Steam Explosion and Dilute Acid Pretreatments to Improve Enzymatic Hydrolysis and Ethanol Fermentation of Green Coconut Fibers

Pursuing a healthy life has driven the coconut industry over the years, and consequently, concerns about the fate of waste have increased. Inadequate disposal of green coconut shells can deteriorate local landscapes and increase the occurrence of disease vectors, so means of recovery are urgent. The present study proposes obtaining sugars and ethanol from green coconut shells through sequenced pretreatments. Steam explosion and acid pretreatments were carried out in the first stage, and alkaline peroxide and alkaline pretreatments in the second one. Enzymatic hydrolysis was conducted under different dosages of cellulases (10, 15, and 20 FPU/g), and the simultaneous saccharification and fermentation (SSF) and semi-simultaneous saccharification and fermentation (SSSF) strategies were carried out to produce ethanol. Due to the removal of hemicellulose and lignin, the application of sequenced pretreatments (sulfuric acid/sodium hydroxide and steam explosion/sodium hydroxide) achieved higher cellulose values (57.69% and 47.55%, w/w) and higher crystallinity indices (49.34% and 45.66%). Using samples from sequenced pretreatments guaranteed reduced cellulase dosage from 20 to 10 FPU/g without compromising the sugar release, with a maximum glucose yield of 50.8% at 20 FPU/g. Using 15% (w/v) solids, the SSSF of the sulfuric acid/sodium hydroxide sample achieved a maximum ethanol production of 18.39 g/L, which corresponds to a yield of 37.5%, using the Saccharomyces cerevisiae PE-2. Therefore, the present study is aligned with efforts to make cellulosic ethanol technology competitive and will contribute to an adequate use of green coconut shells.