Optimization of Dilute Acid Pretreatment for Enhanced Release of Fermentable Sugars from Sugarcane Bagasse and Validation by Biophysical Characterization

Pretreatment of biomass is one of the most challenging steps in the process of second-generation (2G) ethanol and biochemical production. Dilute acid pretreatment is a widely adapted and convenient method to recover pentose (C5) as well as hexose (C6) sugars due to its featured solubilization of hemicellulose and cellulose before and after enzymatic saccharification, respectively. In the present study, dilute sulfuric acid (H2SO4) pretreatment of sugarcane bagasse (SCB) was statistically optimized using the face-centered composite design (FCCD) of response surface methodology (RSM) in terms of acid concentration (0.1–3% v/v), solid loading (5–20% w/v) and residence time (15–60 min) at constant temperature of 121 °C followed by enzymatic hydrolysis using commercial cellulase (Novozymes Cellic CTec2) for enhanced combined sugar yield (CSY) comprising of C5 and C6 sugars in pretreated as well as saccharified hydrolysates. Optimized process parameters found in the study were 2.18% (v/v) acid; 14.35% (w/v) solid loading; and 29.49 min residence time. CSY under optimized conditions was found to be 521.42 ± 7.2 g/kg raw SCB with 72.06 ± 1.0% sugars recovered out of the maximum theoretical sugars present in the raw biomass. Total reducing sugar yields in pretreated and saccharified hydrolysates were found to be 215.28 ± 2.4 and 306.14 ± 5.3 g/kg raw SCB, respectively. Morphological and structural changes in optimized pretreated and saccharified biomass further validated the efficiency of optimized pretreatment applied in the present study. The maximum ethanol concentration, volumetric productivity and yield from released sugars were calculated as 10.82 ± 2.2 g/L, 0.45 ± 0.9 g/L/h and 0.42 g/g-glucose consumed or 71.45 ± 2.5 g/kg raw SCB, respectively. Ethanol yield obtained from the fermentation of dilute H2SO4-pretreated SCB was corresponding to 82.4% of the theoretical ethanol yield.