Integrated bioprocess for bio-ethanol production from watermelon rind biomass: Ultrasound-assisted deep eutectic solvent pretreatment, enzymatic hydrolysis and fermentation

Abstract

Herein, the potential of watermelon rind (WMR) biomass for bio-ethanol production through an integrated bioprocess consisting of combinative sequential ultrasonication and deep eutectic solvent (DES) pretreatments, enzymatic hydrolysis and fermentation was investigated. For the improvement of the efficiency of the WMR conversion into bio-ethanol, the effectiveness of the ultrasonic-assisted DES pretreatment on bio-ethanol yield was studied by optimization, adopting Central Composite Rotatable Design. Glucose was the most dominant sugar in the WMR biomass which enhanced its suitability as cheap carbon source for fermentation process. Maximum glucose and total reducing sugar yields of 60.17% and 83.03% were respectively obtained, and the enzymatic hydrolysis efficiencies ranged from 75.79 to 91.13%. The bio-ethanol yield ranged from 0.276 to 0.458 gg−1 raw WMR, corresponding to fermentation efficiencies of 54.12–89.80%, with the maximum obtained at ultrasonication power 180 W, ultrasonication frequency 40 kHz, ultrasonication time 40 min, DES reaction temperature 120°C and DES reaction time 180 min. Results showed that all the pretreatment factors considered had significant effect on bio-ethanol yield. It was established that combinative pretreatment was efficient for delignification, solubilization and hydrolysis of cellulose and hemicellulose into monomeric sugars by removing hemicellulose from the cellulose fibers. This enhanced fermentable sugar recovery, which consequently increased bio-ethanol yield.