafter. Oven-assisted mixed organic-acid hydrolysis is therefore a workable, laboratory-accessible route for converting cassava peel waste into fermentable sugars and ethanol.